Serial ports
      Serial port/cable hardware
            Signals, by pin
                  input:Rx, output:Tx
                  output:RTS, Request To Send
                  output:DTR, Data Terminal Ready
                  input:DCD, Data Carrier Detect; also CAR, Carrier; CD, Carrier Detect; RLSD, Receive Line Signal Detect
                  input:DSR, Data Set Ready; also LE, Line Enable
                  input:CTS, Clear To Send
                  input:RI, Ring Indicator; also RNG, Ring
                  reference:GND
            RS232 - DTE, DCE, null-modems
            Wiring and signals, by use
                  modem
                  null-modem
                  mice and assorted small serial devices
                  digital control signals (Rx/Tx unused)
                  GPS
                  bootloaders
                  RS485
                  UART
            Drivers and levels
                  mark/space/break
                  UART
                  RS232
                  RS422
                  RS485
                  old terminals/teletypes
      Serial port, software interfacing
            serial port configuration
                  data frame
                  break
                  baudrate vs bitrate
                  speed constraints
                  flow control
                  split speeds
      Serial baud rates

Serial ports

For serial-over-TCP see /projects/sw_SerialOverIP_RFC2217/
For USB-serial chips see /projects/hw_chip_USBinterfaces/

Serial port/cable hardware

Many devices are interfaced via a form of a serial port. The possible implementations are:

barebones internal

UART - low-voltage internal bus, logical levels, near-GND for L, near-Vcc for H; usually 5V or 3.3V, basic communication to line transceivers; common on many embedded modules

with driver

RS-232 - discrete Rx and Tx lines, high-voltage signaling, +3..12V to GND for H, -3..-12V to GND (sometimes just GND) for L
RS-422 - discrete Rx and Tx lines, each as differential-voltage pair
RS-485 - shared multidrop differential bus
USB-serial - a type of USB adapter with UART or RS-something output

Many microcontrollers, from Arduino-class to ESP8266 and ESP32, are equipped with a serial port and a bootloader, allowing in-system flashing of new firmware.

Signals, by pin

input/output from the perspective of the computer ("DTE")

input:Rx, output:Tx

sending and receiving of data, serial synchronous

output:RTS, Request To Send

indicates to modem it can send data
reset for ESP8266/esptool bootloaders
power for mice
RTS/CTS flow control

output:DTR, Data Terminal Ready

indicates to modem it is operational
reset for Arduino-type bootloaders
bootloader mode selection at reset for ESP8266/esptool
power for mice
on null modems, connected to remote DCD-DSR pair

input:DCD, Data Carrier Detect; also CAR, Carrier; CD, Carrier Detect; RLSD, Receive Line Signal Detect

indicates to computer the modem is receiving carrier tone from the other side, that the link is established and alive

deasserting leads to "NO CARRIER" error

on null modems, paired with DSR and connected to DTR output on the other side
Pulse-Per-Second input with GPS receivers, for timekeeping

input:DSR, Data Set Ready; also LE, Line Enable

indicates to computer the modem is operational
on null modems, paired with DCD and connected to DTR output on the other side

input:CTS, Clear To Send

indicates to computer the modem is ready to accept data
RTS/CTS flow control

input:RI, Ring Indicator; also RNG, Ring

asserted by modem when call is incoming, "RING"
computers can have "wake on ring" function, for power saving until communication from outside happens

reference:GND

signal ground

also called SG
beware of ground loops and potential differentials between devices too far apart and connected to different mains socket circuits

protective ground (PG, PGND), shield

cable shielding
usually connects to machine's chassis
improper shield grounding ruins noise immunity, causes hard to chase intermittent bugs

RS232 - DTE, DCE, null-modems

The serial output has two data lines:

Data

Rx - Receive; also RxD
Tx - Transmit; also TxD

Control output

DTR - Data Terminal Ready, DTE/computer ready
RTS - Request to Send

Control input

DSR - Data Set Ready, DCE/modem ready; also LE (Line Enable)
CTS - Clear to Send
DCD - Data Carrier Detect; also CAR (Carrier), CD (Carrier Detect), RLSD (receive line signal detect)
RI - Ring Indicator; also RNG (Ring)

Reference

GND - ground; also SG, signal ground
PG - protective ground, shielding

RS-232 devices have two flavors:

DTE - Data Terminal Equipment - male connector on device, usually computers

from point of view of coders and admins, this is the main reference as seen from within the machine

DCE - Data Circuit-terminating Equipment - female connector on device, usually modems

CAUTION: the maintaining of the Rx and Tx terminology for pins that CONNECT TO Rx/Tx instead of that ARE Rx/Tx is confusing

RS-232 connectors come in a few common variants (and a plethora of ad-hoc vendor-specific ones):

9-pin D-sub: TIA-457
25-pin D-sub: RS-232C (?)
RJ-45: specified by EIA-561; many pinouts possible, no standard or too many standards, a common one is shown

EIA signal 9pin  25pin RJ45   DTE (DCE)  pairs
BB  Rx      2      3    5     IN   out    Tx     serial data receiving
BA  Tx      3      2    6     OUT  in     Rx     serial data transmitting
CA  RTS     7      4    8     OUT  in     CTS    computer asks modem to send data
CB  CTS     8      5    7     IN   out    RTS    modem is ready to accept data - response to RTS, after few msec
CD  DTR     4     20    3     OUT  in  DCD+DSR   computer is operational; when asserted, accepts/continues call; often low by OS when port is unused
CC  DSR     6      6   (1)    IN   out    DTR    modem is operational
CF  DCD     1      8    2     IN   out    DTR    modem indicates carrier present, remote side is sending signal"; presence of remote party and active connection
CE  RI      9     22    1     in   out    n/a    modem announces incoming call; input on computer, omitted on nullmodems
AB  GND/SG  5      7    4     ref  ref           signal ground
AA  PGND           1                             protective ground, shielding

Some devices use RJ45 or RJ11 connectors. Some pinouts listed here.

Wiring and signals, by use

modem

DTE-to-DCE cable: all wires straight through, no crossovers
male on computer, female on modem, corresponding F-M on cable ends

null-modem

DTE-to-DTE, device A to device B: (---- is link via cable, ++ is short link inside connector)
males on both computers, females on cable ends

ALWAYS:          A:Rx-----B:Tx,   A:Tx-----B:Rx,    A:GND----B:GND

simple:          no additional lines
self-control:    A:RTS++A:CTS,    B:RTS++B:CTS,     A:DTR++A:DSR++A:DCD,  B:DTR++B:DSR++B:DCD
part-self:                                          A:DTR++A:DSR++A:DCD,  B:DTR++B:DSR++B:DCD
RTS-CTS:         A:RTS----B:CTS,  A:CTS----B:RTS
partial:         A:RTS----B:CTS,  A:CTS----B:RTS;   A:DTR++A:DSR++A:DCD,     B:DTR++B:DSR++B:DCD
EYN253C,full:    A:RTS----B:CTS,  A:CTS----B:RTS,   A:DTR----B:DSR++B:DCD,   B:DTR----A:DSR++A:DCD
EYN254C:         A:DTR----B:DSR++B:CTS++B:DCD,      B:DTR----A:DSR++A:CTS++A:DCD

mice and assorted small serial devices

DTR and RTS are ganged together (usually via two diodes) and both set to 1 to provide power

digital control signals (Rx/Tx unused)

the port can be used for sending digital signals by eg. optocoupler or relay
CAUTION: many operating systems probe ports on boot for detecting of modems and mice; this may wreak serious havoc

the port starts with DTR=0,RTS=0; then PnP sequence raises DTR=1, pause, raises RTS=1, pause, drops both to zero
RTS=1,DTR=0 combination should not occur during PnP autodetect
an optocoupler with LED from RTS(+) to CTS(-) should therefore not activate during PnP sequence
tested with hw_remotereset, hardware reset of one computer from another

GPS

Rx,Tx - position/time data, usually NMEA 0183 formatted; setting of the receiver
DCD - PPS sync input for precision timekeeping, by RFC2783[ref]

beware of jitter on USB-serial interfaces (up to 125 usec for USB2)

bootloaders

Rx,Tx - communication with the device
DTR

Arduino: reset
ESP8266/esptool: bootloader mode selection on reset

ESP8266/esptool: reset

RS485

Rx,Tx - communication with the bus
RTS - usually used to enable the transceiver's transmitter (needs precision timing, some controllers can have it done in hardware in RS485 mode or have a dedicated output; FT232 family is good here)

UART

often 3-pin connector, with Rx,Tx,GND
4-pin can also have Vcc, useful for level converters

TP-Link AC750: Vcc-GND-Tx-Rx
My Passport Wireless: GND-Rx-Tx-3.3V
DJI Onboard SDK: 9V-GND-Tx-Rx
USB-UART CP2102 module: 3.3V-GND-5V-Tx-Rx-DTR-(RTS)
Electrobot USB-to-TTL: 5V-GND-Rx-Tx-3.3V
CP2102 another: VCCIO-GND-Tx-Rx-RTS-CTS
CP2102 micro: 5V-GND-Tx-Rx-DTR-3.3V
Odroid, CON5: GND-Rx-Tx-Vcc
Raspi, header edge: 5V-5V-GND-Tx-Rx

CAUTION: as demonstrated, there is no pinout standard for board UART connectors!

Drivers and levels

mark/space/break

current-loop implementation, early systems similar to rotary dials
mark - circuit is closed, current is flowing - default idle state
space - circuit is interrupted
break - space longer than framing period, error condition or signaling

UART

bare UART is just TTL signaling (usually 3.3V or 5V)
signals are H when idle, L when active/asserted
MARK = H, level around Vcc
SPACE = L, level around GND
control signals are H when unset (0), L when asserted (1)

RS232

signals are L when idle, H when active/asserted
MARK = H, level around +3..+15V
SPACE = L, level around GND to -15V
drivers invert TTL-level signals and increase voltage
DTR+RTS can be used to power small devices (usually mice); caution, connect via diodes to prevent current flow between pins when in unequal state!
control lines are L when unset, H when asserted

RS422

wiring like ordinary RS-232
separate Rx, Tx, control lines
signals use differential pairs instead of single wire to ground

L level when one wire in the pair is higher voltage than the other, H when the other way
if signal line polarity is wrong, swap the wires in the pair

faster, more resilient to noise/EMI
for longer distances or higher speeds
RS-422 converts to RS-232 just by swapping the line drivers

RS485

shared differential bus for Rx,Tx

multiple devices on bus
wires A,B
MARK = H, when A+ B-, A negative and B positive
SPACE = L, when A- B+, A positive and B negative
CAUTION: datasheets often have the names the other way - BEWARE when assigning wires, be prepared to swap, this one can be wrong too

only one device can transmit at time

RTS used to enable the transmitter

usually no control signals on the bus, comm handled by a protocol

usually a master-slave kind with polling of the slaves

fast, resilient to EMI
for long distances
very common in industrial systems
underlying layer for many other buses

old terminals/teletypes

usually a current loop

bipolar +-20mA (uncommon)
40mA, 80-110V
60mA, 80-110V

current drives an electromagnet, a high-inductance coil

high voltage with series ballast resistor used
counteracts the resistance of the long cables
counteracts the coil's inductance
for local power, even 12V could work at 50 bps in some cases

when circuit idle, current flows through ("MARK")
signalling done by periodic interruption of circuit ("SPACE")

too long space leads to BREAK condition

Serial port, software interfacing

In Linux, the serial ports are character devices. Plain input/output is handled by writing data to the device and reading from it.

"Out of band" operations, like setting baud rate and handling the control lines, is done by performing ioctl operations on the opened file.

The serial port names are usually:

/dev/ttyS* - standard hardwired serial ports on x86 architectures
/dev/ttyAMA* - hardwired serial ports on Arm processors
/dev/ttyUSB* - USB-serial converters, USB CDC, newer than ACM
/dev/ttyACM* - USB-serial converters, USB CDC-ACM, communication/modem class, "Abstract Control Model"
/dev/ttySAC* - on some Samsung systems
/dev/rfcomm* - typ. serial-over-Bluetooth
/dev/ircomm* - typ. IrDA
/dev/ttyHSL* - Bluetooth
/dev/ttyHS* - Bluetooth
/dev/ttyGS* - USB Gadget serial port, USB-OTG peripheral
/dev/cua* - on old unixes or FreeBSD; modems (to differentiate from tty*, the terminals)

Windows have a very similar concept to ioctl, the DeviceIoControl call.

serial port configuration

For the communication, several configuration options are required:

speed, the number of bits per second
data bits, usually 8, can be also 7, 6 or 5, rarely 9

5 - early teleprinters, usually with Baudot/ITA2 code
6 - rarely used
7 - 7bit ASCII, often used with even or odd parity
8 - entire byte, most common
9 - rarely used, unsupported on most hardware, special purposes/protocols

data bit order

most often little endian, starts with LSB (least-significant bit) first
in rare cases with big endian, bit order can be reversed in software

parity, usually last in bit sequence

N, none - no parity bit present
E, even - parity bit is set to make number of 1 bits even
O, odd - parity bit is set to make number of 1 bits odd; more useful than even, forces at least one state transition into character, better in detecting baudrate mismatch induced errors
mark - parity bit is always 1 (unusual, mostly not supported)
space - parity bit is always 0 (unusual, mostly not supported)

stop bits - minimum time of the bus being idle, in bit periods

1 (most common)
2 - pre-1930 electromechanical typewriters, to enforce time needed for letter impact
1.5 - on 5-bit Baudot mechanical teleprinters

rarely supported on modern hardware
supported by eg. 16C650 UART
on non-supporting hardware use 2 on Tx side, and 1 for Rx side

data frame

for 8N1 (8bits, no parity, 1 stop bit) there are 10 bits per byte transmitted; 9600 bps == 960 bytes/sec

A sequence of one lonely byte, compared with a few together, looks

For 8N1 (most common):
 ^^^^^^^^sddddddddS^^^^^^^^
 ^^^^^^^^sddddddddSsddddddddSsddddddddSsddddddddS^^^^^^^^^ structure
 111111110dddddddd10dddddddd10dddddddd10dddddddd1111111111 template
 111111110001010101010100110101100111010001011101111111111 example 8N1
          00101010  10100110  11001110  00101110           payload

For 7E1 or 7O1:
 ^^^^^^^^sdddddddpS^^^^^^^^
 ^^^^^^^^sdddddddpSsdddddddpSsdddddddpSsdddddddpS^^^^^^^^^ structure
 111111110dddddddp10dddddddp10dddddddp10dddddddp1111111111 template
 111111110001010111010100110101100111110001011101111111111 example 7E1
 111111110001010101010100111101100111010001011111111111111 example 7O1
          0010101   1010011   1100111   0010111            payload

For 8E1 or 8O1:
 ^^^^^^^^sddddddddpS^^^^^^^
 ^^^^^^^^sddddddddpSsddddddddpSsddddddddpSsddddddddpS^^^^^ structure
 111111110ddddddddp10ddddddddp10ddddddddp10ddddddddp111111 template
 111111110001010101101010011001011001110110001011100111111 example 8E1
 111111110001010100101010011011011001110010001011101111111 example 8O1
          00101010   10100110   11001110   00101110        payload

For ol' 5N2/ITA2:
 ^^^^^^^^sdddddSS^^^^^^^^^^
 ^^^^^^^^sdddddSSsdddddSSsdddddSSsdddddSS^^^^^^^^^^^^^^^^^ structure
 111111110ddddd110ddddd110ddddd110ddddd1111111111111111111 template
 111111110000011101000011010100110000011111111111111111111 example 5N2
          00001   10000   10100   00001                    payload

  ^ - idle line - UART in H
  s - start bit - 1 cycle, L
  d - data bits - usually little-endian, LSB-first
  p - parity bit - optional, last in data bit sequence
  S - stop bit - 1, 1.5 or 2 cycles, H; functionally equivalent to idle line

  test sequence: ASCII: "Test", 0x54-0x65-0x73-0x74
                 ITA2:  "TEST", 0x10-0x01-0x05-0x10 ("Baudot")

There may or may not be additional arbitrary space (not necessarily a multiple of bit cycle) between the stop bit and the subsequent start bit. The front edge of the start bit synchronizes the receiver's bit clock.

break

A break is a stream of zero bits, usually for 250-500 msec. Usually either

ignored
output as 0x00
translated to 0xFF 0x00 0x00

Sent by IOCTLs:

TCSBRK - send break
TCSBRKP - send break, with argument in duration in 100s msec ("deciseconds")
TIOCSBRK - start break
TIOCCBRK - end break

baudrate vs bitrate

In communication, symbols are fed through the data line at a given rate per second. A symbol can be a discrete value of signal amplitude (or voltage on the line), frequency or frequency combination (FSK - a discrete-level variant of FM), phase (PSK), or combination (QAM, QPSK...)...

There are possible encodings of bits per symbol, ranging from one (or less than one in some cases) to many:

type        bits/baud   type
logical 1/0      1      common serial UART
QPSK             2
8-PSK            3
16-QAM           4
64-QAM           6
256-QAM          8
CIS-45          45      OFDM, 45 parallel tones, 33.33 or 40 Bd (1500 or 1800 bps)
CIS-60          60      OFDM, 60 parallel tones, 35.555 Bd (2133.33 bps)
CIS-93          93      OFDM, 93 parallel tones, 22 Bd (2046 bps)

speed constraints

physics

available RF or cable bandwidth (3kHz for analog telephones, may be way less for narrow-bandwidth radio)
signal-to-noise - eg. noisy environments with the constellation diagram dots smeared all around need modulation schemes with fewer dots further apart (and therefore fewer bits per baud)
distortion in media - cables eat signal edges, reflections from wrongly terminated cables add up and cause spurious signals

electronics

the system may not be able to handle incoming data fast enough, at higher speeds even the mere hauling of incoming bits from the input register to a buffer may be too much for a microcontroller
electromechanical terminals with no buffers and no flow control just can't handle faster speeds

the IBM Selectric had also a low-speed constraint, where characters coming in too slow gave time for its clutch to disengage; repeated engaging of clutch for each character at 110 bps led to premature wear, so 134.5 bps (where the data flow rate was just about equal to the printing) became a standard

flow control

To give the receiving system the ability to tell the sender to back off a little because it is sending data too fast, flow control schemes can be deployed. The most common are:

none - let it go and when there's too much of data just lose them; data delivery rate can be throttled down by the baud rate of the link
in-band - XON/XOFF; send a symbol to the other side to stop (XOFF) until a symbol to start (XON) is sent again; usually they are ctrl-S and ctrl-Q
out-of-band - hardware, usually RTS/CTS; RTS output goes active when recipient is ready to receive data, goes down when the recipient is processing and can't handle further input
CAUTION: improper setting of flow control can block data line - everything should work but the sender thinks it should not be sending anything

check the XON/XOFF setting
check the RTS/CTS setting, the state of the CTS input on the sender side

split speeds

In many cases the speed needed in one direction is much lower than for the other direction.

A terminal attached to a computer can benefit from high speed on the downlink but the same speed would be wasted on the meat-in-the-chair's keystrokes.

Hence, split speeds (downlink/uplink); eg.:

600/75
600/150
1200/75 - French Minitel, German BTX
1200/150
2400/300
2400/600
4800/300
4800/600

Similar principle applies for eg. ADSL lines, where the assumption is that the downlink bandwidth (web browsers, video, assorted downloads) will be needed to be way higher than the uplink (webpage requests, occasional email or chat message).

Serial baud rates

The speed of the line, the bitrate (or baudrate, for usual UART/RSxxx/currentloop one-bit-per-baud they are equal), can vary widely.

The bit clock is generated by binary division of a master clock source, usually a crystal oscillator. (PLL can be also employed but these are rare with ordinary serial ports.)

Many devices have fixed list of supported bitrates. Some have the unused once-common-standard speeds remapped to more modern ones missing in the tables. Eg. some Amiga computers have the 31250 bps rate (common for MIDI) in the setting for 134 bps - the target audience had more musical equipment to talk with than IBM Selectrics.

 c_flag = bitrate identification number for unix termios structure
 DIV = divisor from 1.8432 MHz clock divided by 16 = from 115200
 DIV1 = divisor from 1 MHz
 DIV12 = divisor from 12 MHz (FTDI chips use 3MHz clock reference)
 CAN = CAN Bus
 NICAN = National Instruments NI-CAN hardware - CAN bus bitrate, less common
 Lantronix PFAL = Lantronix CAN bus interface speeds [ref]

 CP2102,CH304,PL2303 = common USB-UART chips supporting the bitrate

 s=standard speed
 S=standard speed, common
 c=CAN bus speed
 C=CAN bus speed, common
 v=vehicles, not-CAN
 r=radio link speed
 t=teletypes
 T=teletypes, common
 m=modems
 i=industrial

            rate   c_cflag  DIV DIV1 DIV12
           OTHER   0x1000                   requires termios2, the speed is then in c_ispeed, c_ospeed
               0   0x0000                   no connection
            1/60                            IRIG timecode D
               1                            many [link?:time signal] radio stations, eg. DCF77, TDF time signal; IRIG timecode H
    v          5                            initialization for OBD-II ISO9141-2 and ISO14230-4 car diagnostic protocol (K-line)
              10                            IRIG timecode E
tr            45                            RTTY 45 baud, common amateur radio standard, 5-bit (eg. 14075 KHz USB); often a shorthand for 45.45(45)
Tr       45.4545                    264000  TTY V.18, "60-speed" teleprinters (USA), 22msec pulse, 5-bit/ITA2, 5N2; amateur-radio RTTY; Teletype Model 15, most audio recording of teleprinters; some HAM wireless
tr            50   0x0001   2304    240000  TTY V.18, "66-speed" teleprinters (Europe), 20msec pulse, 5-bit/ITA2, 5N2; common Telex, news agency wires, from year 194x; CH340; GPS signal, legacy C/A PRN, LNAV, L5 CNAV
tr            56.875                        "75-speed" teleprinters, 5-bit/ITA2
tr            74.2                          "100-speed" teleprinters, 5-bit/ITA2
Tr s          75   0x0002   1536    160000  5-bit teleprinters; reverse channel in some V.23 1200bps modems; RTTY 75 baud, 5 bit (eg. 10536 kHz USB), common standard in weather data transfer; CH340,PL2303
 r           100           10000    120000  packet radio, AMTOR; IRIG timecode B (eg. IRIG B122); CH340; GPS signal, L1C CNAV2, GLONASSL1OC and L3OC
T ms         110   0x0003                   typical teleprinter speed; Bell 101 modem; CH340,PL2303
T            134.5 0x0004                   IBM2731 mechanical teletype terminal (1.5 stop bit); IBM Selectric typewriter - at 110 bps the internal clutch kept engaging/disengaging and wearing out mechanism[ref]; CH340
  ms         150   0x0005    768     80000  2*75; acoustic coupler modems, typical reliable speed; CH340,PL2303
    v        160                            vehicles; ancient (1980) General Motors on-board diagnostics, PWM; Assembly Line Diagnostic Link (ALDL)
 r           200   0x0006    576     60000  some terminals; unusual; packet radio, PACTOR; some automotive K-line diagnostics
  ms         300   0x0007    384     40000  modem, V.21 (full duplex, 300baud, FSK); acoustic coupler modems, 1972; Pennywhistle modem; Hayes Smartmodem (audio FSK, 1981), Bell 103 modem (also packet radio, eg. 14105 kHz USB, 147.060 NFM); "300bps,N,8,1"; common packet radio speed below 30 MHz; IRIG timecode J-12; CP2102,CH340,PL2303
             450             256            some terminals; unusual
 r           512             225            POCSAG 512 baud, radio, paging format originating in the UK, used widely in the US
  ms         600   0x0008    192     20000  modem, V.22 (full duplex, 600baud, PSK); Golay Motorola radio paging signal; IRIG timecode J-13; CP2102,CH340,PL2303
  m          700                            one of CDMA2000, reverse packet data control channel
  m          800                            one of CDMA2000, reverse acknowledgment channel
             900                            CH340
            1000                            IRIG timecode A
  ms        1200   0x0009     96     10000  modem, V.22 (full duplex, 600baud, QPSK), Bell212A (QAM), Bell202 (1200baud, FSK), V.23 (half-duplex, FSK, sometimes 75bps reverse channel, eg. French Minitel, German BTX); acoustic coupler practical upper limit; Bell 202 modem AFSK (1200Hz mark, 2200Hz space, half-duplex; packet radio (eg. 144.39 NFM, 145.030 NFM), HART); DALI[ref] bus; POCSAG 1200 baud; various telemetry; IRIG timecode J-14; one of CDMA2000; CP2102,CH340,PL2303
  m         1350                            one of CDMA2000
  m         1500                            one of CDMA2000
  m         1600                            FLEX, radio paging (929 and 931 MHz US)
  m         1800   0x000a     64            less common; some leased-line modems, Bell 202C/D/R/T (FSK); about the ceiling of FSK on phone line; one of CDMA2000; CP2101,CH340,PL2303
            2000                      6000  some terminals; unusual
! mS        2400   0x000b     48      5000  modem, V.22bis (full duplex, 600baud, QAM), V.26bis (1200baud, PSK); POCSAG 2400 baud; DPSK common modem modulation; some multimeters; IRIG timecode J-25; CP2102,CH340,PL2303,HT42B534
  m         2700                            one of CDMA2000
 rm         3200                            FLEX, radio paging (929 and 931 MHz US); one of CDMA2000
 rm         3600              32            0.5*7200; modem, Bell 203A/B/C (vestigial sideband); unusual; various mobile radio trunking control (Motorola Type 1); one of CDMA2000; CH340,PL2303
  m         3800                            one of CDMA2000
            4000                  250 3000  unusual; CP2102
!rmS        4800   0x000c     24      2500  modem, V.32 (half-duplex, 2400baud), V.29 (PSK/QAM), V.32bis, V.27ter (1600 baud, PSK, 1650Hz carrier), Bell 208A/B (8phi-PSK); wireless, dispatch control systems, Automatic Train Control System; some slower GPS; various mobile radio trunking control; ISO9141-2/ISO14230-4 in cars (K-line, optional speed); IRIG timecode J-26; one of CDMA2000; CP2102,CH340,PL2303,HT42B534
    c       5000                  200 2400  NICAN
    c       6150                            NICAN
 r          6250                            ERMES paging (European Radio MEssage System), wireless (169MHz in France, Hungary, Malaysia)
 r          6400                            FLEX, radio paging (929 and 931 MHz US)
 rm         7200              16            modem, V.29 (PSK/QAM), V.32bis, Bell 203A/B/C (vestigial sideband); one of CDMA2000; unusual; CP2102,PL2303
    c       7812.5                128 1536  1/2 15625, 1/16 125000; NICAN
    c       8000                  125 1500  NICAN
    v       8192                            vehicle diag, ancient (1986); GM XDE-5024B; upgraded ALDL; half-duplex [ref]
!rmS  i     9600   0x000d     12      1250  modem, V.32 (half-duplex, 2400baud, QAM, 1650Hz carrier), V.29 (PSK/QAM), V.32bis, Bell209A (QAM); common factory default (8N1) for slower speeds; very common general communication rate for lower-speed devices (GPS, multimeters, MODBUS, Profibus...); ISO9141-2/ISO14230-4 in cars (K-line, optional speed); various mobile radio trunking control; IRIG timecode J-27; CP2102,CH340,PL2303,HT42B534
    c      10000                  100 1200  NICAN, Lantronix PFAL; IRIG timecode G
    v      10400                            some OBD-II car interfaces, bidirectional serial line, "K-line"; ISO 9141-2 (async 8n1) is primarily used in Chrysler, European, and Asian vehicles; ISO 14230 KWP2000 (Keyword Protocol 2000, async 8n1), Ford UBP; default speed for ISO9141-2, ISO14230-4; ELM327 protocol 2, SAE J1850 VPW (mostly GM vehicles); ELM327 protocol 3, ISO9141-2, 5baud init; ELM327 protocol 4/5, ISO14230-4, 5baud/fast init
           11520              10            1.2*9600
  m        12000?                     1000  0.5*24000; modem, V.32bis (TCM/Trellis); fax, V.17 (fax, TCM/Trellis modulation)
    c      12500                   80  960  NICAN
  m        14400               8            1.5*9600; modem, V.32bis (TCM/Trellis), V.33 (4-wire), HST (USRobotics); fax, V.17 (TCM/Trellis modulation); mobile GSM CSD (2G), GPRS (2.5G) upstream, one of CDMA2000; CP2102,CH340,PL2303
    c      15625                   64  768  1/2 31250, 1/8 125000; NICAN; SAE J1708 (62500/9600)
    c      16000                  62.5 750  NICAN; CP2102
  m        16800                            modem, US Robotics HST version (unusual)
! mS  i    19200   0x000e/EXTA 6       625  modem, V.32terbo/V.32ter (TCM/Trellis, non-ITU-T, AT&;T, unusual), V.34; some faster multimeters, MODBUS, Profibus, LIN; CDPD, Cellular Digital Packet Data; IRIG timecode J-28; EXTA for external baudrate generator A; CP2102,CH340,PL2303,HT42B534
    c      20000                   50  600  NICAN, Lantronix PFAL
  m        24000                       500  modem, V.34
    c      25000                   40  480  NICAN
  ms       28800               4            3*9600; modem, V.34 (3200 baud, TCM/Trellis), V.FC/V.FastClass/V.FAST (non-ITU-T, Hayes/Rockwell); GPRS (2.5G) upstream, one of CDMA2000; CP2102,PL2303
  m        31200                            modem, V.34+/V.34bis
!  Sc      31250                   32  384  1/4 125000; serial MIDI, NICAN; Profibus PA (trapezoidal biphase Manchester encoding)
           32000                       375
    C      33300                            SWCAN, single-wire CAN, SAE J2411; eg. Tesla ChargePort; Lantronix PFAL
  m        33600                            modem, V.34+/V.34bis (3429 baud, TCM/Trellis), upstream for V.90/V.92 when digital off; near Shannon limit for 3kHz bandwidth; CH340
!  s       38400   0x000f/EXTB 3      312.5 38400 baudrate often needs a specific kludge; EXTB for external baudrate generator B; IRIG timecode J-29; CP2102,CH340,PL2303,HT42B534
    c      40000                   25  300  NICAN
  m        40800                            some wideband modems; one of CDMA2000
    c      41600                            CAN, Ford SCP; ELM327 protocol 1, SAE J1850 PWM (mostly Ford vehicles); maybe ISO9141-2,ISO14230?
     i     45450                   22  264  Profibus
  m        48000                       250  2*24000 modem, upstream for V.92
  m c      50000                   20  240  some wideband modems; NICAN, Lantronix PFAL; ELM327 protocol C, USER2 CAN, 11bit ID (id/speed adjustable); fault-tolerant CAN in Fiat/Alfa/Lancia, not OBD2 standard; singlewire CAN?
           51200                            unusual; CP2102
  m        53300                            modem, V.44
  m        56000                            56 kbit/s line; modem, V.90 (8000/3429 baud, 56k/33k6, digital), V.92 (8000/8000 baud, 56k/48k, digital), various "56K"; frame relay; CP2102,CH340,PL2303
! mS       57600   0x1001      2            1.5*38400, 3*19200; common higher speed lower than 115200; mobile HSCSD downstream, GPRS (2.5G) downstream, one of CDMA2000; CP2102,CH340,PL2303,HT42B534
    c      62500                   16  192  2*31250, 1/2 125000; NICAN; SAE J1708 (62500/9600)
  m        64000                      187.5 ISDN single, DS0/E0/T0; frame relay; CP2102
           72000                      500/3 3*24000
           74880                            ESP8266 post-reset/bootloader (26 MHz crystal) (115200*26/40) (2.5% off 76800, which usually works)
           75000                  40/3 160  something industrial
  m  i     76800             2/3            2*38400; BACnet (Building Automation and Control) MS/TP networks; one of CDMA2000; CP2102,CH340
  m        79200                            one of CDMA2000
    c      80000                  12.5 150  NICAN
    C      83300                            SWCAN, single-wire CAN, SAE J2411, high-speed for diagnostics; CAN_GEN_SW (singlewire); Lantronix PFAL
     i     93750                  32/3 128  3*31250; Profibus
    c      95200                            CAN bus, less common; Lantronix PFAL
    c     100000                       120  NICAN, Lantronix PFAL; TIRA-1 infrared transceiver; WiDEN; Futaba S.Bus (8bit, 2 stopbits, even parity, inverted)
!  S      115200   0x1002      1            3*38400, 2*57600; common factory default (8N1) for higher speeds; IrDA SIR; very common bootloader and general communication rate for higher speed devices (arduino...); typical max speed of older RS232; one of CDMA2000; CP2102,CH340,PL2303,HT42B534
    C     125000                    8   96  4*31250; MIDI (less common), NICAN, CAN (Fault-tolerant CAN ISO 11898-3, truck-trailer CAN ISO 11992); CAN_GEN_LS (lowspeed); Lantronix PFAL; ELM327 protocol B, USER1 CAN, 11bit ID (id/speed adjustable)
  m c     128000                            CAN, ISDN dual (2x 64k); frame relay; CP2102,CH340,PL2303
          134400                            PL2303
          144000                            ISDN Basic Rate Interface; 2x 64kbit/s data bearer channels (B channel), 1x 16kbit/s control channel (D channel)
    c     153600             4/3            4*38400; CAN; one of CDMA2000; CP2102,CH340; maximum of 16550 UART?
  m       156000                            one of CDMA2000
    c     160000                        75  NICAN
          161280                            PL2303
     i    187500                        64  Profibus
          192000                            frame relay (3*64000, 5*38400)
    c     200000                    5   60  NICAN
          201600                            PL2303
!  S      230400   0x1003    1/2            2*115200; AppleTalk/LocalTalk (RS485); one of CDMA2000; highest "usual" UART bitrate; top for FTDI at 12 MHz?; CP2102,CH340,PL2303,HT42B534
!  SC     250000                    4   48  802.15.4; common higher-rate speed (eg. Marlin CNC), DMX512, MIDI; CAN (ISOBUS (ISO 11783), SAE J1939), NICAN, Lantronix PFAL; CP2102; ELM327 protocol 8/9, ISO15765-4, 11bit/29bit ID; ELM327 protocol A, SAE J1939 CAN, 29bit ID (adjustable speed)
    c     256000                            CAN; frame relay; CP2102,PL2303
  m       259200                            one of CDMA2000
          268800                            7*38400; PL2303
  m       307200                            8*38400; one of CDMA2000
  m       309600                            one of CDMA2000
          320000                            frame relay
          384000                            frame relay
    c     400000                        30  NICAN, less uncommon
          403200                            PL2303
          448000                            frame relay
!  s      460800   0x1004    1/4            4*115200; common fast for microcontroller UART; one of CDMA2000; CP2102,CH340,PL2303,HT42B534
  m       463200                            one of CDMA2000
!  sC     500000   0x1005           2   24  CAN, NICAN, Lantronix PFAL, Ford HS CAN, Ford MS CAN; Profibus, max of Fieldbus; CP2102; ELM327 protocol 6/7, ISO15765-4, 11bit/29bit ID
          511500                            GPS signals, GPS L2C CL and CM code bitrates, multiplexed together to 1,023,000 bps
  m       512000                            frame relay; ADSL(G.lite) upstream
  m       518400                            one of CDMA2000
          576000   0x1006    1/5            5*115200; IrDA MIR slow; CP2102
  m       614400                            16*38400; one of CDMA2000; PL2303
  m       616800                            one of CDMA2000
    c     666600                            CAN bus, less common, Lantronix PFAL
          691200             1/6            6*115200
   sC     800000                        15  NICAN, Lantronix PFAL
          806400                            PL2303
  m       921600   0x1007    1/8            8x115200; common fast for microcontroller UART; default speed of higher speed powerline modems; CP2102,CH340,PL2303
  m       924000                            one of CDMA2000
!  sC    1000000   0x1008           1   12  common RS485; CAN 2.0 maximum (Highspeed CAN, ISO 11898-2, SAE J2284), NICAN, Lantronix PFAL; CAN_GEN_HS (highspeed)
         1023000                            GPS signals, GPS L2C CL and CM code bitrates multiplexed together
  m      1024000                            frame relay; ADSL(G.dmt) ITU G.991.2 upstream
  m      1036800                            one of CDMA2000
         1152000   0x1009   1/10            10x115200; IrDA MIR fast
         1200000                        10  CP2102
         1228800                            32*38400; PL2303
  m      1231200                            one of CDMA2000
         1382400            1/12
  m      1440000                            ADSL2 ITU G.992.3 upstream
     i   1500000   0x100a                8  Profibus; CP2102,CH340
  m      1536000                            USB 1.0 low-speed; frame relay; ADSL(G.lite) (1536k/512k)
  m      1538400                            one of CDMA2000
  m      1544000                            ISDN T1 based; 23x 64kbit/s B channels, 1x 64kbit/s D channel for control
         1612800            1/14
         1700000                            HT42B534
         1843200            1/16            original 8250 UART clock; ESP8266?
  m      1845600                            one of CDMA2000
! m  i   2000000   0x100b         1/2    6  common RS485; top for FTDI at 48 MHz?; MSDSL; CH340
  m      2048000                            frame relay; ISDN E1 based; 30x 64kbit/s B channels, 1x 64kbit/s D channel for control, 1x free timeslot for sync
         2073600            1/18
         2304000            1/20            20*115200; HT42B534
  m      2320000                            SDSL
         2457600                            64*38400; PL2303
         2500000   0x100c         2/5
         2686400                            ESP8266 theoretical maximum?
         2764800            1/24
         2768000?                           serial UART maximum?
     i   3000000   0x100d         1/3    4  ceiling of some USB-serial chips with 12 or 48 MHz clock; Profibus; PL2303, FT232R maximum
         3072000
         3400000                            HT42B534
         3500000   0x100e
         3584000                            ADSL2+ ITU G.992.5 upstream
         3686400            1/32
         3916800            1/34
         4000000   0x100f         1/4    3  IrDA FIR
         4608000            1/40            40*115200, ESP8266 top UART speed?
  m      5690000                            SHDSL ITU G.991.2
     i   6000000                  1/6    2  Profibus; PL2303
  m      8192000                            ADSL(G.dmt) ITU G.991.2 (8192/1024 kbps)
     i  10000000                 1/10       RS485 maximum at 12m; 10base2 and 10baseT ethernet
    Ci  12000000                 1/12    1  Profibus maximum; CAN FD max (ISO 11898-2 2015), CAN_GEN_FD; PL2303
  m     12288000                            ADSL2 ITU G.992.3 (12288/1440 kbps)
    C   15000000                            CAN FD maximum
  m     24576000                            ADSL2+ ITU G.992.5 (24576/3584 kbps)

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