Serial Protocols

RS-422

USB

IEEE 1394 Firewire

I2C

1) voltages

a) logic high, logic low

b) single ended, differential

c) distance

d) supply power to another device

2) read/write/tri-state

3) connectors

4) pinouts

5) collision arbitration

1) bits per byte, parity

2) lsb first, msb first

3) bit speed

a) fixed baud (start bit, number of stop bits)

b) clocked

4) character encoding (ASCII)
packet layer

1) address

a) receiver device

b) sender device

2) number of bytes

3) checksum

4) collision arbitration
EIA RS-232
between DTE (data terminal equipment) and a DCE (data communication equipment = modem)
originally current loop driven, later changed to voltage level driven
UART (universal asynchronous receiver transmitter)

software UART emulation

hardware UART buffer size
inverted logic

logic HI = 1 = mark = -3V to -25V

logic LO = 0 = space = +3V to +25V
originally D25 connectors
point-to-point connections use one receiver per transmitter

no collision detection needed

DTE = D9 male TD pin 3 RD pin 2 signal GND pin 5 chassis GND pin 9

DCE = D9 female TD pin 2 RD pin 3 signal GND pin 5 chassis GND pin 9

DCD (data carrier detect) into DTE pin 1

DTR (data terminal ready) from DTE pin 4

DSR (data set ready) into DTE pin 6

RTS (request to send) from DTE pin 7

CTS (clear to send) into DTE pin 8

fixed baud rate (bits per second) = 300, 1200, 2400, 4800, 9600, 14400, 19200

can now use 28.8 K, 38.4 K, 57.6 K, 115.2 K

old teletypes = 110 baud (2 stop bits)

drop line to logic LO for start bit with duration (1/baud) seconds

transmit data bits at fixed rate with lsb first and msb last

transmit parity bit if needed

usually use prearranged baud rate between devices

can have auto baud recognition
be careful of data over-run if UART does not have a buffer or CPU cannot keep up

may need to flush UART buffer to get most recent information

50 feet maximum

only one receiver per transmitter

does not explicitly provide power to DCE (some “port powered” devices parasite from RTS)