SERIAL Port (RS-232)

SERIAL Port (RS-232)
By default - a START-STOP ports does not have clocking-pins (pin-15 Transmit / pin-17 Receive - Clock).

The ASYNC-Serial Interface (RS-232) is designed to work at operation speeds:

from 50-bps up to max. 19200-bps on some older machines (8250 ACIA-type)
from 50-bps up to max. 115200-bps on all other machines

For a DIRECT connection (PC-to-PC, DTE-DTE ) use a NULL-Modem cable. (see below 25-pin sample)
For a MODEM connection (PC-to-MODEM, DTE-DCE ) you should use a 1-to-1 cable.
(DTR-to-DTR, DSR-to-DSR, TD-to-TD, RD-to-RD, CTS-to-CTS ... )

Be sure (directly connected) machine setup, is defined for same SPEED (baud-rate), WORD-length
and PARITY (default use: 8-N-1 = 8-databit NO-parity 1-Stopbit)
if connected together with a Null-modem (PC-to-PC) cable.

See > Modem Setup Strings for AT (Hayes) AT-commands for various MODEM-types.
See > Some about MODEMs

The CABLE-length/speed, depends on various facts as:

Transmisson-speed (always try a lower speed, if 19200 bps or faster does not work) ?
Cable length/type used (single wires / flat cable) ?
Is the cable shielded ?
Are TD/RD lines twisted with ground (against crosstalk) ?
Is any electr. noise coupled into the cable (induction/environment) ?
Ground problems / FRAME-GROUND connected ?
(Line-frequency noise, because Line-filters installed but w/o frame ground connected).

Normally a well behaved cable should work, if the total length is <= 12 meters (36 feet). And all electric/environment factors are OK.

Voltage Interchange Information

Interch. Voltage Binary State Condition Interf.-Ctrl. Funct. Positive = Binary 0 = Spacing = ON Negative = Binary 1 = Marking = OFF

Interch. Voltage	Binary State	Condition	Interf.-Ctrl. Funct.
Positive =	Binary 0	= Spacing	= ON
Negative =	Binary 1	= Marking	= OFF

Valid EIA Voltage (signal) Levels: (SIGNAL-GROUND to PINx)

+3 up to +15 Volt DC = ON Function
-3 up to -15 Volt DC = OFF Function
-3 up to +3 Volt DC = INVALID (defective) LEVEL

25-Pin D-shell (DB-25) Connector Pin-layout (Async- w/o clock)

PIN Signal Name I/O PIN Signal Name I/O 2 Transmitted Data TD O 7 Signal Ground SG 3 Received Data RD I 8 Carrier Detect DCD I 4 Request to Send RTS O 20 Data Terminal Ready DTR O 5 Clear To Send CTS I 22 Ring Indicator RI I 6 Data Set Ready DSR I

PIN	Signal Name		I/O	PIN	Signal Name		I/O
2	Transmitted Data	TD	O	7	Signal Ground	SG
3	Received Data	RD	I	8	Carrier Detect	DCD	I
4	Request to Send	RTS	O	20	Data Terminal Ready	DTR	O
5	Clear To Send	CTS	I	22	Ring Indicator	RI	I
6	Data Set Ready	DSR	I

9-Pin D-shell (DB-9) Connector Pin-layout (Async- w/o clock)

PIN Signal Name I/O PIN Signal Name I/O 1 Data Carrier Detect DCD I 6 Data Set Ready DSR I 2 Receive Data RD I 7 Request To Send RTS O 3 Transmit Data TD O 8 Clear To Send CTS I 4 Data Terminal Ready DTR O 9 Ring Indicator RI I 5 Signal Ground SG

PIN	Signal Name		I/O	PIN	Signal Name		I/O
1	Data Carrier Detect	DCD	I	6	Data Set Ready	DSR	I
2	Receive Data	RD	I	7	Request To Send	RTS	O
3	Transmit Data	TD	O	8	Clear To Send	CTS	I
4	Data Terminal Ready	DTR	O	9	Ring Indicator	RI	I
5	Signal Ground	SG

Examples:

Serial 25-Pin IBM-PS/2 to PRINTER Cable (for Printer XON-XOFF Protocol)
(Serial Printer Cable IBM FRU-P/N 8509386)

COMPUTER PRINTER PIN SGNL SGNL PIN 2 XMIT->------------+ +--------<-XMIT 2 +---+---+ 3 RCV -<--------+ +------------>- RCV 3 4 RTS ->-þ----------+ +--------<- RTS 4 | | | 5 CTS -<-+------+ +---+-------->- CTS 5 | +---+-------+ 6 DSR -<-+--+ | +------------>- DSR 6 | | | 7 GND ---+------+---+-------------- GND 7 | | | 8 DCD -<-+ +---+---+ DCD 8 | | 20 DTR ->------------+ +--------<- DTR 20

Serial 25-Pin NULL MODEM Cable

COMPUTER-A COMPUTER-B PIN SGNL SGNL PIN 2 XMIT->------------+ +--------<-XMIT 2 +---+---+ 3 RCV -<--------+ +------------>- RCV 3 4 RTS ->------------+ +--------<- RTS 4 +---+---+ 5 CTS -<--------+ +------------>- CTS 5 6 DSR -<-þ------+ +------þ->- DSR 6 | | | | 7 GND ---+------+-------+------+--- GND 7 | | | | 8 DCD -<-+ +-------+--+ +->- DCD 8 | | 20 DTR ->----------------+ +-----<- DTR 20

Serial 25-Pin LAPLINK Cable (c) Traveling Software, Inc.

COMPUTER COMPUTER PIN SGNL SGNL PIN 2 XMIT->------------+ +--------<-XMIT 2 +---+---+ 3 RCV -<--------+ +------------>- RCV 3 4 RTS ->------------+ +--------<- RTS 4 +---+---+ 5 CTS -<--------+ +------------>- CTS 5 6 DSR -<--------+ +------------>- DSR 6 | | 7 GND ----------+---+-------------- GND 7 +---+---+ 20 DTR ->------------+ +--------<- DTR 20

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