The Basics of the Interface

A 25-wire interface:

Pin	Abbreviation	What it does
1	FG	Shield or Protective Ground
2	TxD	Transmitted Data
3	RxD	Received Data
4	RTS	Ready To Send
5	CTS	Clear To Send
6	DSR	Data Set Ready
7	GND	Signal Ground / Common Return
8	DCD	Data Carrier Detect
9	DC+	Positive Voltage
10	DC-	Negative Voltage
11	--	Unassigned
12	(S)DCD	Secondary Data Carrier Detect
13	(S)CTS	Secondary Clear To Send
14	(S)TxD	Secondary Transmitted Data
15	TC	DCE Transmitter Signal Element Timing
16	(S)RxD	Secondary Received Data
17	RC	Receiver Element Timing
18	LL	Local Loopback
19	(S)RTS	Secondary Request To Send
20	DTR	Data Terminal Ready
21	SQ	Signal Quality Detector
22	RI	Ring Indicator
23	DRS	Data Signal Rate Selector
24	XTC	DTE Transmitter Signal Element Timing
25	TM	Test Mode

But All You Will Really Ever Use:

Pin	Abbreviation	What it does
2	TxD	Transmitted Data
3	RxD	Received Data
4	RTS	Ready To Send
5	CTS	Clear To Send
6	DSR	Data Set Ready
7	GND	Signal Ground / Common Return
8	DCD	Data Carrier Detect
20	DTR	Data Terminal Ready

The Absolute Minimum (assumes software handshaking):

Pin	Abbreviation	What it does
2	TxD	Transmitted Data
3	RxD	Received Data
7	GND	Signal Ground / Common Return

Length of the Connection

The Standard says 50 feet
But our testing in 1996 has shown that newer equipment has improved this (better UARTs). We have gotten 9600 baud over 1000 feet of cable. Campbell confirms this, and states that he has gotten 19,500 baud over 750 feet (Page 62).
His test and ours (we shamelessly copied his experimental design) was to set up a computer with both ends of 1000 feet of UTP Cat-5 cable (he used 3-conducter unshielded 22 AWG wire) connected to it and run a program that sent the ASCII letter U (01010101). The program kept count of the discrepancies between what it sent and what it received. The fault tolerance was 1 million characters without error.


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