OOPs.... let's try that rs232 table again.

[Kevin Szabo]

Well, I forgot that some mailers/news programs don't like lines
that begin with a dot or other obscure characters. So here is
a version with an X put in front of any line that starts with
a dot. To remove just edit out the first character of those lines
( in vi: ... :g/^X\./ s//./ )

					Kevin
X.nr PO 0.5i
X.nr LL 7.0i
X.LP
X.SH
RS-232 Signal Conventions
X.LP
I believe the signal threshold is +- 3 volts, with the preferred signal
levels around 12 volts. Military is 24 volts.
For data `1'=Negative Signal, `0'=Positive Signal. For control signals
ON=positive, OFF=Negative. DTE stands for Data Terminal Equipment, 
DCE stands for Data Communication Equipment.
X.LP
The signal flow is: Computer <-> DTE <-phoneline-> DCE <-> Terminal.
X.LP
I am not really sure how the signal CI/CH works, the data signal rate
selector. I believe all the timing signals provide a clock, as opposed
to a code representing the transmission speed.
X.LP
X.TS
center, box;
n | c s | c s | c s | c | l
n | c c | c c | c c | c | l.
Pin	Data	Control	Timing	Circuit	Description
	DCE	DTE	DCE	DTE	DCE	DTE		
_
1							AA	Protective GND
_
2		\(bu					BA	Transmitted Data
_
3	\(bu						BB	Received Data
_
4				\(bu			CA	Request to Send
_
5			\(bu				CB	Clear to Send
_
6			\(bu				CC	Data Set Ready
_
7							AB	Signal Ground
_
8			\(bu				CF	Received Line Sinal Detector
_
9							-	Reserved for Data Set Testing
_
10							-	Reserved for Data Set Testing
_
11							-	Unassigned
_
12			\(bu				SCF	Secondary Signal Detector
_
13			\(bu				SCB	Secondary Clear To Send
_
14		\(bu					SBA	Secondary Transmitted Data
_
15					\(bu		DB	Transmission Signal Element Timing
_
16	\(bu						SBB	Secondary Received Data
_
17					\(bu		DD	Receiver Signal Element Timing
_
18							-	Unassigned
_
19				\(bu			SCA	Secondary Request To Send
_
20				\(bu			CD	Data Terminal Ready
_
21			\(bu				CG	Signal Quality Detector
_
22			\(bu				CE	Ring Indicator
_
23			\(bu	\(bu			CI/CH	Data Signal Rate Selector
_
24						\(bu	DA	Transmit Signal Element Timing
_
25							-	Unassigned
X.TE
X.LP
The Data, Control and Timing Columns denote the Signal source, 
i.e. Data Set Ready is asserted by the DCE.
X.LP
X.SH
How To Make A Null Modem
X.LP
Generally this connection scheme works, but be careful since some
manufacturers have applied their own meaning to the signals Request
to Send and Clear To send, especially when the terminal tries
to do BOTH soft and hard flow control. Some do this by dropping
request to send when they are practicing hard flow control. The
loopback scheme shown here will prevent the terminal from resending XON
to restart soft flow control, thus locking up the terminal.
If you have a terminal which only
does one type of control at a time this connection should work.
X.LP
The source of this information is from DEC's handbook for PDP-11,
that describes the H312, and asynchronous null modem.
X.LP
For connectors A and B, connect the following signals together:
1A to 1B (protective ground). 2A to 3B (data), 3A to 2B (data).
4A to 5A to 8B (loop back the Request To Send to Clear To Send and to
Carrier Detect). 4B to 5B to 8A (same again). 20A to 22B to 6B (Data
Terminal Ready to Ring Indicator to Data Set Ready) 20B to 22A to 6A
(same again).
X.LP
-- 
Kevin Szabo  watmath!wateng!ksbszabo (U of Waterloo VLSI Group, Waterloo Ont.)