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From: hull@haouucp.UUCP (Howard Hull)
Newsgroups: net.micro.amiga
Subject: Re: AEGIS DRAW / AMIGA / APPLE PLOTTER  (NEED HELP!!)
Message-ID: <123@haouucp.UUCP>
Date: Wed, 18-Jun-86 12:03:04 EDT
Article-I.D.: haouucp.123
Posted: Wed Jun 18 12:03:04 1986
Date-Received: Sat, 21-Jun-86 12:44:04 EDT
References: <3311@vax4.fluke.UUCP>
Distribution: net
Organization: High Altitude Obs./NCAR Boulder, CO
Lines: 102
Summary: RS232 Standard and Applications, Applied


Outlined below is the RS232 handshaking sequence, followed by a diagram of a
full null modem cable for connecting a terminal and a printer.  It is normally
assumed that the computer is DTE (Data Terminal Equipment) and that the device
at the other end of the cable is DCE (Data Communications Equipment), i.e., a
modem.  Usually with a computer and a printer such is not the case, and that
is why one needs a null modem cable.  However, be warned: one frequently finds
that the printer output RS232 connectors on CRT terminals, for instance, are
wired as DCE devices so that a null modem cable is not desired.  The problem
becomes one of discovering how the computer manufacturer wired his RS232 port.
Was it wired as one would wire a terminal, or was it wired as one would wire a
modem?  The same problem exists for the printer.  It has been done in every
possible combination by someone at some time.  Recently the standard seems to
be to wire everything except modems as DTE.  If that is done, then a printer
cable needs to be a null modem cable.  Next, there is the matter of the group
of handshaking sequence.

Pin #	Signal Name			Description

 1	Protective ground		(mutual)
 2	Transmit Data (TD)		(from T to M)
 3	Receive Data (RD)		(from M to T)
 4	Request to Send	(RTS)		(from T to M)
 5	Clear to Send (CTS)		(from M to T)
 6	Data Set Ready (DSR)		(from M to T)
 7	Signal Ground			(mutual)
 8	Carrier Detect (CD)		(from M to T)
20	Data Terminal Ready (DTR)	(from T to M)
22	Ring Indicator		(from M to T)

Notes:	The "from" device asserts the signal.
	The "to" device reads the signal.
	Data Terminal "T" is the computer or a crt terminal (DTE).
	Data Set "M" is a modem, or a null-modem to a terminal (DCE).

	The RS232 standard lists a half-dozen or so handshaking methods.
The most complex protocol is used from DTE to DCE to switched telephone
network to DCE to DTE.  In this case, the switched network must be induced
to connect before the DTE/DCE path can be initialized.  In such a case,
the first signals exchanged are those associated with the readyness of the
DTE/DCE combination of each end of the switched network.  For the pure
example, at power-up, Each assumes it is an answering station unless set
otherwise; the DTE asserts DTR.  (The DTE may at its discretion assert
RTS and the DCE may initiate an ASCII task request protocol.)  After a
time, one of the DTE's is programed to make a call.


The call is initiated by the calling DTE holding or setting DTR as is
appropriate, followed by holding or assertion of RTS and delivery of the
ASCII calling instructions, if appropriate, or waiting for the operator
to dial the call and put the DCE on the line.  The call placing sequence
involves taking the line off-hook, determining that there is a dial tone,
dialing the number, connecting the DCE to the line, and then issuing DSR.
Nothing then happens until the phone rings at the called site, where the
DCE then asserts RI to let the called DTE know that the phone is ringing.
The called DTE then responds by holding or asserting DTR, as appropriate. 
The called DCE responds by asserting off-hook to the switched network,
effectively answering the call and then placing the answer tone on the line.
The calling DCE then asserts the originate tone.  Some timing pulses are
exchanged by the DCE's, and then they together assert CD (carrier detect,
or what is in the standard named the "Received Line Signal Detector").
This completes what is known as the Group 1 transaction.

For the Group 2 transaction, each end conditions itself to handle data.
At the calling end, the DTE response is to hold or set RTS, and the DCE
responds with CTS.  At the called end, the DCE sets CTS, and the DTE sets
RTS if write permissions exists for the port.


DEC		TERMINAL / COMPUTER RS-232 NULL-MODEM CABLE

Cable Type:	Belden 9504	4 pair overall shielded
Outside Dia.:	0.265 inches	(6.75 millimeters)
Connectors:	DB-25

Terminal								Computer
Pin #	Signal Name		Wire Color	Signal Name		Pin #

 1	Protective ground-------Shield Drain----Protective ground	 1
 2	Transmit Data---------> red ----------> Receive Data		 3
 3	Receive Data <--------- white <-------- Transmit Data		 2
 4	Request to Send--:----> violet -------> Carrier Detect		 8
 5	Clear to Send <--: 5 jumpered to 4
 6	Data Set Ready <-:-----	green <-------- Data Terminal Ready	20
22	Ring Indicator <-: 22 jumpered to 6
 7	Signal Ground--white's blk & red's blk--Signal Ground		 7
 8	Carrier Detect <--- violet's blk <--:---Request to Send		 4
			    5 jumpered to 4 :-> Clear to Send		 5
20	Data Terminal Ready -> green's blk -:-> Data Set Ready		 6
			   22 jumpered to 6 :-> Ring Indicator		22

Notes:	This cable is wired like a DEC H312 Null Modem Card.

The pins that must be connected for a printer depend on whether the printer
uses a group 1 or a group 2 handshake.  For group 1, the important cross
connects are Pin 8 --> Pins 4 & 5.  For group 2, use Pin 20 --> Pins 6 & 22.
It is even possible to encounter a situation that requires group 1 passing
from device A to device B, but group 2 passing from device B to device A.
The full implementation uses all four: group 1, A to B; group 2, A to B;
group 1, B to A; group 2, B to A.

								!hao!hull