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From: mike@relgyro.stanford.edu (Mike Macgirvin)
Newsgroups: comp.sys.next
Subject: Re: dumb terminal dumb
Summary: RS422 -> RS232
Message-ID: <232@helens.Stanford.EDU>
Date: 16 Jun 89 16:41:05 GMT
References: <9001@phoenix.Princeton.EDU> <4076@ima.ima.isc.com>
Sender: news@helens.STANFORD.EDU
Reply-To: mike@relgyro.STANFORD.EDU (Mike Macgirvin)
Organization: Stanford Relativity Gyro Experiment (GP-B)
Lines: 26

In article <4076@ima.ima.isc.com> johnl@ima.UUCP (John R. Levine) writes:
>In article <9001@phoenix.Princeton.EDU> andy@phoenix.Princeton.EDU (Andrew M. Milburn) writes:
>>I'm having some difficulty connecting a dumb terminal to my cube.
>>A perfectly good null-modem cable results in data travelling succesfully
>>from the NeXT to my adm3a, but nothing goes the other way.
>
>The problem is that the NeXT is RS422 while your terminal is RS232.  I had
>exactly the same problem trying to hook up an IBM PS/2-50 as a terminal.
>Turns out that the NeXT is using levels of (roughly) +5 and ground, while the
>terminal wants +5 and -5.  Many RS232 receivers are forgiving enough to
>accept +5/0 but others aren't.  Another terminal might work better.

	I once solved this problem for (don't laugh) a Commodore 64. I
might be able to dig up my notes if someone is interested, but the general
idea is to hook up one or two MC1488/MC1489 line driver/receiver chips
(I forget which is which right now) and a +12/-12V supply (or two 9V
bateries). 
A simple terminal interface can be done with two chips, a modem interface 
probably requires at least one more. See the Motorola Data sheets on these 
chips. It costed me all of three bucks and two hours work on the C-64.

#include <std.disclaimer>


		Mike Macgirvin
		mike@relgyro.stanford.edu (36.64.0.50)