[comp.sys.next] dumb terminal dumb

andy@phoenix.Princeton.EDU (Andrew M. Milburn) (06/15/89)

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.
Has anyone else struggled with this, or is it as easy as it should
be, implying that I've got a hardware problem somewhere?

johnl@ima.ima.isc.com (John R. Levine) (06/16/89)

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.
-- 
John R. Levine, Segue Software, POB 349, Cambridge MA 02238, +1 617 492 3869
{ bbn | spdcc | decvax | harvard | yale }!ima!johnl, Levine@YALE.something
Massachusetts has 64 licensed drivers who are over 100 years old.  -The Globe

mike@relgyro.stanford.edu (Mike Macgirvin) (06/16/89)

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)