[net.periphs] Line Drivers vs. Short Haul Modems

mark@cogent.UUCP (Mark Steven Jeghers) (10/23/86)

We are trying to decide whether to use Line Drivers or Short Haul Modems
in an upcoming installation.  Has anyone had any experience in this area?
We would greatly appreciate input on the pros and cons of the two options
and any recommendations.

Thanx in advance.
-- 
+----------------------------------------------------------------------------+
|     Mark Steven Jeghers - the living incarnation of "Deep-Thought"         |
|     ("You won't like the answer ... you didn't ask it very well.")         |
|                                                                            |
|     {ihnp4,cbosgd,lll-lcc,lll-crg}|{dual,ptsfa}!cogent!mark                |
|            ^^^^^^-------recommended------^^^^^                             |
|                                                                            |
|     "A poodle-free world within a decade.  We can do it...together!"       |
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| Cogent Software Solutions can not be held responsible for anything said    |
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embick@tetra.UUCP (Edward M. Embick) (10/24/86)

Mark,

We have had very good experience using bo-sherel mx-2 asynchronous short
haul modems.  They mux 8 ports over two twisted pair.  We are running them
between two buildings.  I have vt100's, vt220's, h19's, pc's, and several
vt100 and h19 compatibles running at 9600 baud to our VAX 11/780 with
Emulex dmf type ports (Emulex CC32 subsystem).  

I chose the mx-2's because I could run eight ports over two twisted pair,
rather than having to have four copper wires for each terminal.  I've
got communications for 40 ports (5 pairs of mx-2's) between the two
buildings using only 20 wires (10 pair).  Cost wise the mx-2's are just
under $500 a piece ($1000/pair).  They have been very reliable (no failures
in over a year of operation).  All eight ports can run full bore without
any perceptable degredation in response or character overruns.

The only problem I have is that if the transmit line (pin 2) is not 
properly terminated, the receive line will cross talk.  Since my mx-2's
at my VAX are always hot, the situation of UNIX sending out the login
message and talking to itself, resulting in a getty failure message,
can occur.  I cured that by making up connectors for the terminal end
so when a users terminal is disconnected, I jumper pin 2 to pin 7 (signal
ground).  This cures the problem and allows me to connect and disconnect
terminals at will in the remote building without having to go to the host.

We are running our terminals to the mx-2's through three wire cables
(transmit-pin 2, receive-pin 3, and signal ground-pin 7).  We jumper
pins 4-5 and pins 6-8-20 at both ends.  All of our terminals are cabled
into a central communications room where the mx-2's are located.  Some
runs are almost 300 ft.  I've had no problems with this setup.

FYI bo-sherrel co. is at 36133 Niles Blvd., Fremont, CA 94536.  Their
phone is (415) 792-0354.  I have no connection with them except for
being a satisfied customer.
------------------------------------------------------------------------------
Ed Embick    (the more paths I make, the more paths they break! waaaaaaa....)
Computer Sciences Corp.                embick@noscvax.UUCP  or
4045 Hancock St.      {akgua,allegra,decvax,ihnp4,ucbvax}!sdcsvax!noscvax!embick
San Diego, CA 92110                MILNET:  EMBICK@NOSC
(619) 225-8401 x516