Path: utzoo!utgpu!water!watmath!clyde!burl!codas!mtune!rutgers!umn-d-ub!umn-cs!peiffer
From: peiffer@umn-cs.cs.umn.edu (Tim J. Peiffer)
Newsgroups: comp.dcom.modems
Subject: Re: IBM-C128 hookup
Summary: Null Modems
Message-ID: <3955@umn-cs.cs.umn.edu>
Date: 16 Feb 88 14:14:33 GMT
References: <2695@cup.portal.com> <3154@cup.portal.com>
Organization: University of Minnesota, Computer Science
Lines: 44
Posted: Tue Feb 16 08:14:33 1988

In article <3154@cup.portal.com>, Brendan@cup.portal.com writes:
>  You may want to buy a 'null modem', which acts in place of the phone company
> and phone lines between the computers...also, I think I saw something in one
> of the old Transactor magaziines..I only have sketchy knowledge about the null
> modem, and I'm sure there are others out there who could elaborate...
> UUCP: ...sun!portal!cup.portal.com!Brendan_Patrick_Kehoe

    You are correct in assuming that what is a null modem.  Your local Radio
Shack stocks one that cost's approximately $20.00.  If you do not want to
pay through the nose, you can hook up one as the following

             1   chassis gnd        <->           chassis gnd    1
             2   txd                <->           rxd            3
             3   rxd                <->           txd            2
             4   rts                <->           cts            5
             5   cts                <->           rts            4
             7   sig gnd            <->           sig gnd        7
             6 & 8 (dsr & dcd)      <->           dtr            20
             20  dtr                <->           (dsr & dcd)    6 & 8

     All other connections are unnecessary for most RS-232 application
standards.  Just remember that the big three are txd,rxd, & sig gnd.  This
should hold for just about every application that you need.

     Commodore does not implement RS-232 very well.  What they implement is
the method of control, not the voltage level; most certainly not the pin-outs.
RS-232 voltages range from -25vdc (or -15) to +25vdc (or +15).  Commodore chose
to leave out the required line drivers and receivers.  Their voltage levels
range from 0vdc to +5vdc.  You may elect to get around this problem by
purchasing the required converter from JameCo Electronics.  The unit retails
for approximately $39.95.

    I have used this method to do direct machine to machine transfers up to
19200 Baud.  It is reliable in the area of data, but handshake can become
a little shakey (pardon the pun).


Tim Peiffer
Computer Science Systems Group
Computer Science Department
University of Minnesota, Minneapolis, MN 55455.
(Ding-aling Net 612-625-0876) 
peiffer@umn-cs.cs.umn.edu
peiffer@umn-cs.UUCP