Path: utzoo!mnetor!uunet!lll-winken!lll-lcc!mordor!sri-spam!rutgers!mtune!icus!gil
From: gil@icus.UUCP (Gil Kloepfer Jr.)
Newsgroups: comp.sys.atari.8bit
Subject: Re: Linking two computers
Message-ID: <266@icus.UUCP>
Date: 23 Feb 88 06:14:12 GMT
References: <578NU109703@NDSUVM1> <2178@bsu-cs.UUCP>
Reply-To: gil@icus.UUCP (Gil Kloepfer Jr.)
Organization: ICUS Computer Group, Islip, NY
Lines: 35
Summary: Use the 'old shift register

In article <2178@bsu-cs.UUCP> cfchiesa@bsu-cs.UUCP (Sir Xetwnk) writes:
>Therefore you need
>TWO joystick ports to transmit ONE 8-bit data byte.
>
>Or do you?
>
>I would give some thought to the possibility of using only ONE joystick
>port, and performing 4-bit I/O over the port-to-port line, reassembling the
>original bytes after both halves ("nybbles") are received.
>Chris Chiesa

Here's a method I have used a lot and it works well:  Rather than trying to
assemble the nybbles (a hard thing to do really, if you sit down and think
about it) -- use an 8-bit shift register (serial-to-parallel).  You use one
line of the joystick port to clock the shift register and another line to
set the ones/zeros on the data input of the shift register.

Why do it this way?  Well, companies like MPP have developed nifty little
ways of generating a certain baud rate by timing loops and tying down the
CPU (more than it is already by the screen).  With the shift register, you
can clock the data out at any rate you feel like, whenever you can get it
out.

I used this on an old Commodore PET (the original 8K one with the chicklet
keyboard) to set-up a modem interface.  I used the shift-register idea to take
full advantage of the 8-bit parallel port that the PET had (the only one at
that!).  I sent data to an 8251 UART chip which had a baud-rate generator
IC built into it.  I was able to use the board successfully at 1200 baud with
no problem, and I'm sure I could do better if I had the opportunity.

+------------------------------------+----------------------------------------+
| Gil Kloepfer, Jr.                  | Net-Address:                           |
| ICUS Computer Group, Systems Dvlp. | {boulder,ihnp4,talcott}!icus!gil       |
| P.O. Box 1                         | Voice-net: (516) 968-6860              |
| Islip Terrace, New York  11752     | Internet: gil@icus.UUCP                |
+------------------------------------+----------------------------------------+