Date   : Thu, 07 Feb 1991 16:08:04 GMT
From   : mcsun!unido!horga!veeble!proppi@uunet.uu.net (Paul Lenz)
Subject: Is my machine too SLOW for 2400bps modems?

In article <316@fxrs.intel.com> jmasters@fxrs.intel.com (Justin Masters)
writes:
>
>I have a Radio Shack model 4 w/ Montezuma Micro's CP/M 2.2. 
>[...]
>I'm using Qterm 4.2f and I've also used MEX 1.12 (or some old version like
>that).  Both worked beautifully for me when I had 1200 baud capability, but
>now it seems that 2400 baud is too fast.  Is this true?  How do I fix it?  Is
>it a problem of my machine waiting too long at the end of the line before
>continuing on to the next line?

Yes!!!

I had had the same problem. My machine scrolls only when the monitor
finishes a picture and the electron ray goes to the upmost line.  When
my BIOS wants to scroll, it is waiting in an ENDLESS loop for a flag.
I found that the vert sync signal makes an interrupt. In the interrupt
subroutine this flag is set. The CPU escapes from the endless loop and
does the scroll.

There are two ways to fix this:

1. I copied the complete scroll routine from the BIOS into the modem
program and added a SIO polling routine to the endless loop. In this
routine the CPU saves the bytes coming from the modem into a special
area. So when the CPU waits for the scroll, it gets every byte from
the modem. After the scroll the modem program checks this special
area and prints the characters.

2. After that, I finally succeeded to make my Z80 SIO working in
interrupt mode. So of course the CPU gets every byte from the modem.

Proppi

PS: Don't mind my bad english!

proppi@veeble.UUCP = Paul Lenz / Friesenstrasse 22 / D-3000 Hannover 1
                     !unido!horga!veeble!proppi
voice: +49 511-342451 / And remember: Rock'n'Roll and CP/M never die!!