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From: clay@sask.UUCP (Clay Cederstrand)
Newsgroups: comp.sys.m6809
Subject: homebrew coco serial port problems
Message-ID: <822@sask.UUCP>
Date: Tue, 18-Aug-87 17:59:48 EDT
Article-I.D.: sask.822
Posted: Tue Aug 18 17:59:48 1987
Date-Received: Sat, 22-Aug-87 05:54:51 EDT
Organization: University of Saskatchewan
Lines: 33
Keywords: 6551



I recently added a homebrew UART cct (6551 ACIA) to my coco3, specifically to
the disto disk controller on the coco3. It works basically OK except for
two teeny tiny problems, OK OK you dragged it out of me, one teeny tiny
problem, and one that is a real pain in the rear.

Because I did not as yet hook up the interrupt line from the 6551 I can
only use 300 baud operation through a modified /t3 descriptor ( only the
address of the device was changed ) and the modpak driver which uses the
virtual interrupt handler routine. The 300 baud is a limitation imposed
by the VIRQ handler, the clock interrupts every 16 msec, any speed higher
than 600 baud would result in lost data. This brings me to the teeny tiny
problem, since the NMI is the only interrupt line brought out the cart port
and since /T2 is interrupt driven, it looks on the surface as though there
could be lost data problems with the disk controller masking interrupts at 
at high baud rates. Anyone have any knowledge of, or experience with the
rs232 pak and its operation at high baud rates, especially with programs
such as kermit ?

The pain in the rear problem occurs when I use kermit to download files,
when the file is downloaded the stinking drive it was accessing continues
to be selected and runs constantly. You can exit kermit and use OS9
normally as before, even access any drives, but when a disk access is done
the drive that kermit used will be reselected and continue running. Any
idea's what is happening here?  It can't be a hardware problem since the
drives can still be accessed after and otherwise function normally.

Any info greatly appreciated.

Clay Cederstrand
Technician at large ( or is it a large technician..I can never remember!)
University of Saskatchewan