[Coco] DW and 6551

[Allen Huffman]

> On Feb 10, 2015, at 10:12 PM, Aaron Wolfe <aawolfe at gmail.com> wrote:
> 
> On Feb 10, 2015 11:00 PM, "RETRO Innovations" <go4retro at go4retro.com> wrote:
>> 
>> On 2/10/2015 9:26 PM, Stephen H. Fischer wrote:
>>> 
>>> And there is the 6551 flaw if I remember correctly voids the entire idea
> of using it.
>> 
>> What flaw is that?  The CBM folks use 6551s
> all the time with no issues.
>> 
>> JIm
> 
> I don't know the details, but I'm pretty sure the problem is limited to the
> Tandy Deluxe rs232 pak.  There are other 6551 based serial cards for the
> coco that can operate at higher speeds.  Unfortunately the Tandy card is
> the most common.

There was an issue where it would lose a byte, but a simple fix worked around that. For the StG-Net BBSes, we used the CDI device — bought in kit form and installed in a DB25 pass thru connector. I forget the details, but it seems like it just moved a wire somewhere and caused it to assert hardware flow control.

SockMaster’s webpage has more details and his fix for high speed RS232:

http://users.axess.com/twilight/sock/cocofile/rs232mod.txt

He writes:

"Well, it turns out that that is technically correct, but the 6551 has a
*lousy* way of dealing with CTS line changes.  Every time the modem would raise
the CTS, the 6551 would stop dead in it's tracks - right in the middle of
sending a byte! For low speed callers, that meant every 200th byte or so was
truncated and percieved as trash”
--
Allen Huffman - PO Box 22031 - Clive IA 50325 - 515-999-0227 (vmail/TXT only)
Sub-Etha Software - http://www.subethasoftware.com - Established 1990!
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