From: gerrit@laosinh.stgt.sub.org (Gerrit Heitsch)
Subject: Re: 6551A and 6551 compatibility
Distribution: world
Organization: Lao-Sinh project (private UUCP site)
Lines: 31

In article <C5owvs.pr@world.std.com>, Internet Surfer writes:

> Does any one know if the 6551 is timing/pin compatible with the 6551.. 

No, the 6551A is able to operate in a 2 MHz system, the 6551 can only
take 1 MHz without problems.

If you see a 8551 made by MOS or CSG, take it, its a 6551A.

> It seems the 6551 has in iheirent bug with cts/rts handshaking and i need
> a suitable pin replacement to put in my serial card... possibly a buffered
> version perhaps?

I know no fixed version of the 6551. There are different possibilities
to work around this bug. The easiest is to tie _CTS to GND and
use _DSR or _DCD as _CTS. It should be easy to fix the software,
_DSR is bit 6 and DCD ist bit 5 of the Status Register (Read 6551
with RS1 = low and RS0 = high).

Using the _CTS-line can lead into big trouble. The 6551 _instantly_
stops transmission if _CTS goes high. This means, that you may
get only a half byte...

Gerrit

-- 
         Gerrit Heitsch  Moenchweg 16  7038 Holzgerlingen  Germany
  Logical adresses: UUCP: gerrit@laosinh.stgt.sub.org FIDO: (2:2407/106.9)
If we will ever be visited by Aliens, it will be very hard to explain, why a
 lifeform, that is intelligent enough to build atomic weapons can be stupid
   enough to do it. (taken from GEO special about space, page 88-91)
