Re: Fast modems and the SSC

[foegelle@ccwf.cc.utexas.edu (Michael Foegelle)]

In article <1992Nov16.074456.13677@actrix.gen.nz> David.Empson@bbs.actrix.gen.nz writes:
>Regarding the Super Serial Card, which uses a 6551:
>
>The 6551 has two very nasty features with its flow control inputs.  If
>the CTS input goes inactive, the transmit data signal is IMMEDIATELY
>cut off.
>
    >Stuff about DCD and DSR deleted<
>
>The CMOS 6551 (65C51) is somewhat better: CTS cuts off the transmitter
>after the current byte has been completely transmitted.  It has the
>same behaviour for DCD.
>Its other advantage is that disabling receive interrupts also disables
>the DCD and DSR change of state interrupt.  This cannot be disabled on
>the original 6551, which is why a IIe will usually hang if you unplug
>the modem from the SSC while you aren't in a comms program.
>
Be careful with your descriptions.  The 65C51A works as you describe, but the
65C51 works identically to the 6551 in terms of CTS.  At least that's the way
the GE/Harris/RCA version works.  I believe there's also a 6551A available 
which may solve the problem as well.

>BUT, I don't think you can simply replace a 6551 with a 65C51.  We
>tried this in one of the products my company produces, and it didn't
>work reliably, because the two chips expect different types of crystals.
>-- 
I'm using a 65C51A in my SSC with my 14.4Kbaud Supra with no problems.  It 
also solved the problem that the 6551 had with RTS low forcing CTS low 
internally (a problem mentioned several months back).

For about six bucks from a mail order electronics house, it's a pretty easy
fix.

>David Empson
>
>Internet: David.Empson@bbs.actrix.gen.nz    EMPSON_D@kosmos.wcc.govt.nz
>Snail mail: P.O. Box 27-103, Wellington, New Zealand


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