Re: Fast modems and the SSC

[David.Empson@bbs.actrix.gen.nz]

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.

The other feature is that while the DCD signal is false, the 6551 will
not receive any bytes.

The DSR input doesn't interfere with either receive or transmit
operation.

This usually means you should avoid using CTS: use DSR instead.  DCD
is practically useless, as if the modem turns off its carrier signal,
you won't be able to receive any bytes (i.e. no command-mode output).



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.

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.
-- 
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