Re: Hi-Speed Modems and //c

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

In article <c8922369.722093903@jupiter.newcastle.edu.au> c8922369@jupiter.newcastle.edu.au (Leon James Garde) writes:
> c8922369@jupiter.newcastle.edu.au (Leon James Garde) writes:
> 
> >>Since it seems that high-speed modems require hardware handshaking to
> >>operate properly, am I correct in assuming my //c will not properly handle
> >>a 9600 bps modem?  Ya know, with my crippled little 5-pin serial port and
> >>all...
> >I have the 6551  pinouts,   what should I do to connect a 25 pin to the chip 
> >direct ?
> 
> in answering my own question, I looked at my //c's board.
> the 4 serial port wires connect the AMI S6551P chips, through 
> SIB and SOB, assumably for serial input buffer and serial output buffer.
> 
> what are MC1488 (sob) and MC1489 (sib) ??? what do they do ?

The 1488 and 1489 convert between TTL signals (as used by the 6551)
and RS-232 signals (as used between computers).  The voltages involved
are quite different.  RS-232 uses -3 to -25 volts to indicate a logic
"1" and +3 to +25 volts to indicate a logic "0", while TTL uses 2.0 to
5.0 volts for a "1" and 0.0 to 0.7 volts for a "0".

If you don't know this sort of thing already, you shouldn't be
fiddling around with a soldering iron on RS-232 signals!

Trying to feed RS-232 signals directly into a 6551 will NOT work, and
you will damage the chip.  You have to go through the level converters
(1488 and 1489) but I suspect you will find that there are no spare
converters in a IIc.  The IIc only has four transmit and receive
signals (two of each for each port) so probably has a single 1488 and
1489 which are shared by the two ports.


What do you want to do this for, anyway?

You will not be able to get a high speed modem working with a IIc,
because the 6551's output flow control lines interfere with transmit
and receive data and interrupts (depending which one you use).
-- 
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