Re: Hi-Speed Modems and //c

[c8922369@jupiter.newcastle.edu.au (Leon James Garde)]

David.Empson@bbs.actrix.gen.nz writes:

>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!

Um. I did know the RS--232 standard was like that, I didnt think there
was going to be a need for the bufffers.


>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
yes, of course !
>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.
thats right, one sib, one sob.



>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).
yes, i ve hearf that. Pity the only cmos in a //c is the 65c02.
(I tested them all, all chips were hot at operating temperature, except
the cmos processor ! :)


leon