Re: //c modems at > 2400

[dempson@actrix.gen.nz (David Empson)]

In article <1994Jul13.201424.29734@cobra.uni.edu>,
 <fishern3485@cobra.uni.edu> wrote:
> 
> It seems the considered opinion that the //c has a fundimental hardware
> flaw in that the lines that prevent uncontrolled data flow are used
> by the //c for other purposes, causing data from the modem to fill
> the buffer, and then continue to flow, overflowing the computer's
> buffer. (128 bytes if I recall correctly; if not that, then 256)

Worse than that.  If you use the flow control lines that the IIc does
provide, you will disrupt the flow of data in the OTHER direction.
The IIc's outgoing flow control line (RTS) prevents it from sending.
Its incoming flow control line (DCD) prevents it from receiving.

The intended function of the handshake lines with a high-speed modem
is to tell the OTHER device to stop sending.
 
> After examening the ROM, I think it may be possible to build a special
> interrupt handler that can use the modem and printer ports in tandum
> to control the flow. (using the printer's send line to transmit the
> critical DTR signal necessary to stop the modem)

RTS, actually (but not in its official function, according to RS-232 or
V.24).  DTR tells the modem to drop carrier or enter command mode,
depending on the setting of the AT&D command.

> If anyone has detailed information on the protocol for setting up
> ACIA interrupts, or information on making modem drivers for the //c,
> please drop me a line.  If it's in hardcopy form, I'd be happy to
> pay for photocopy and mailing costs.  Thanks all for your help!

I have this information in the Rockwell data sheet for the 6551.  I've
typed it in before, but I don't think I kept a copy (it was entered
directly into the editor and posted to news or mail).


There is another problem with some IIcs (original motherboard): the
6551s are clocked at the wrong speed, and you can get data corruption
and loss, especially at 1200 bps or faster.
-- 
David Empson
dempson@actrix.gen.nz
Snail mail: P.O. Box 27-103, Wellington, New Zealand