Re: Apple //e --> Modem

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

In article <48gk0r$s8r@golden.ncw.net>, Chuck Orem <progreen@ncw.net> wrote:
> 
> Conventional wisdom has it that a cable that has pins 2,3 and 7 wired 
> straight through will work for you.

Only if you don't need hardware handshaking, and have no need to do
forced hangups or detect carrier.

> In fact, the recommended IIe modem cable in the ProTerm manual has
> got 2-8 and 20 wired straight thru. This configuration doesn't work
> for me, for any comm program except Kermit. 
> 
> My personal theory is that it has to do with the DSR line on the 
> straight thru cable. Try clipping off pin 6 from your cable and see if it 
> helps.

I think it is more likely that the DCD signal at the computer end (pin
8) is the problem.  The DCD input of the 6551 is used to enable the
receiver.

If you have the carrier detect signal enabled on the modem (AT&C1),
the modem will only activate carrier while it is connected.  This will
prevent the computer from receiving anything from the modem while it
is NOT connected (i.e. before dialling, while in command mode).

For example, this prevents the computer from receiving the "OK"
response from the modem when the initialization string is sent.  This
could cause the comms program to think the modem is not present or has
failed to initialize.

The DSR signal from the modem (pin 6) is almost completely useless -
in asynchronous modes, all it does is tells the computer that the
modem is there and is switched on.  Who cares?

> The cable configuration that I've had 100% success with, is one that I 
> made when I ran a BBS off of my IIe. Here are it's pinouts:
> 
>   SSC end                             Modem end
> (Male DB-25)                         (Male DB-25)
>     1-------------------------------------1  Chassis GND
>     2-------------------------------------2  TXD
>     3-------------------------------------3  RXD
>     4-------------------------------------4  RTS
>     6-------------------------------------8  DSR
>     7-------------------------------------7  Signal GND
>     20-----------------------------------20  DTR
>     5-------------------------------------5  CTS
>        |  Yes, Pins 5 & 8 are jumpered on
>     8--      the SSC end of the cable.

I'll second that, except that you have mislabelled pin 8 at the modem
end (it is DCD, not DSR).  This is similar to the cable we used on my
user group's BBS when it was running on a IIe with a Super Serial Card.

There is one catch with the above configuration: if you are using
hardware handshaking (RTS/CTS), then the computer will ignore incoming
characters while the RTS line is deactivated (because it is looped
back to DCD).  This could cause a character to be lost as RTS is dropped.

I'd suggest looping pin 20 (DTR) back to pin 8 (DCD) instead, as DTR
should never be dropped, except while doing a forced hangup.


-- 
David Empson
dempson@actrix.gen.nz
Snail mail: P.O. Box 27-103, Wellington, New Zealand