jeffm@mmintl.UUCP (Jeffrey Miller) (11/17/86)
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A while back I read a note from someone asking if it is possible to get 1200
baud on the original Apple Communications Card. Someone else responded by
saying no. Well it is possible (I hope I am not duplicating a subsequent
response). I will quote from the manual.
"MODIFYING THE COMMUNICATIONS INTERFACE CARD FOR HIGHER BAUD RATES
The Communications Interface Card may be modified for higher BAUD rates
by rewiring signals on the two 74C161 counters.
In order to prevent cutting and wiring on the printed-circuit board, all
rewiring should be done on the devices themselves. If the IC's are stacked
on 16-pin DIP headers, then the parts can be easily changed for differenty
BAUD rates.
Because of the count rate required, the only counter IC's that should
be used are 74C161 (National Semiconductor) of 40161 (Fairchild).
The Communications Interface Card will default to the higher modified
BAUD rate when you activate it, but will give to lower modified BAUD rate
when the card is set for 110 BAUD, as described in the operating manual.
For instance, with the 1200/300 modification, setting 110 BAUD will give 300
BAUD.
THIS INFORMATION IS PROVIDED FOR THE CONVENIENCE OF THE SOPHISTICATED
USER. ANY PRODUCT MODIFICATIONS (INCLUDING THIS ONE) VOID THE
MANUFACTURER'S WARRANTY ON THE MODIFIED HARDWARE."
The manual then shows pictures of the 2 chips before and after modification
for 1200/300 and 4800/1200 (the other possible modification). Since I don't
remember what the wiring was before modification, I have no way of knowing
what on this picture is different. So I'll try to describe what appear to
be changes (I'll mail copies to anyone that's interested as long as it
doesn't turn out to be hundreds of people). Chip A1 (74C161) has pins 6,5,
and 1 tied together. The all go to the spot that 1 goes to, and 6 and 5
have no other connection (at least in the picture). Pin 4 goes to pin 8 and
has no other connection. Pin 8 is still connected to its original spot.
Pin 11's trace is cut and has no connection. Pin 13 is rerouted to the spot
Pin 11 used to go to. Chip A2 (74C161) has pin 5 going to pin 8 and nowhere
else. No other changes on A2.
To reiterate, chip A1:
pins 4, 5, 6, 11, 13 cut from original traces
pin 4 to pin 8
pins 5 & 6 to pin 1
pin 13 routed to pin 11's trace
pin 11 has no contact
chip A2:
pin 5 cut from its trace and connected to pin 8
There is another set of mods for 4800/1200 but I don't feel like typing them
in at the moment. They're on the same sheet tho if anyone's interested in
that odd combination.
Hope this helps,
Jeff
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* Jeff Miller *
* Multimate International Corp. *
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* East Hartford, CT 06108-9911 *
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