Re: Appletalk on IIC

[nparker@cie-2.uoregon.edu (Neil Parker)]

In article <3365065f.0@news1.ibm.net> wshek@ibm.net writes:
>In <david.862027332@wraith>, david@uow.edu.au (David E A Wilson) writes:
>>
>>wshek@ibm.net writes:
>>>A recent post stated that appletalk work was not completed on 
>>>IIC. Not true! Tech ref book shows a large segment of rom code
>>>in slot 7 dedicated. If you have the UNIdisk rom (second one)
>>>then you have appletalk support.
>>
>>However you do need additional hardware - probably a box with a 65c02 and
>>a z8530 that would connect to the serial port of the //c and have an
>>Appletalk port - and no one has one of these to my knowledge.
>>--
>>David Wilson	Dept CompSci Uni Wollongong Australia	david@cs.uow.edu.au
>
>I'm not understanding why you need extra hardware, the mac doesn't use it?
>I started AT on my macs using just the extra printer cable I have
>joining the serial ports of 2 machines. (works great)
>As for processor not being enough.... on a //C it's not doing anything else 
>anyway and as for memory .... where do you put stuff you load from a disk
>drive? The //c serial buffering as implemented should be sufficient.

There are two reasons why you need extra hardware.

1.  Baud rate.  Appletalk runs at 230400 baud, which is 16 times faster than
the maximum speed supported by the Apple IIc's 6551 serial chip.

2.  Appletalk is a synchronous protocol--i.e. each packet is sent as a single
stream of bits, with no intervening start/stop bits at the byte boundaries.
The 6551 (like most serial chips) was designed solely for asynchronous
communication, and cannot be made to transmit bytes without start and stop
bits, nor can it be convinced to receive bytes that don't have start and
stop bits.

Problem number 1 could probably be cured with a "TurboASB-like"
modification to the IIc motherboard.  Problem number 2 could only be cured
by using a different serial chip (one that supports synchronous
communication, such as the Z8530).

The Mac (and the IIGS) needs no extra Appletalk hardware because its built-
in serial chip is the Z8530.

As I understand it, the idea behind the IIc Appletalk implementation (had it
ever been completed) was that there would be an external box with two or
three serial ports.  One would connect to the IIc, and would speak some
kind of slow, asynchronous, not-quite-Appletalk protocol. The other ports
would be controlled by Z8530s, and would speak the synchronous Appletalk
protocol at full speed.  The box would contain firmware to translate
between the two protocols.

               - Neil Parker
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