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External LocalTalk / serial port clocks


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I know that, in theory, one can supply an external synchronous clock to the SCC by supplying it on the clock in pin on the 8-pin DIN of the appropriate serial port.

 

Does anyone know if this is sufficient to get that clock used, or whether I have to do anything in software? (I sort of expect to have to?)

 

I am looking at running LocalTalk at a different data speed (lower), for various rather silly reasons.

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This may be of interest to people hoping to run Localtalk at faster than 230kbps as well.  A few third party hardware makers back in the old days achieved around 1Mbps or better - Farallon and Asante are the names that come to mind.  Though if I recall, they both required you to use their hardware at both ends.

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5 hours ago, cheesestraws said:

Does anyone know if this is sufficient to get that clock used, or whether I have to do anything in software?

You should configure the SCC in software. I have attached the Zilog manual that you will need. It is difficult to understand this manual at first, as it is not so well organized, but after you look at some sample source codes too, you will understand it better.

 

The file SCC.SDLC.Asm is what I wrote earlier this week for the SCC. Turns out I don't need this code for my particular application, but I'll leave it here as an example for others.

 

 

Also, look into Async AppleTalk (or Asynchronous AppleTalk). This was a project back in the late 1980s or so to run AppleTalk over RS232. Someone bought a source license to the AppleTalk .MPP driver from Apple and rewrote the ALAP. I don't have any files for this, but I'm still looking.

 

 

Z8530UM.pdf

SCC.SDLC.Asm.txt

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56 minutes ago, Dog Cow said:

It is difficult to understand this manual at first, as it is not so well organized, but after you look at some sample source codes too, you will understand it better. 

I have the SCC manual and I think more or less understand it—I've also got a technote which I've distributed before about implementing LocalTalk on a Zilog microcontroller with a built-in SCC (which you are welcome to a copy of if you would like).  What I'm trying to work out is how Apple wired it up, rather than how the chip itself behaves.  Perhaps to clarify, I was always given to understand that the provision of an external clock line to make synchronous serial, and especially SDLC, go faster was a deliberate design decision, and so it's perhaps not unreasonable to think that there is a "neat" way to enable that external clock line without having to go near hardware.  Especially given that I would rather not pierce that abstraction unless I absolutely have to.

 

I'm curious especially about that Farallon stuff that used an external clock. If anyone has any of that, what software did it use?  Is there a copy available anywhere?

 

The (potential) use case (well, useless case?) I have here is very definitely synchronous, and has an external clock, and is SDLC framed.  So the only thing I need to do is to slave the existing implementation to an external clock.

Edited by cheesestraws
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2 hours ago, cheesestraws said:

 Perhaps to clarify, I was always given to understand that the provision of an external clock line to make synchronous serial, and especially SDLC, go faster was a deliberate design decision, and so it's perhaps not unreasonable to think that there is a "neat" way to enable that external clock line without having to go near hardware.

OK, now I understand your objective better. The clock can be supplied through one of the pins on the Mac's serial ports. It's one of the handshaking pins.

 

The first-generation Macintosh hard disks like the Tecmar MacDrive provided a clock to operate at around 700 or so kilobaud.

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8 hours ago, Dog Cow said:

The clock can be supplied through one of the pins on the Mac's serial ports. It's one of the handshaking pins.

Pin 2 on a mini-DIN, if memory serves.

 

My question is what I need to do to enable this from an OS perspective, if anything.  And especially how the Farallon devices did it.

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  • 7 months later...

Did you figure out how to create an external clock?  Since I figured out how to get AppleTalk over a null modem connection, I've been wondering how to increase the speed.

My USB to serial adapter is 921.6kbit/s (115.2KB/s), way faster than the 57.6 kbit/s (7.2KB/s) my LC 475 does.

I'm wondering if I could use an external clock to push the LC's end of the connection to a faster speed (limited by the CPU's ability to keep up of course)

The 900 kbit/s (112.5KB/s) for the IIfx and Centris/Quadra AV thanks to their Input/output processor (IOP) ports is pretty close to the speed I'd like to hit.

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Not sure if this helps at all, but all the simple passive MIDI interfaces work like this. MIDI is a serial port at 31.25KHz, which the Mac can't do natively, so to do MIDI you need a breakout box that supplies a 1MHz clock. I have schematics for something like this somewhere but my Macs aren't hooked up due to moving. It's extremely simple, though, literally just the clock on the right pint (and some components normally required for MIDI, which is a current loop, not a TTL connection). Unfortunately I don't think there's driver sources available for this.

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13 hours ago, paws said:

Unfortunately I don't think there's driver sources available for this.

 

Yeah, the hardware is reasonably straightforward.  It's the software side that I don't know how one would go about doing :-(.

 

I wish I had one of those Farallon jobbies that had their own clocks...

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