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GPIB to Serial or parallel printer adapter with the PET

Eudimorphodon

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...although I'm a little worried about the floating inputs on U12B, maybe he forgot to finish the diagram. They should at least have had tie resistors on those pins. With that sort of thing, it may not be viable to build it as the documentation is questionable.

Having the manual to this device would help a *lot*, because, ho boy, that is an amazing pile of spaghetti around those DIP switches.

Just eyeballing it that section with the mysterious hanging inputs looks like it's part of a circuit designed to enable some kind of translation on input data; unless I lost the plot trying to follow it the output from that section goes into one of the dip switches, and from there goes to a circuit appearing to enable selective gating of the D7 line on the parallel port. Maybe this is a filter to prevent "normal" printers from getting confused by PET screen dumps where the high bit is set on reverse video characters? Although maybe it's more complicated than that, since some of the qualification is coming from lower data lines.

Other than those hanging chads this looks to me like it's... mostly there? But maybe not? On the handshaking side the "NDAC" line appears to be almost directly used as the Centronics "strobe", and there's some kind of state machine going on cycle the NRFD and DAV lines based on Centronics "BUSY". What it making my head hurt is how this thing sorts out the IEEE ATN/Listener mechanism, but it doesn't help that I'm pretty fuzzy on how that actually works. I was under the vague impression that for an MVP implementation of that you'd need to have some kind of flip-flop on this somewhere that would store the result of decoding what listener the talker was designating so it'd know to process subsequent data bytes? I *think* those 'LS30s are there as part of the IEEE command processing, IE, to grok a listen command and its address, but again... I can't see how it would *store* this "I'm the listener until I see someone else told they're supposed to be listening" status? That's the huge question mark for me here.
 

Ruud

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I found my box: Black box, 232-488. I googled on this and it is easy to find. Except, I have a different layout on the the front and a different power switch on the back.
 

Hugo Holden

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I found my box: Black box, 232-488. I googled on this and it is easy to find. Except, I have a different layout on the the front and a different power switch on the back.
Do you have more details on it and the schematic ?
 

daver2

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That's the one. My phone is a little 'hit and miss' as to whether it opens a PDF or not!

Pages 133 and 157 for the circuits.

Dave
 

Eudimorphodon

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The downside of the Centronics driver circuit in that book is it’s what the book describes as a “non-standard” device, IE, the circuit doesn’t do any device number resolution and thereby will try to print whatever gets sent to *any* attached device. (In other words, if you have this hooked up alongside a disk drive or whatever you’ll probably want to turn it off when you do any disk activity.)

There don’t seem to be schematics in the book for a full individually-addressable IEEE-488 target. I assume it’s because the command processing part is going to be pretty ugly, although reading the requirements for the CMD and string PRINT# commands it looks like all you really need is to be able to handle the ATN listen/unlisten for your chosen device number to toggle on and off. I assume that’s what most of the spaghetti in the ADA1800 schematic is for but, again, I don’t see the flip-flop I would expect it would need to have to store that?
 

daver2

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Good point.

Over on the Tektronix 4051, 4052 and 4054 thread - @nikola-wan and @WaveyDipole have been 'messing around' (and enjoying themselves in the process) with an IEEE 488 interface - but I think they contain microcontrollers rather than pure hardware.

Dave
 
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Eudimorphodon

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Honestly if you wanted to make one of these from whole cloth today I think slapping an AVR CPU on the lines would be the get out of jail free card, as I suggested before the… serial unpleasantness. The command processing part then just turns into a few lines of code. A 40-pin variant like the 324 would have enough I/O lines to completely buffer a Centronics port separate from the IEEE-488 lines, and you’d have enough CPU grunt to, if you wanted to, emulate the fancy stuff real Commodore printers can do. (Check out the manual for a printer like the 4023, they could do tricks like automatic column formatting.)

… or the diet version of the above would be to build the ‘dumb’ Centronics adapter from the book and hang a small MCU off it to do the command processing.
 

MikeS

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Wow, thanks, I didn't know the ADA1800 existed. It may well even be rarer than the ADA1450. It turns out it has been reverse engineered, I found the schematic on zimmers ( Attached). I could make that very easily.

Would you happen to know if the user manual is around for this one , DIP settings etc ?
I'll look around but don't hold your breath :)
I do have an ADA1800 though.
 

Hugo Holden

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I'll look around but don't hold your breath :)
I do have an ADA1800 though.
Since the ADA1800 is all discrete logic, it would be good to have an accurate schematic. Can you look inside it and find out where pins 3, 4 & 5 of U12 B connect ?
 

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daver2

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U5C and U5D are the 'listen' S/R flip-flop.

Switch S1 I am still working on. Pins 1 & 8 of SD1. Something to do with DIO8.
Switch S2 inverts (or not) DIO1. Pins 2 & 7 of SD1.
Switch S3 inverts (or not) DIO2. Pins 3 & 6 of SD1.
Switch S4 enables (or disables) passing through of DIO8 to Centronics D7. Pins 4 & 5 of SD1.

U7 is the 'listen' decoder.
U9 is the 'unlisten' decoder.

Dave
 
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Hugo Holden

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>>> Yes, a classic bowtie flip-flop.

Just not drawn classically!

Dave
I agree.

One interesting thing, is the open collector NAND gate U4B, also sharing its output with another open collector NAND ,making sense as both those outputs would be normally high, assuming a pullup resistor on the /NRFD line in the PET (I haven't checked that)...but pin 4 of U4B is held low by the output of an '04 inverter, which has its input connected directly to VCC. That is a little odd, as that would disable U4B and prevent its output changing state and going low. I wonder if there is a drawing error in that area.

I cannot see a physical pullup resistor on /NRFD, unless it is intrinsic to the 6522.

These sorts of things are making me worry a little more about this schematic. But we should be able to get it fixed as long as there is the real ADA1800 to examine.
 
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Eudimorphodon

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U5C and U5D are the 'listen' S/R flip-flop.

Doh! I was looking specifically for a 74LS76 or something.

The switch to strip the high data bit makes sense, it kind of reads from that PET interfacing book that the PET has a habit of sending characters with the high bit set in otherwise normal ascii. I wonder if those other switches are trying to massage CR/LFs.
 

daver2

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Hugo,

The Centronics BUSY is routed via U14D, U5A and U4D - so I suspect the 'disabled' gate is not used at all.

Dave
 

dabone

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Here's all I got today with a ADA 1450 an IEEE to serial printer adapter for the pet.

Photos of the Case, PCB and Manual are located here. (Click the word here.)

Adjusting the baud rate with a trimmer, wild.....
 
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