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| Logic analyser suitable for serial decoding of old HP calculator bus? |
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| Bill Woodbridge:
I’m currently trying to fault-find an HP-67 calculator by examining its serial address/instruction bus. The machine transfers address, instruction and display data on this bus synchronously using fixed time slots within the 56-bit so-called ‘Woodstock word’. These timeslots are 12, 10 and 8 bits respectively, simple HI = 1 LO = 0 levels with a brief return to zero between each bit. There is a separate clock line (in fact two different phases) and a separate sync line indicating the start of each word. My Keysight DSOX3014 makes a good job of capturing the bus in either analogue or digital (MSO) acquisition mode. But none of the built-in serial protocol decoding capabilities are useful despite the Woodstock word structure being very simple, and as far as I can see there are no user-defined options for custom decoding. So I’m having to squint at the display counting 1s and 0s, then writing these into a spreadsheet, etc. My question is: does anyone know of a logic analyser that has enough user-defined decode options to extract and present data automatically from the ‘Woodstock word’? Just stripping out the three data items from each word would be nice, with extra points awarded for reversing the LSB-first order, converting the instructions against a lookup table of microcode mnemonics and presentation in an address – instruction table. Even more extra points if there is a classic HP model that would do this, to add to my vintage test equipment collection. Full disclosure – I’m pretty sure I’ve already found the actual fault, but I’d still like to dig deeper into analysis of what’s going on. Many thanks if anyone can help. |
| Le_Bassiste:
you may want to reach out to forum member "teenix" over at https://www.hpmuseum.org/forum/index.php. he has done extensive work on HP calculator hardware, especially woodstocks. hth |
| Bill Woodbridge:
Thanks for the suggestion - Tony (Teenix) has already been incredibly helpful in helping me pinpoint the fault (a failed ROM/RAM IC, so unobtanium unfortunately as often with old HP calculators unless anyone happens to have a spare 1818-0232 they could sell me). It's true I haven't asked him this question though - I thought I'd post here as the calculator project has now crossed over somewhat into my vintage test equipment enthusiasm. Thanks again, Bill |
| aeg:
The HP logic analyzers were intended for parallel data buses, not serial. However, if you really want to decode synchronous serial, you can write a custom inverse assembler and upload it to the analyzer. Your triggering capabilities will be limited to triggering on sync or on the first few bits after sync. You could also make a hardware "preprocessor" to convert the serial bus to parallel - then you get the full capabilities of the analyzer. |
| Bill Woodbridge:
Thanks very much - I wasn't aware of the possibility of custom inverse assemblers for HP logic analysers. I'll look into that some more. Hardware preprocessing might be a step too far for me though! |
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