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Thread: HP 21mx - Plans & Ideas

  1. #1
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    Default HP 21mx - Plans & Ideas

    Hi Folks!

    So, one random Ebay purchase later and I've got an HP 21mx (2108A) on it's way to me.

    I know the machine is fitted with 2x64K and 1x128k RAM boards along with the DCPC, MEM, MEM Protect and MEM controller boards. For IO, it has a single BACI (Serial) board.

    My aim for this machine is pretty simple. I'd like to hook it up to my model 33 Teletype, running BASIC (Or other language) and use paper tape for storage. 1/2" tape drive would be nice but...

    Is this possible with this machine / configuration? If not, what extra hardware would I need? What software/OS do I need to look at?

    Cheers,
    Dave

    PS. There is also the potential problem that there may be a fault on the machine but fixing things is half the fun

  2. #2

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    You might check which boot ROMs it has. Different ones work with different I/O. There is usually a list on the panel someplace but it never hurts to hunt around for an undocumented addition.
    Dwight

  3. #3
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    Yes, I'd read that there were different boot ROM's available. They are fitted on a board under the main CPU board aren't they? I see the ROM images are also on Bitsavers so I should be able to burn one if needed.

    Machine won't be with me for another week or two yet but I'll post what's fitted when It arrives.

    Cheers,
    Dave

  4. #4

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    They almost always have a serial one.
    It has been a while since I played with my machine. There is a lot I've forgotten.
    If it has DRAM memory boards, it is likely to be the setup that is intended to have the external backup batteries. There will be a connector on the back that you need to add a resistor to or it won't power up.
    I believe the BASIC that was available was intended to run with two computers stacked together. I never fully looked into this.
    Someone did make a Forth like programming language for these.
    One can always toggle stuff in from the front panel.
    Dwight

  5. #5
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    Quote Originally Posted by Dwight Elvey View Post
    They almost always have a serial one.
    Yes, it seems this was a standard option.

    Quote Originally Posted by Dwight Elvey View Post
    If it has DRAM memory boards, it is likely to be the setup that is intended to have the external backup batteries. There will be a connector on the back that you need to add a resistor to or it won't power up.
    Thanks! I didn't know about that. I'd seen the connectors in diagrams / photos but didn't think anymore about it.

    Quote Originally Posted by Dwight Elvey View Post
    I believe the BASIC that was available was intended to run with two computers stacked together. I never fully looked into this.
    I've just been reading up on this. It was the 2000/Access System (Time sharing) that used a pair of processors. One for character I/O and the other for disk & applications. There seemed to be a couple of other versions that would run on a single processor (Both multi-user and single user).

    Being new to this, it all seems rather complicated. Going to take a while to get my head round the various options, etc.

    Quote Originally Posted by Dwight Elvey View Post
    One can always toggle stuff in from the front panel.
    Indeed!

    Cheers,
    Dave

  6. #6
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    Take a look at HPDrive - http://www.hp9845.net/9845/projects/hpdrive/ - a very clever and useful setup that will let you emulate many types of HPIB attached mass storage. David Bryan has worked out how to use HPDrive with the HP 1000 series machines.

  7. #7
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    Thanks Jack. I'd seen the page and it does seem to be the most obvious way to get the machine up and running. Will probably end up going down this route.

    Sadly, the system has a dead power supply at the moment. Need to pick up an ESR meter to check the capacitors.

    Cheers,
    Dave

  8. #8

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    If it is a DRAM machine, you need the resistor on the battery connector!
    Dwight

  9. #9
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    The resistor is only needed if the internal battery support circuitry is present. This is missing on my system. I also think its missing the optional microcode board that normally resides under the CPU board. Strange that it does have the extended RAM controllers though.

    The system did power up briefly but now just blows fuses. Seems to be a short on the 5V line but I've not had chance to spend time with it yet.

    Cheers,
    Dave

  10. #10

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    OK
    Then you might want to go to VCF West in August. I will be giving a talk on finding shorts.
    Dwight

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