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Identify this mystery machine (SS30/SS50 system?) aka EPSILON computer (?)

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    Identify this mystery machine (SS30/SS50 system?) aka EPSILON computer (?)

    I found this thing some time ago, and it caught attention of the SS50 & SS30 Computer Bus Chat Facebook group admin and since then we are discussing it there, although it's very small group...

    Right now I already have checked the PSU, cleaned/inspected floppy drives, traced a portion of the I/O board and I try to get some serial output out if it (no success so far). I also dumped the EPROM contents.

    Photo documentation is at:

    I'm looking for any information/documentation regarding this machine.
    Check out my YouTube channel - the Rusty Bits

    Judging by the eurocard connector and the dates on the newer logic ICs, I would guess that this is possibly a PLC. Based on the variety of date codes, flux, and lack of solder mask, I think it might not have been a widely available commercial product.
    Also, there is something unusual about the way the EPROM binary is laid out. If I disassemble the entire 4K at $F000, I get an odd looking reset and NMI vector:
    svec_NMI FDB hdlr_NMI ;FFFC: 17 65 '.e'
    svec_RST FDB hdlr_RST ;FFFE: 30 4E '0N'

    But, If I just disassemble the first 2K at $F800, I get a much more reasonable looking reset and NMI vector:
    svec_NMI FDB hdlr_NMI ;FFFC: FD 8C '..'
    svec_RST FDB hdlr_RST ;FFFE: F0 00 '..'
    With the full 4K binary, those are sitting at $F7FC. So, I think some logic may be inverting one of the address lines (A11?) for the EEPROM, or possibly doing other tricks with how it is addressed. Everything is standard logic, so I would figure out how the EEPROM is really addressed before spending any more time disassembling it.
    Last edited by epooch; September 14, 2021, 05:44 PM.


      The jumper from the 74S288 (PROM) to the EPROM is controlling A11 and possibly inverting it. The other jumper (that forms an X) possibly restores the normal A11 signal to the other ROM IC sockets. That PROM probably controls the memory map of the device, similar to how a PAL or GAL would normally be used.