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Thread: RX8E 74xx clone works

  1. #1

    Cool RX8E 74xx clone works

    This week I got my RX8E 74xx clone boards, and with two hours of soldering the first one was ready.
    After building the VC8E clone boards I've learned to test all chips before putting them into a board.
    It is a bit of work but it can save a lot of debugging time... Especially when you are using old and
    sometimes used chips like I do.

    RX8E clone and original.jpg

    After putting it in my 8M I ran the DIRXAC maindec and it passed it's test without a problem.

    My own RX02 drive has some problems which I have to fix first. But tonight I got
    another one and from that drive the left drive seems to work. So I put in some
    OS/8 disks and see what happens. And it boots OS/8

    boot os8 rx8e.jpg

    My plan is to put two of these controllers into both of my machines. One at 75 and one at 76.
    At 75 I want to connect the RX02 emulator from Don North as primary device.
    Just emulate an OS/8 boot disk and a diagnostics disk. I expect it to be quite fast because there
    is no seek time. The second controller at 76 can be connected to a real RX01 or RX02 drive.
    And in this way I can always use the machine as a stand alone machine...

    I want to do a stress test before I release the files. But I think that it is a very nice and cheap
    way of expanding your PDP8 machine... Just with standard 74xx logic chips... No DEC chips...

    I'm really happy with the results so far, especially because I found a few errors in the RX8E schematics.
    Having an original RX8E really helped me to filter these errors. It gives me a bit confidence to
    make the TU60 controller. I will order these boards this week...

    And yes, all files will be free to make your own boards if you want to. Give me a bit time
    to make the partlists etc.

    Regards, Roland
    WTB: Case for Altair 8800 ...... Rolands Github projects

  2. #2


    Nice work Roland! It's always good to see new OMNIBUS boards in the works; hopefully this should make the hobby more accessible too now that the original boards are getting expensive.
    Wanted: TU56, RK05, VT52/100, Altair 8800 CPU card, KD11-A, KE8E (M8340 M8341), memory for 11/40.
    Available for trade: AlphaStation DS10, DS15, Numerous PC ISA, PCI, and VLB boards and motherboards from the 5150 to P4 era, DEC Storageworks HDDs, misc DIP ICs, QBUS cards, 8080/85/86/88/Z80 CPUs, VT420, Misc radio valves (vacuum tubes), C64, Amiga 1200, Sinclair/TRS-80 spares, Mac Classic + Classic II.

  3. #3
    Join Date
    Jun 2012
    UK - Worcester


    Another nice job Roland!


  4. #4


    I'm impressed with your high pace! Well done, again!

  5. #5
    Join Date
    Jul 2014
    Rhode Island


    Very nice! Omnibus boards are going for very high prices on eBay lately.
    Member of the Rhode Island Computer Museum

  6. #6


    Thanks for the nice compliments! I've put some documents on GitHub
    I will put the part list on a later moment. First I want to do some more testing.

    For testing and primary boot device I want to use the nice RX02 emulator of Don North
    since his emulator also passes the Maindec in combination with the RX8E.

    The only thing which bothered me on the emulator project was the price of the four layer board.
    At JLC the minimum amount is 5 boards. 5 boards are 28 Dollar which is just a bit to
    high for free importing from China to the Netherlands. Customs and shipping
    brings it to approx 53 Dollar.

    So I made a 2 layer version which is a lot cheaper. And because of that also no
    customs to get the board here. So 5 of these boards are now $8.87 including shipment.

    RX01 - RX02 emulator.jpg

    I have no idea how this goes to the US or other countries. But if people like it
    I can share these files as well. (I have to built the boards first to test them off course)

    Regards, Roland
    WTB: Case for Altair 8800 ...... Rolands Github projects

  7. #7
    Join Date
    Aug 2010
    Silicon Valley USA
    Blog Entries


    Yeah 2 layer should probably work just fine. I was just being lazy and wanted to try a cheap 4 layer project, so my $3 vs $6 per board cost was a non issue for me.

  8. #8


    My RX8E M8357R clone card has been running the DIRXAC for a few hours on my RX02 drive and passed it with flying colours!
    So I've added the component list at GitHub and in the meanwhile I've soldered two extra boards as well...

    Three controllers.jpg

    Now if you want to have a floppy controller you don't need Ebay any more. Just make your own...
    All the files you need are here:
    You can easily drag and drop the Gerber zipfile at a PCB manufacturer like JLCPCB

    And my two layer version of Don's Emulator shield runs fine. I will put these files on GitHub as well...
    RX02 Emulator.jpg

    Since I have no RX02 disks I could emulate it trough Don's Emulator. Now I can confirm that the
    bootstrap 10 (RX8 / RX01) and Bootstrap 11 (RX28 / RX02) on the M847 extended edition are both working and booting OS/8.

    One of the five RX8E M8357R boards goes to Mattis. That leaves one spare which can go to another one person here.
    I think $17.50 ex shipping is fair for the bare board. Getting a little of my costs back...

    And please, if you are going to built a controller. Let me know! I love to have some feedback especially
    because it does cost a lot of time, energy and a bit of money. Feedback gives motivation to go further.

    In the meanwhile I've ordered the TA8E boards and a few others. To be continued...
    I'm currently not working at new designs because I want to have the current projects done first.
    These will be done in a few weeks.

    And I will make an update of the original RX8E schematics because there are a few errors in it.

    Regards, Roland
    Last edited by Roland Huisman; August 14th, 2019 at 01:17 PM.
    WTB: Case for Altair 8800 ...... Rolands Github projects

  9. #9


    I've updated the original RX8e schematic. In the original file were these three errors:

    Label "D3 MAINT (0) H" is wrong and must be "D3 MAINT (1) L" at pin 1 and 12 from E28 (7420).
    Label "D2 XDR IOT L" is wrong and must be "D2 XFER IOT L" at E41 (7442).
    And pin 13 of E28 is wrongly connected to pin 5 of E28. Pin 13 of E28 must be connected to pin 2 of U28.

    I've updated the schematic and Al has put it on Bitsavers already:

    Regards, Roland
    WTB: Case for Altair 8800 ...... Rolands Github projects

  10. #10


    If anyone is interested, I have a few spare bare boards left... Due a mistake the factory sent me another 5 of them... Just sent me a pm.
    WTB: Case for Altair 8800 ...... Rolands Github projects

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