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Thread: "Building a 32K RAM Board for the PDP-8/e" article published on Tronola.com

  1. #1

    Default "Building a 32K RAM Board for the PDP-8/e" article published on Tronola.com

    Hi folks,

    I'm pleased to announce that we have just published a new vintage computing article on Tronola.com: "Building a 32K RAM Board for the PDP-8/e". You can read it here:
    http://www.tronola.com/html/ram_for_pdp-8e.html

    It includes a new look at interfacing memory to the Omnibus.

    -- Comments and corrections are welcome.

    Thanks,
    Steve Lafferty
    http://www.tronola.com/index.html

  2. #2
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    Steve,

    Very nice article. Yours looks a lot like mine : http://www.vintage-computer.com/vcfo...chmentid=12029 which I built earlier this year, and the older one http://www.vintage-computer.com/vcfo...achmentid=2430 that I built in 2008. The ones I built were designed by Charles Dickman in 2006, but I had a bit of troubleshooting to do. Chuck's work is here: http://www.chdickman.com/pdp8/ under the "hardware projects" section. The marked up schematic scans are my handiwork. I learned how the omnibus worked to do the troubleshooting from Volume 1 of the 8/e maint. manual.

    All my wiring is on the front side. I too made all the mods to the etches on the Douglas board. It is interesting that you also found the lower right corner to be the optimal place to put everything.

    You should write a little more about running the extended memory maindecs to gain confidence in the build.

    Chuck's design does support data break operation. I use it in my 8/e with an RL8A, which performs all transfers to/from disk via data break.

    Tell us some more about your system! If you were running OS/8 in 8kW, it's either from TC08, RX01, or RK05.

    Lou
    Last edited by Lou - N2MIY; May 24th, 2013 at 03:46 PM.

  3. #3

    Default

    Thanks, Lou,

    That's a very nice build that you did on your RAM board. I see from the schematic markups that it uses the 62256 chip also. (I assume that it's the same as the Cypress part.) It's nice to have some confirmation that it's popular. The PDP-8/a looks great.

    Yeah, that little gotcha on the Douglas board--- The first time I plugged it in, I quickly discovered that the +15V supply had pulled down to +5V! Nice little robustness test for the PDP-8/m's switching regulators. Geez.

    I'm envious of your PDP-8/a, with the RL8A controller, supporting the RL01/02 drive. Of course, there isn't an RL01/02 controller for the four-slot version of the Omnibus. I restored an RK8E controller board set and an RK05 drive to get OS/8 running. The RK8E does data break with the RAM board, of course. The drive needed a couple boards repaired, a power supply fixed, blower replaced, absolute filter replaced and foam replaced. Also made a prefilter assembly for it. Feel like I earned that sucker!

    Thanks for the suggestion about writing up the memory diagnostics. I actually thought the same thing but the article was getting long and I wanted to get it out. I'll go back and add that and a few other things.

    I appreciate your bringing Chuck Dickman's design and your fine project to my attention. I will add coverage of those to the article as well.

    I have a few things on the projects list before I get back around to the PDP-8/m but will be bringing up an Extended Arithmetic Element set and a Power Fail/Autoboot for it in the near future. There is also that stuck bit in the 8K core board set to fix. And, there is a second RK05 sitting here, waiting to be restored. Not to mention the Tek 4012 graphics terminal begging for attention. So many toys, so little time. I guess you know the feeling.

    Steve L.

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    Very nice article!

    I have two restored PDP-8m systems in working condition, both with 16K core, EAE, and RX01/2 for storage. I have extra core, but no more slots. Makes me think about building a RAM extender board (altho with so much board space left, I would probably add a CF/IDE socket, and FPGA controller, to emulate a standard DEC disk, like an RK05 or RL02).

    BTW, lining up all the wires looks nice, but it is absolutely the worst thing you can do in terms of generating crosstalk. The random point to point option with wires crossing only at angles, and never running parallel, is electrically much better. Looks worse, tho.

    Lucky the OMNIBUS is so slow in terms of signal edge rates, and your wires are short enough, that crosstalk will be minimal.

    Don

  5. #5

    Default

    Thanks, Don,

    Yeah, with half the slots of the otherwise similar PDP-8/e, we 8/m owners have to be miserly with space. (But hey, I can lift an 8/m!) When I get the 8K core, EAE and power fail/autoboot stuff back into mine, there will be only one slot will be left! Like you, I've had dreams of what to do with all the space that's left on the Douglas prototyping board. Doing an FPP-12 floating point processor emulator is a fond fantasy.

    I agree that going out of your way to bundle up wire runs is worse from the crosstalk point of view. However, when multiple wires are going between similar locations, there isn't a lot else that makes sense. I guess you could take pains to separate them but as you say, the practicality is that for a situation like this, it's not an issue.

    Steve L.

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    Quote Originally Posted by shlafferty View Post
    Of course, there isn't an RL01/02 controller for the four-slot version of the Omnibus.
    Steve,

    I didn't let that stop me. See attached: http://www.vintage-computer.com/vcfo...chmentid=10439

    So the RL8A is really in my 8/e. The 8/a only has an RX02 for mass storage at the moment. I do have a RK8E boardset that I need to repair (haven't touched it yet) and two RK05s to be restored. You can see them a little better here: http://www.vintage-computer.com/vcfo...chmentid=10438 . I have to make the adapter board from the 40 pin berg connectors to the dual height card that goes into the little backplane in the RK05. So yes, there's still a lot of work in the queue here too! In the long term, the RK05s will stay with the 8/e and the RL8A will properly go into the 8/a.

    Keep up the good work!

    Lou

  7. #7

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    Quote Originally Posted by Lou - N2MIY View Post
    Steve,

    I didn't let that stop me. See attached: http://www.vintage-computer.com/vcfo...chmentid=10439

    So the RL8A is really in my 8/e. The 8/a only has an RX02 for mass storage at the moment. I do have a RK8E boardset that I need to repair (haven't touched it yet) and two RK05s to be restored. You can see them a little better here: http://www.vintage-computer.com/vcfo...chmentid=10438 . I have to make the adapter board from the 40 pin berg connectors to the dual height card that goes into the little backplane in the RK05. So yes, there's still a lot of work in the queue here too! In the long term, the RK05s will stay with the 8/e and the RL8A will properly go into the 8/a.

    Keep up the good work!

    Lou
    Wow -- who needs a hex expansion chassis?!! Well cheers for the gumption! :) Made me look at the 8/m. Seems that, if I outboarded the fans, there would room for another pair of Omnibus slots (which I happen to have), to make it hex wide... nah :)

    By the way, as you probably know, the adapter board, cable and berg connectors which goes from the RK05 backplane to the RK8E is the M993. I purchased one (designated M993YA) from http://www.jtcomputer.com/ in December 2012 for $100 plus shipping.

    Thanks, Steve

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    Steve,

    There is really no need for the E and F sets of fingers in any omnibus machine we will use today. All that is provided there are three more address bits for the 128kW option that there is virtually no software that makes use of. So, if you pull out your power supply, you need not put any backplane in those E and F positions. You would not have been the first to go that route. I read about that idea in a post by Charles Lasner on alt.sys.pdp8 a long way back. The same can be done in an 8/e also, but the power supply is really heavy! I was too lazy. I should post an updated picture, but I cleared out the space above the 8/e chassis in the rack and put in a blank cover. The RL8A jerry-rig can only be seen when the cover is removed.

    Thanks for the tip on the M993. I had planned to make my own since there isn't much to it.

    I did check out tronola.com and I think several of the folks here will enjoy your kinds of projects (I know I do.)

    Lou

  9. #9

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    Hi Lou,

    I knew that there wasn't much in the extra hex Omnibus slots but I had no idea that there was that little! Unlike the PDP-8/e, the 8/m mounts the power supply in the back of the box, rather than the side. So literally the only things occupying the side space are the two fans. Those could be mounted outboard, leaving the space free for hex cards to hang out. Without even the need to mount bus slots, it's very tempting indeed! Perhaps my dream of someday having an FPP8A is more doable than I realized!

    Clever solution to packaging your RL8A into the rack, by the way.

    >I did check out tronola.com and I think several of the folks here will enjoy your kinds of projects (I know I do.)
    --- Thanks, Lou.

    Steve

  10. #10

    Default i8255 in PDP-8

    Hi, all !

    I have already read the article: "Building a 32K RAM Board for the PDP-8/e".
    http://www.tronola.com/html/ram_for_pdp-8e.html

    I'm not familiar with Omnibus signals, so can anybody explain, how to modifier
    this scheme:



    if I want to conduct to the PDP-8 scheme parallel adapter i8250 ?

    So, to drive it with I/O comands?

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