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PDP-8 Peripherals?

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    PDP-8 Peripherals?

    OK... moving on to one of my favorite machines of all-time, the PDP-8...

    I've had PDP-8s of one flavor or another for about 20 years or so. Back in college, I ponied up $600 for an RL8A and another $250 for an RL01 (with shipping) and so have had a hard disk on a PDP-8 for quite a while now. I realize that back in the day, the RK8E was much more common (since it fit in any OMNIBUS machine, unlike the hex-height RL8A). For those folks out there with hard disks on their -8s, what do you have?

    To date, I have two sets of DF/DS-32s (partially tested, but the plating is probably scraped off the platters), an untested RK8E, and the RL8A. Everything else I do with the -8s is papertape or 8" floppy.

    Oops... I forgot about the "modern" machines - a DECmate II with an ST225 (RD31) and an SBC6120 with an 8MB CompactFlash emulating
    an IDE disk. I _have_ gotten plenty of time on the SBC6120 lately - it's a lot more portable than any other -8 I have, especally with the Planar Systems ELT-320 terminal. When I use a laptop as the terminal, I have the entire rig running off of batteries.


    DEC hard disks

    My 8e has a dual RK05 setup. I have not got it fully working yet, but I have run the basic test and saw that it was reading and writing. I have no bootable pack yet, but am working on making an OS/8 pack soon I hope.
    My 8a has just the floppies. I stole the RK05 from the 8a. I also have an RL02 drive, but no hex wide controller for the 8a. That will probably get hooked up to the 11/34a someday.


      PDP-8 Peripherals?

      I have exactly one RK05J 16-sector pack (and one RK05F 16-sector pack), that may or may not be bootable. I am still in the (long) process of repairing my -8/e PSU and will turn to debugging the CPU once I get power restored. After that, I can see if the RK8E works.

      Once you get the hardware qualified, there are tools out there on sites like and that will help you feed your -8/e from a PeeCee loaded with 12-bit software to spoon down the console line. I have not done that myself, but it shouldn't be too bad. Since you have floppies, you are not totally dead in the water - if you have a complete OS/8 distro, you should have the tools and drivers you need to init an RK pack.

      Keep us appraised of your RK05 progress. I've done a bit with an RKV11D and a couple flavors of Qbus PDP-11, but I haven't yet run an RK05 on an -8.



        RK05 packs

        I have used (or tried to use) those programs to download via the serial port to my 8e. For some reason I get checksum errors on trying to load any fairly long virtual paper tape image to the bin loader. I am still working on what is causing this problem. If I can get this solved I should be able to load an image to a pack and boot.


          Any suggestions to get my 8e back to life

          I was so close to having my 8e loading from my new (old) Dectape drive.
          I think one tape was bad, and the second looked like it would read a bit and then the machine would halt. What I finally think my problem was was that I was only running with 8k, and it wanted at least 12k. On putting in the second 8k, it decided to just not work. None of the switches do anything. All the lights are on. I have removed all cards and looked for shorts. I will physically have to remove the Omnibus so I can look closer for shorts. I don't think it is a short as it was working for quite awhile. Anyone got any ideas. I am working on getting troubleshooting gear (logic probe, oscillloscope, etc). Thanks.



            Well, I now have added an 8f to my collection. This one is known tested.
            My 8e just refuses to load tapes quite right. I seem to have some odd
            timing problem. Well, on trying to load tapes to the 8f it seems my cable
            must not have all the wanted lines. A simple echo program works Ok.
            But on running the full ASCII echo it seems to buffer up a bunch and then
            output it. This is probably on the PC side I would guess? So I think this
            is also probably happening on trying to load a paper tape image.
            So at this point my 8i does pretty much nothing as I have no TTY for it and the paper tape reader seems to have a bug. The 8e is not quite right and the 8f is still not reading tapes right. But on the positive side my 11/34a boots and runs from RL02 and my 11/84 runs, and I hope to make
            it bootable tonight.

            If I can ever get a tape loaded into the 8f I plan to make a bootable RK05 for it.
            Tim Radde


              I have a few '8s.

              I have the Intersil super sampler board with a 6100 and memory extended
              to 4k.

              Several Decmate-IIIs that are stock save for an added external reset switch.

              And my grand dame is a PDP8f wtih 16k of core and running. However while its well loaded with memory the only IO is a serial console. At some point I'll have to use one of the quad width protoboards I have to make some kind of mass staorage. Im thinking of a 256k*word) ramdisk paired up with a 256k(word) EEprom disk. For a PDP8 thats enough to run plenty of stuff and for OS8 it's a enough to emulate a fair number of standard devices.

              Good, Fast and Cheap pick any two.


                My systems now include two pdp-8i's, 2 pdp-8a's, and one pdp-8e. The 8e still is not fully functional but will someday have the RK05 that I previously mentioned attached to it. I am working on adding an RL02
                to the 8a, since it only has floppy now. I wish I had the programmers
                panel for it. But I managed to snag a dual RK8 disk drive setup for
                the 8i last year. I guess this is or was the precursor to the RK05? It
                has a similar looking pack but holds half the RK05 data. Weeee!
                It also is not working, so someting appears to have happened when I
                transported it. It was working when I saw it.

                My favorite 8 is the 8i. Maybe if I had worked with straight 8's that
                would be but I never did.


                  PDP-8 peripherals

                  Originally posted by tradde
                  My systems now include two pdp-8i's, 2 pdp-8a's, and one pdp-8e. The 8e still is not fully functional but will someday have the RK05 that I previously mentioned attached to it. I am working on adding an RL02 to the 8a, since it only has floppy now.
                  I have used my RL8A with one or two RL01s extensively, and not yet with an RL02, but in the back of my head is a dim memory of some annoying issue with OS/8 and the RL02. Since the VT278/DECmate I had a (rare) RL02 interface, I am pretty sure the RL02 will work, but there might be something strange about using it (partition size, number, something like that).

                  I wish I had the programmers panel for it
                  I picked up one of those (at last) some months back for $50. ISTR it took the seller a couple of months to get it to me, but he eventually did.

                  But I managed to snag a dual RK8 disk drive setup for
                  the 8i last year. I guess this is or was the precursor to the RK05? It
                  has a similar looking pack but holds half the RK05 data. Weeee!
                  It also is not working, so someting appears to have happened when I
                  transported it. It was working when I saw it.
                  Sounds like the ancient RK8 with RK01 or RK02 drives. Do your drives look like Diablo model 30s? (IIRC, the '30' _was_ the RK03; same density as RK05, but non-DEC)

                  I don't know much about stuff from that era, but it's similar, electrically, to the RK05, just fewer cylinders.

                  My favorite 8 is the 8i. Maybe if I had worked with straight 8's that
                  would be but I never did.
                  My favorite is the 8/L, but that's almost certainly due to the fact that it was my first one. I'm still trying to get OS/8 up on one... I'm helping (from the sidelines) test a modern (2005) Posibus RX01/RX02 interface we've dubbed the RX8. It doesn't work yet, but when it does, it will bring RX8E functionality to the -8/L and the Posibus-equipped -8/i. The design will be published and freely available for folks to make their own, once it's debugged and working.



                    My disks and status

                    I have made some progress with the RK8. I found out I had it cabelled
                    not quite right. Now I can see status register changes if I toggle in
                    a specific IOT to load certain registers and then loop. This is a good thing, but I am still having some other issues. I will continue to work
                    with this. I am hoping to get some documentation on the RK8 controller.

                    I am almost to the point of checking the alignment on my RK05 as I
                    now have an alignment disk and the terminator needed. Just have to
                    get a 1x1 scope cable to hook to the external input from the drive.

                    I am also hoping to have 2 DS32 (posibus version) drives soon. Not
                    sure what I will do with them in the short term as there are issues with power cycling and the heads and platter. I am hoping Vince will have a DS32 replacement for just the drive some day and I can help test it.
                    Tim Radde


                      Re: pdp-8 peripherals

                      Well, the 8i is dead and has been for some time now. I shorted something while scoping for another problem. Now I have no memory
                      access. The PC still increments on examine and deposit, just don't
                      get any data back. So I have gone off to try to get my 8a working.
                      Got a programmer's panel for it, and it works (once cabelled right).
                      With that I have been able to test it, and it passes all the tests I ran.
                      So I plugged the RK8 into it, and it fails the diskless test. Not sure why
                      yet. I do have the 8a running with one RX01 floppy. I am going to try to help a fellow pdp-8'er get his RL02 up and running. I will be borrowing an RL8a, and with my working RL02 I can try to make a bootable disk.

                      How did you make your bootable RL01? Or do you boot from floppy and just use the RL01 with OS/8? If I can boot OS/8 from a floppy can I
                      then create a bootable image on the RL02? TIA.
                      Tim R


                        Parallel adapter for PDP-8

                        Hi, all!

                        Excuse me, if you find my question naive...
                        I want to build a classic PDP-8 minicomputer replica like this:

                        But I doubt if I can connect to such a computer my own peripherals such as
                        PPA i8255 for example.
                        Or simple parallel register to drive LEDs using PDP-8 assembler.

                        Can anybody explain how to connect non-standart peripherals to PDP-8?
                        Or, may be, is there any manuals or sourses to read them?


                          The problem is solved

                          Originally posted by iLavr View Post
                          I doubt if I can connect to such a computer my own peripherals such as
                          PPA i8255 for example.
                          Or simple parallel register to drive LEDs using PDP-8 assembler.
                          The probem was solved with the help of David Gesswein:
                          Special thanks to David !

                          He has recommended the PDP-8 Small Computer Handbook:
                          > A chapter at the end of the small computer handbook for the model shows how
                          > to interface to the machine.

                          The PDP-8 Small Computer Handbook seems to be very useful to me!
                          So I've found that the connection of i8255 to the PDP-8 would require
                          the introduction of additional logic, which will prescribe through the
                          C0-C2 lines the CPU performance of its two operations at least:

                          - Clear the AC & OR a device register into the AC.
                          - Write AC to a device register.

                          And because i8255 PIA is a flexible device, the design will be quite

                          If anybody wants to drive your oun parallel register using PDP-8 assembler,
                          there is a very good schematic example in the Small Computer Handbook:

                          Source: PDP-8 Small Computer Handbook p. 9-35.
                          Attached Files


                            Holy cats this is an ancient thread.
                            [Need something to waste time on? Click here to visit my YouTube channel CelGenStudios]
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                            = Excellent space heater


                              Originally posted by NeXT View Post
                              Holy cats this is an ancient thread.
                              May be...

                              Originally posted by NeXT View Post
                              I have always wanted to run a PDP with some sort of disk (RK-05?) or tape (TU-56?) subsystem.
                              Try to look through this material: