32K Memory + Bootstraps for Omnibus

This page describes a replacement for core memory, which also provides bootstrap support for a variety of devices. The memory provided is 32K, switchable to enable or disable each 4K memory bank, allowing you also use any vintage memory you might have.

The memory implementation is based on a design by Stephen Lafferty.

The bootstrap implementation is based on the work of Roland Huisman.

While supplies last, you can obtain a kit by clicking here, and letting me know of your interest. I'm asking $95 plus shipping (which is typically another $15 within the USA).

The older 32K Memory card, without bootstrap circuitry, is described here.

Some of you have asked, so here is a shortcut to the CAD drawings. Of these, the "Roland+mem1" drawings are the board as sent off for fabrication.

is the Eagle board drawing.
is the Eagle schematic. (The action is on sheets 2 and 3.)
is the board drawing, as a PDF.
is the schematic the board, as a PDF (you want pages 2 and 3).

Here are some assembly hints for the Omnibus memory.

  1. Here is a photo of an assembled board, for reference below. (You can click on the photo to get a bigger version.)

    You should receive a snack size ziploc with discretes, one anti-static bag with chips and sockets, and a board, for each kit you have ordered. You should also get a printed sheet with the Bill of Materials itemizing the parts, and possibly release notes if there are issues discovered before your board was shipped.

    I use eutectic (63/37) no-clean rosin core solder in the 0.031 inch (0.8mm) thickness, and a fairly hot iron (325 degrees), for an easier soldering experience. The boards come in a tin-lead finish, so soldering this is about as easy as soldering gets.

    The front side is the one with the components marked, and also is the side from which the components are inserted.

  2. Start out by inserting the IC sockets. Pin one goes on the end with a notch, so point the notch TOWARD the edge connectors! (Yes, this is the opposite of the previous memory board.) Flip the board over and tack-solder diagonally opposite sides of the socket. Then pick up the board, and while pressing down gently on the socket, reheat the solder so that it is tacked as far into the board as it wants to go. (Don't burn your fingers!) Then finish soldering the socket into place.

    You should receive wide sockets for the memory chips, and a narrow socket for the ATMEL processor, and possibly another for the I/O expander as well.

    Your sockets may have the chips in them. If you leave them there, you can skip the "insert chips into sockets" step later.

    One note about these sockets. They are machine pin sockets, but the contacts aren't gold -- they are tin/lead. I think they are fine, but if you really wanted the gold ones, you'll have to provide them yourself (they are about twice the price of these).

  3. Sockets are not provided for the inexpensive components or those unlikely to be removed from the board. In the unlikely event those need repair, the safest thing is to chop them out, clean the holes one at a time, and solder in a new part. On the other hand, that does mean you have to get pin one aimed at the gold edge connectors, and all the pins in their holes, before soldering them!

    Work your way through the other chips, being sure you have the right part with the right orientation. Tack opposite corners, then press and reheat, as you did for the sockets.

  4. Solder in the pushbutton switch, SW2. The orientation is important! If you squint, you can make numerals etched into the metal. These markings should match the orientation of the silk-screen "SW2". (It is acceptable to install it rotated 180 degrees, as I have done, but NOT to rotate it 90 degrees!) If you are in doubt, make sure that the left and right sides are shorted only when the button is pressed.

  5. Solder in the DIP switches. The orientation shown will correctly make the "disabled" position to the left of the larger one.

    The smaller DIP switch is perhaps arguably installed 180 degrees from the optimal orientation in my photo. The configuration shown should be read as choosing the bootstrap numbered "16" in octal. Either way, "ON" will represent a "1" in the binary bootstrap number, and the MSB is furthest from the edge connectors.

  6. Solder in the electrolytic. This is a polarized component, so be sure that the "-" end is to the left, and the ridged end is to the right, as shown. Bend the long leads slightly after inserting, so that the part doesn't fall back out. Trim the leads after soldering. (You will want to use a similar technique to hold in the remaining discretes while you install them.)

  7. The Schottky diodes are also polarized components. Solder them near the memory chip sockets, with the black side (cathode) matching the silk-screen stripe, as shown.

  8. The non-Schottky diode is also polarized. Solder it near the ATMEL processor

  9. The 10K SIP resistor pack must also be installed with the correct orientation. Pin 1 has a dot on the left if you are reading the markings, and as with the chips, pin 1 goes closest to the edge connectors.

  10. At this point I went ahead and soldered in the non-polarized capacitors, paying attention to which value goes in which spot. I like to orient the markings on the components to match the markings on the silkscreen.

  11. Proceed with the discrete resistors, being sure to get the right values in the right spots.

  12. The battery holder goes as shown. Be sure to get it flat to the board, and yes, the "+" lead hole is oversized. You might also need a little more heating time to make sure these don't end up with cold solder joints. Be sure to trim the "+" lead flush when you are done.

  13. It wasn't really necessary to delay putting in the transistors, but let's do them now. Install the 2N3904 at Q3 to match the silkscreen. Bend the center lead back gently to get it into the hole, and be sure the flat side lines up with the silkscreen.

    Q1 and Q2 must NOT be installed unless the rest of the boot loader part is populated. (Your PDP-8 will be stuck in RESET if you do.) The silkscreen doesn't match the package quite as well, but bend the center lead out toward the round side, away from the flat side, to form a "V" shape. Then insert the leads in the holes provided.

  14. The one wonky bit is the crystal. I had originally planned a larger crystal, resting on it's side. We've got low profile crystals, but there is an interference fit with R18. Just mount the crystal on it's side, as if it were bigger.

  15. That's basically it for the soldering.

    Insert the socketed chips, unless you left them in their sockets wile soldering. Be sure to get pin one pointing toward the edge connectors. The chips are a little static sensitive (it shortens the life of the chips), so don't work in an environment where static electricity can build up, and try to minimize handling.

    To get the chips in, place the chip on it's side on the board, and rock it slightly, bending all the pins at once until they point more or less straight down from the chip body. Place the chip in it's socket (pin 1 toward the edge connectors!), and make sure then thin part of each pin is started in the socket hole before applying pressure. When everything is looking good, press firmly on the chip body to press the pins into the grippers inside the holes. You should be able to tell when it is seated.

  16. Insert the battery from the left, writing side up, and press down until the plastic tab on the left clicks over the top of the battery.

  17. Lastly, you may wish to cover the exposed "+" battery wiring at the battery terminals and at the diodes with a couple of layers of non-conductive tape. This limits the likelyhood of shorting the battery when setting the board down. If you want to be completely paranoid about it, cover pin 28 of IC3 and IC4, too. (Sorry, no handles come with your kit.)

    You've done it!

Last updated on 02/25/23 02:21