Poly88 replica, entry one (PCB)

Since my post several years ago getting a pile of scrap to run as a functional computer I have gotten several requests for people who want to buy my Poly88 machines. I’ve resisted these offers because I know that if I let them go I will probably never be able to get another. Since I have these machines, I’ve decided to do my best to not just document them but to recreate them in CAD so anyone who wants one can just make it. They’re not that big and with manufacturing these days being so easy I see no reason to pine over an old sheet metal box, just make a new one. For this project I have also been sponsored by PCBway so all the parts will be available for purchase through their website as I design them. Part one here will cover the first part of the journey: the circuit boards.

I have two machines and naturally that means I have two different revision motherboards. They’re not very different, but I did bother to do both variants in CAD so you can pick your variant. The PCBWay project has the rev 2.1 board uploaded because that’s the one I ordered, but my github has both. You can find the schematics either on my github or in the manuals, but the only really noteworthy thing is that there’s an optional section for generating a signal on pin 55 for mains frequency based clocks. One of my original circuit boards had that section populated, the other did not, I believe that had to do with these chassis being able to connect for extra capacity. I’m going to detail some of the quirks I found while designing the board.

That red jumper wire had to be added to the rev e board because there was a trace missing for connecting one of the 8v legs from the transformer. That is mostly why I had the rev 2.1 boards manufactured, because they seem to have fixes in place.

This is the above section captured by me in KiCAD (shown from the top view, so mirrored)

This is the same section on the rev 2.1 PCB. You can see the connection is made, but there’s also some alternate connector for the 8vac pins. This is odd to me because you can’t just use those three pins since you need the +/- 16v for the other two rails on the s100 connector. I may have a hint for that however.

Notice the diode I labeled D8 and marked as Do Not Populate. The truth is you can populate it and I don’t think it will do anything harmful because this is how I have it on the schematic:

This diode sends voltage from the 8v rail to the 16v rail. That means that in a fully built system no current will flow as 16v is greater than 8v. If you choose to build the system with only an 8v rail however, this puts that on the 16v rail as well. There may be some use cases where you have an s100 system and you don’t need the full 16v on that rail and don’t need the negative 16v rail at all. I think that’s what the extra unpopulated 3 pin connector is for and this diode. I have not used them in my build but I left them in because I kinda like the reconfigurability of this board.

I have a note up there about the RTC circuit only being on one backplane as mentioned above, but I also note that 1, 2, 51, and 52 do not connect between backplanes. That is implemented on the male side of the connector. That means that the backplane, when acting as an s100 card, passes the signals back and forth but not the power rails. The ground is shared however. This means that you can expand these chassis up to however many slots you want, but each chassis’ cards are powered by the transformer in that chassis. I also said it’s implemented on the male side. Let’s take a look at that.

You can clearly see the pins making no connection with the rest of the board. Why is it important that it’s on the male side? because this means you can plug in an s100 card outside of the chassis on the female side and have a total of 6 cards connected to a single poly88 chassis at once. Why is this important? well, I may want to use the chassis with the lids on with an exidy sorcerer s100 chassis adapter card. Eventually. I have so many projects…

In an earlier picture you saw brown and white twisted wires going off the board, I made a note that this is used for the fan upgrade to the poly88 (not all of them had this feature, it was extra). Those wires go off to a 24vdc fan. That fan has three wires and a 5.6uF 250v cap on it so there’s something they’ve done to make it happy running off the 32v from the linear power supply. I may get into that more later as I replicate more of these computers.

That’s really all there is to these circuit boards, Your biggest cost will be the s100 connectors and if you want them to be exactly right your biggest headache may be the 0.093″ 5 circuit board mount polarized connector pictured above for the power input. On the PCBWay project I have linked to a set of wire to wire connectors that you can use with the board side being a pigtail instead of PCB mount, but you are of course free to use whatever pigtail you like. In the future I will probably be showing alternatives (I’ll certainly have alternatives to the power supply transformer, which probably means you can skip all the big caps and diodes too as my supplies will be regulated DC.

Part two involving the lids will be linked here.