Mike's PCjr Page

Use this circuit instead. Same as one above but changed to 7404 (non-open collector). Cheers.

https://www.dropbox.com/s/tw4t20aizm59f ... 4.png?dl=0

How about using a NAND instead of an OR? That way you can remove the inverter.

Yeah, it's a pretty simple circuit and I'm sure it could be done in a better way, or at least a lot of ways. I just had those parts by chance in the bin. The bi-direction parallel port mod uses gates already on the card if I had schematics It would be interesting to see if there are any spare gates and figure out if they could be used.

FYI, all the gates you need are probably on the board already.

I looked for spare gates on the schematic in the PCjr technical reference and only saw one NOR gate available. However, when looking at a board I have on hand as well as photos of other parallel port boards, I see a 74LS00 quad NAND chip that isn’t on the schematic. I suspect that most parallel port attachments out there are of a revised version that includes this chip. I’ve circled the chip in this photo:

The board I have only seems to be using one of the four gates on the chip, leaving three available. I’d suggest using two, one for the logic and one as a buffer. LS TTL is only rated for sourcing 0.4 mA but can sink about 8 mA, so it’s better to sink the LED current. Here’s a schematic where I’ve referred to the 74LS00 as “ZM12”, which is probably its reference on IBM’s revised schematic, wherever it is.

Pretty sweet there! Now I guess I have to find another one on eBay and try that . Let us
know if you go there. It's actually a very cool mod and I'm glad I did it.