Mike's PCjr Page

In modern times there really isn't much use for the Jr Parallel Port Side Car other than for a Zip Drive. After performing the Jr bidirectional mod I was thinking while I had the board out I might try my hand at a small circuit to add an LED to the sidecar to show activity much as I have on my jrIDE.

My choice was to pick off the STROBE and BUSY signals on the parallel port. I hooked up the oscilloscope and it looked reasonable. Busy is used by the peripheral and STROBE is used by the host. Both are active low signals and to really show all the activity easily the signals needed to be OR'd together. I threw together a breadboard with a 74LS32 and a 74LS05 a few resistors and an LED as I had those in the parts bin.

Here is the first test, I'm pretty happy with the results.

https://youtu.be/omlV6ZCJmf0

Obviously, I'm NOT a YouTube star

If anyone is interested I'll refine and post the design.

Looks perfect! Curious how you'll get the PAL/GAL/whatever that stuff is on the board into the sidecar though.

It's a very simple TTL Logic design. Just two chips, two resistors, and an LED. A good digital engineer could probably get it down to one off the shelf IC instead of two. But even with the current design, it's like $2 to make one. The inputs come from the LPT connector (you can see it in the video with the little clamps on it. Other than that it just needs power and ground. I'll post a design. The parallel card has tons of room on it for something to hang or stick to it. Someone with schematics (Chuckphd53) might even be able to find enough unused gates already on the board to do it. Either way, pretty cool and easy to make

I'm a big fan of the parallel port, especially on the PCjr. Besides adding Zip drives it can also be used for the Xircom PE3 to add Ethernet, various SCSI adapters, CD-ROMs, external hard drives, floppy drives, and on occasion even a printer!

I wouldn't mind doing that to my parallel ports. For Ethernet it would be a good activity monitor.

Yes, I'm strangely interested in the network adaptor. If I understand correctly from reading your content you have a TCP/IP stack for the Jr. I assume its a TSR utility but, to be honest, I can't quite remember. Might give that a try! Just ordered three serial port proto cards off eBay. Not sure why, but it seems reasonably hackable and might be kinda fun? I've spent the last year or so in the C64 world but enjoying the challenge and opportunity of the Jr world. A little more capable

The Xircom PE3 series of adapters gives you a real Ethernet adapter through the parallel port. They are not as fast as an ISA card would be because of the bottleneck created by the parallel port, but they are far faster than the serial port or the floppy drive data transfer rate. (A Xircom on a bi-directional parallel port is good for 40KB/sec data transfers in both directions.)

The networking code is not a TSR - they are just programs that include all of the TCP/IP that they need. Check http://www.brutman.com/mTCP for details. I explicitly designed all of the code to run well on a PCjr, knowing that if it works there it will work on anything. It's also a hoot to log onto an IRC server with a 36 year old DOS machine. Other programs include a DHCP client, FTP client, FTP server, HTGet, HTTP server, ping, telnet, netcat, SNTP, etc.


Mike

I spent some more time on this project, this morning and settled on this circuit.
It works really well in both directions reading and writing data to the ZipDrive.

https://www.dropbox.com/s/82znn4pm6cicr ... D.png?dl=0

I'll probably build a little board and try it out.

Ok, so it's just a matter of inverting the two signals, and OR'ing those together. The TTL that drives the LED provides enough current to do so? Also, isn't the resistor dependent upon the specific type of LED being used?

Have you considered a variant where you don't OR the inputs together and use two different LEDs (or a two color LED) to be able to tell between sending and receiving?

TTL chips can easily drive an LED. I use 3.3V led's as they are a little brighter. In practice, I find you can get away with a lot of different LED resistor values. I can't remember how much current the circuit was pulling but it was minimal, maybe 20ma or so. You could easily do a few LED's but in practice, I'm not completely sure the two signals exactly always map to reads and writes. I just verified the combination seems to match the ZipDrive light exactly. The big contribution of the ACK signal was when writing to the zip drive, the buffer would fill on it and it would more slowly write the buffer to the physical disk. The zip drive would hold the ACK low until it flushed its buffer then raise it back to its normal state and that kept the LED on until the final write was done. Going to take a closer look at the parallel port board to see if the big empty part of the board has a ground plane printed on it or not. If not I might be able to just drill holes for the pins and mount sockets directly to the board. If there is a ground plane will probably just glue sockets to the board itself or maybe drill through the board and mount a little board with nylon screws. Fun little project!

Finished the parallel port sidecar LED. Made a short video to show it in action.

https://www.dropbox.com/s/1o9te1zh3apx0 ... 9.MOV?dl=0

I ended up going with the plan of drilling holes in the forward part of the parallel port card and mounting two sockets for the chips. Then wiring all the connections. I think if I did it again I would go the daughterboard route, but it did work out. Front of the board is pretty clean but the back with the wires is a little messy for my liking. One byproduct of this mod is the blue parallel port drive light comes up in a random state at initial power on. Once Palmzip loads it's all normal and even reboots seem to keep the state correct. I'm sure I could fix that with some reset circuit but I'm pretty happy with the mod as is. I keep the zip drive out of site and it all seems pretty period correct (which I like).

Board photos here:

Front:
https://www.dropbox.com/s/yz8anb3ldruxn ... 6.JPG?dl=0

Back:
https://www.dropbox.com/s/inqufyfzsmvzx ... 5.JPG?dl=0

I'm making on mod the circuit. I initially used a 7505 (open collector inverter). That works and a simpler circuit could be done using only that chip and switching to wired OR layout and a pullup resistor. But, I already had all the components on there and noticed the output of the inverter was only like 2V due to no pullup resistor. I saw that I had a 7404 in my parts bin that is the same chip without the open collector so I changed it out (sockets!) and found the logic level was a normal 5V as was intended.

Again, the takeaway is you can do this mod with one chip and the wiring would be WAY simpler. I'm not going to post that version as I haven't personally tested it but contact me if you like and I'll tell you how to breadboard and test it!