Mike's PCjr Page

Alright, I've decided for the initial testing, I'd rather do my own PCB. I bought a cheap laminator at Wal-Mart to assist with the toner transfer method of etching. Hopefully, it'll work. I've bumped the trace thickness and clearance up to 15 mils so I can get a clearer print.

Two obstacles remaining:

1. It's a two-sided design. I've read some decent methods for aligning the sides on the internet, though, so hopefully it won't be too much of a problem.

2: There are lots of holes to drill. There's no easy way around this for me, as I don't have a drill press or anything, and I'll have to do it by hand. I do have small enough drill bits already, though.

Anyway, I guess I'll be giving it a shot this weekend. We'll see how it goes!

Good luck. You'll wish you had a drill press - trust me. There's always craigslist....

As far as the toner transfer method, I haven't done exactly that. I have done the photographic method, and even with that, it helps to have an etchant resist marker to fill in any traces that aren't fully colored as they should be.

No drill presses in my area. Just an all in one workstation with attached drill press that the dude wants $400. Not happening.

OK, I'm a little steamed right now. I just went down to Radio Shack to pick up the copper clad board I was planning on using. They stopped carrying it last week! Along with most of the perfboard, too.

I think it's safe to say Radioshack has permanently lost me as a customer today. I really hate the direction their stores are headed.

I read through this thread and I am not quite sure what it is you are building here. Is it a PCB that will replace the CPU and add in the extra 640kb of RAM, or are you building a small daughter board that will be wired into the memory sockets on the board?

I'd be very interested in buying your finished product either way.

It's a daughter board that will go in the CPU socket - the CPU, in turn, will seat on top of the daughter board.

The thing at the beginning of the thread is a little too complicated to install and a little too unstable to be a finished product, even though it worked as a proof of concept. The main thing that's holding me back right now is that I don't have any money (and I need to be sure my idea will work before I take orders or anything).

Would your design accommodate 640kb and allow us to get the full 768k of RAM into the thing?

Lutiana wrote:Would your design accommodate 640kb and allow us to get the full 768k of RAM into the thing?

Yeah, it shouldn't be too difficult to get that in. Although it's 736k the PCjr supports, not 768k. There will be 640k SRAM on the board (I'm routing a version with that right now), but only 608k will actually be usable.

EDIT: Oh, and an update on design - I've added 0.1uF decoupling capacitors from VCC to GND for all ICs on the board except the 8088 now, and I've increased the width of the VCC and GND traces to 30 mil. If I'm going to be making this thing for other people, I want to make sure it's a robust design.

jmetal88 wrote:Although it's 736k the PCjr supports, not 768k. There will be 640k SRAM on the board (I'm routing a version with that right now), but only 608k will actually be usable.

O right, sorry I knew that.

Would a V20 work on this design? I am guessing it would. and how far are you from getting the circuit boards fabricated?

I'll check with the guys over at VC forum to see if any of them would be interested in this, and perhaps we can come up with some scratch to get some PCBs put together. Also, do you have a BOM?

Yeah, I have a V20 in my machine, so it should work with one.

I actually had a prototype of the plug-in design hand-wired that ended up not working. I think it should have, so I probably had a bad IC or some bad soldering or something, but I'm still kind of nervous about fabricating boards since I'm not 100% sure of my design because of this.

As for a BOM, my PCB program can generate one pretty easily, but I'll want to go through it and pick specific part numbers (preferably all from Jameco, as they have good SRAM chips at good prices).