32KW 12 Bit RAM Board with Lamp Register Build

Post Reply
User avatar
glitch
Posts: 176
Joined: Mon Nov 28, 2011 12:43 am

32KW 12 Bit RAM Board with Lamp Register Build

Post by glitch »

Getting close to being able to test the porthole and IM-6100 capabilities of the 560Z board, so I needed as 12-bit RAM board. I've got a couple of blank OSI 420 boards, but I didn't want to have to build one up, and then deal with the reliability and power hungriness of 2102 SRAMs. I've already built a 32K 8-bit RAM board, and then expanded it to 64K, but instead of modifying that board, I went ahead and built another on a reproduction OSI 495 board. I wanted to switch to 74LS240 inverting buffers instead of 8T26 buffers since there will be a production PC board/kit and I want to avoid using hard-to-find/expensive components when possible!

I'll do a proper writeup on the board at some point, and post it to my site, but for now, here's a picture of the front:

Image

The board is installed in my Challenger 3, which is in the extra-deep C2-OEM chassis that has room for 8" floppies. They're not currently installed, the picture was taken through the opening for the disks. Here's the lamp register:

Image

Twelve old red LEDs, some 330R resistors, and three 74LS175 quad D-type latches. The 74LS175s have inverting and non-inverting outputs. I used the non-inverting outputs, but you could use the inverting ones for IMSAI style "programmed I/O" register. The lamp register is addressable on any byte location in 64K address space, using two 74LS688 comparators and two 8-position DIP switches. I have had enough of wiring point-to-point address busses for a while :) Speaking of wiring, here's the back of the thing:

Image

Everything is point-to-point, the color code is the same as the 32/64K board:

* Red is OSI data bus (upper left) and DIP switch interconnects
* Yellow is buffered internal data bus
* Blue is OSI address bus
* Green is control wiring (PHI2, VMA, chip selects, et c.)
* Purple is output latch wiring to lamps

Only one real issue with the board: I forgot to pull up the clear lines on the flip-flops! You'll get random behavior from them if you just leave it floating.
Check out The Glitch Works
OSI Challenger 3, 510 CPU, 8" floppies, 23 MB hard disk system starting to work!
Parts bin Challenger 3 board set, never had a chassis in its time
User avatar
glitch
Posts: 176
Joined: Mon Nov 28, 2011 12:43 am

Re: 32KW 12 Bit RAM Board with Lamp Register Build

Post by glitch »

Got the porthole working on the 560Z, so I was able to test the 12-bit features of this board. Memory and lamp register both work fine! Here's a shot of 1111 octal loaded into the lamp register:

Image
Check out The Glitch Works
OSI Challenger 3, 510 CPU, 8" floppies, 23 MB hard disk system starting to work!
Parts bin Challenger 3 board set, never had a chassis in its time
Post Reply