32KW 12 Bit RAM Board with Lamp Register Build
Posted: Fri Feb 24, 2017 2:15 pm
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:
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:
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:
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.
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:
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:
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: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.
