And it all seems too much like living in the past, rather than looking forward. But anything built from scratch now can give only mediocre results. Very few people built computers before microprocessors, but but they got something not all that inferior to commercial computers. On the other, there’s nostalgia for things not experienced. On one hand, people seem isolated from the past, so nothing counts unless it’s on the internet. But why use diode matrixes once there were microcomputers, which made it simpler but also you could add features? It’s much easier reprogramming than rewiring a diode matrix to send canned messages. If you want to build something retro, the old magazine articles are there. I do recall seeing a Morse keyboard that used an old typewriter for the keyboard. Very few hobbyists made mechanical keyers. As I recall, they got permission for using “hi” as the callsign, that’s not going to happen today. And it was bound to use germanium transistors, a bit harder to find at this point. So it was simpke, but also not likely to be useful for other characters. Oscar 1’s keyer was really simple, send 6 dits, with a space after four. Posted in computer hacks Tagged ALU, Bit serial, mosfet, PDP-8, Q2, raspberry pi, rust Post navigation We’ve featured many similar projects over the years here’s a nice one, a really small 4-bit one, and a really big one. Game of Life and even Pong, add some really lovely touches. In terms of the project write-up, there is plenty to see, with a Verilog model available, a custom programming language calls Q2L, complete with a compiler and assembler (written in Rust!) even an online Q2 simulator! Lots of cool demos, like snake. Discrete transistor D-type flip flop with indicator. A 40-pin header was added, for programming via a Raspberry Pi in case the front panel programming switches are proving a bit tedious and error prone. An LCD screen on board is a nice touch, augmenting the ‘front panel’ switches used for program entry and user input. Memory is courtesy of two battery-backed 6264 SRAMs, with the four 12-bit general purpose registers built from discrete transistors. Supply current was kept to below 500 mA, allowing the board to be powered from a USB connector. This was done in order to keep the propagation delay within bounds for the cycle time without excessive power usage. By looking at the logic fanout, spotted areas with large fanouts, and reduced the pull-up resistors from 10 K to 1 K. This is not a fast way to build a circuit, but it is very compact.
The machine is constructed from 1094 transistors, with logic in an NMOS configuration, using 10 K pullup resistors. For this machine, the instruction cycle time is either 8 or 32 clocks anyway, and at a maximum speed of 80 kHz it’s not exactly fast (and significantly slower than a PDP-8) but it is very small. a single ALU operation will take 12 clock cycles. Like the PDP-8, this machine uses a bit-serial ALU, which allows the circuit to be much smaller than the more usual ALU structure, at the expense of needing a clock cycle per bit per operation, i.e. Also, the DEC has core memory, but the Q2 resorts to a pair of SRAM ICs, simply because who wants to make repetitive memory structures with discrete 2N7002 transistors anyway? SMT components for easy machine placement Like the DEC PDP-8, this is a 12-bit machine, but instead of the diode-transistor logic of the DEC, the substantially smaller Q2 uses a simple NMOS approach. Has an interest in computers-of-old, and some past experience of building computers on perfboard from discrete transistors, so this next project, Q2, is a complete implementation of a PDP8-like microcomputer on a single PCB.