Tynemouth Software Unveils the Four-Chip, Bit-Bashed Software Serial Card for the Minstrel 4th

Tynemouth Software's Dave Curran has designed a new serial board for the Minstrel 4th and RC2014-compatible microcomputers, which manages to achieve high-speed serial communication in just four main logic chips — with no hardware serial controller or microcontroller in sight.

"It may look like there is something missing," Curran writes of the new board's design. "Let's just check. No UART. No ACIA. No DART. No SIO. No microcontroller. No Raspberry Pi. So how is it doing it? The essential part is I wanted a card that used all new parts, but there was no serial chip in production that I thought was suitable. A dangerous thought occurred to me. Can I bit-bash RS232? The answer is yes, of course I can, that was actually one of the first projects I did with the early microcontrollers back in the 1990s (see a previous post), although that was a [Microchip] PIC with something like 4MHz clock and the serial was probably 9600 baud or something like that."

The card is built primarily with the Tynemouth Minstrel 4th in mind, a Zilog Z80-based single-board computer designed to be compatible with the ill-fated Jupiter Ace — though featuring 49 times as much memory and more than six times the ROM space. Its use of the RC2014 bus, however, means compatibility with the wider ecosystem of RC2014-based microcomputers out there — though, as a software-based serial card, it requires supporting software on the host to run, which is still a work-in-progress for anything other than the Minstrel 4th.

The board's main components are four small 74-series logic chips, in place of the usual hardware serial controller or modern microcontroller. "I decided I would prefer to go for the simplicity of plain logic gates," Curran explains. "I went for address $FF, since I could do that with just two ICs, a 20 and a 32. I decided not to give the option of changing the address, since that would need different versions of the ROM. [For input] I went for a 74HC125, this is four way, but they are individually controlled, and I have another use for the two spare gates. [For output] the 175, this is a four way latch with reset and Q and /Q outputs. It's not a widely used chip, I had to hunt around and the only one I found was from 1980!"

The finished board allows for software-driven "bit-bashed" serial transmission and reception at 57,600 and 115,200 baud, support for protocols beyond RS232 eight-bits no-parity one-stop-bit "if you want to write the code," the ability to echo the system's screen over serial, directly input text over serial, and save and load TAP tape images over serial. "I would like to get this working with Grant Searle's modified Microsoft BASIC ROM, so it could be used on standard RC2014 systems," Curran says of its compatibility outside the Minstrel 4th. "That uses the interrupts, so need more than a trivial change to make it work."

A detailed walkthrough of the board's development is available on the Tynemouth Software blog, while kits and assembled boards are available to order via Tindie starting at $25 plus $10 if you want an updated ROM chip for your Minstrel 4th.