Building an Acorn System 1
Posted: Mon Jul 16, 2018 12:44 pm
Among the Acorn System range, I find the original System 1 holds appeal for its sheer simplicity (compared to it's successors): no operating system, no complex video or DRAM refresh circuitry, just a 6502, 1 KB or so of SRAM, some buttons and a calculator display (plus tape interface). Sadly, as they sell on eBay for well over £1000, I'm never going to buy an original.
Some time ago, while reading about the history of the Acorn System range, I came across this photograph of Sophie Wilson's original prototype ('Hawk') through the mos6502 Google Plus group:
That's pretty much the whole System 1 on one board: it's missing the tape interface, and doesn't have the pair of 2114 SRAM chips fitted (though the sockets are seen in red), just the 128kB provided by the INS8154 I/O chip (similar to the 6522). It probably has a EPROM chip (or a second INS8154) hiding behind the calculator display.
That photograph, in turn, reminded me of a disused circuit board I saw lying on a shelf at work:
Both are prototyping boards sold by Vero, using the Verowire (or Road Runner) system. Varnished copper wire dispensed from a plastic wiring pencil (essentially a hollow biro with a spool of wire on top) is wound along plastic guides and around component legs, where they are soldered together. The varnish burns off under the heat of the soldering iron, making an electrical connection. See this view of the board's underside:
Best of all, I'd also found a wiring pencil and spool of wire! At this point I started doing some serious plotting...
Most of the components of the System 1 were either already to hand or readily available. I had some 6502s and most of the logic ICs, and an old faulty Rockwell 8R of mine happens to use a display with the exact same pinout. I also had some spectra-strip and loads of DIN 41612 connectors. The major issue would be obtaining some INS8154s, as at least one other person has found. There were some places off in the far East that claimed to have some NOS parts lying about, but at a price.
You may have noted that my Verowire board is smaller than Sophie's. Mine is Eurocard-sized (160x100mm) with five rows for ICs, Sophie's is slightly taller with six. The original System 1 used two 160x100mm Eurocards. I wouldn't be able to fit a whole System 1 on my single Eurocard, but I did want to keep open the possibility of expanding my System 1 into a full System 2/3/4/5 at some point in the future.
One approach was to leave the cassette interface off (saving 7 ICs) and put it on another card if I wanted it. This is something that was done with the Acorn System: the System 2 used the System 1's front board for the cassette interface only, accessed through the euro-connector. Unfortunately, try as I might, I couldn't find a layout to fit the rest of the ICs (along with the keyboard and display). Fitting the keyboard buttons along my card's five rows (while keeping a sane layout) was particularly troublesome.
Dropping the 74S571 chips that hold the monitor program (which could be put on the EPROM instead) didn't give me enough space to make things work. Equally, I wanted to keep the 2716 EPROM in order to have somewhere to store finished programs that didn't require loading from cassette every time.
In the meantime, I started to clean up my old board. I wanted to reuse the plastic guide strips, and cutting/desoldering all the wire was a particularly fiddly job:
I also came up with a layout that would fit all but the keyboard and display on my Eurocard:
Components labelled in Yellow are from the 6502 board, those in green are from the Cassette/Display board. The empty space near the 6502 is for the clock crystal and associated components. There's likewise space around the cassette ICs for the various resistors and capacitors needed for that part or the circuit. The holes labelled in pink are for pin headers which a separate keyboard & display board will mount. The extra board will be smaller than the normal Eurocard, and probably made from stripboard. The address selection socket will be accessible with the keyboard on, but it will have to be removed to change the EPROM. If the board gets placed in a Eurocard rack, I can simply remove the keyboard. A notable omission is the LM340 voltage regulator: regulated 5V power supplies are ten a penny these days, so it was an early sacrifice in making this layout work.
I've made a slow start on wiring the card, and progressed far enough that I felt ready to write up all the above for you to read. I'll make another post later this week to show you the progress to date.
Some time ago, while reading about the history of the Acorn System range, I came across this photograph of Sophie Wilson's original prototype ('Hawk') through the mos6502 Google Plus group:
That's pretty much the whole System 1 on one board: it's missing the tape interface, and doesn't have the pair of 2114 SRAM chips fitted (though the sockets are seen in red), just the 128kB provided by the INS8154 I/O chip (similar to the 6522). It probably has a EPROM chip (or a second INS8154) hiding behind the calculator display.
That photograph, in turn, reminded me of a disused circuit board I saw lying on a shelf at work:
Both are prototyping boards sold by Vero, using the Verowire (or Road Runner) system. Varnished copper wire dispensed from a plastic wiring pencil (essentially a hollow biro with a spool of wire on top) is wound along plastic guides and around component legs, where they are soldered together. The varnish burns off under the heat of the soldering iron, making an electrical connection. See this view of the board's underside:
Best of all, I'd also found a wiring pencil and spool of wire! At this point I started doing some serious plotting...
Most of the components of the System 1 were either already to hand or readily available. I had some 6502s and most of the logic ICs, and an old faulty Rockwell 8R of mine happens to use a display with the exact same pinout. I also had some spectra-strip and loads of DIN 41612 connectors. The major issue would be obtaining some INS8154s, as at least one other person has found. There were some places off in the far East that claimed to have some NOS parts lying about, but at a price.
You may have noted that my Verowire board is smaller than Sophie's. Mine is Eurocard-sized (160x100mm) with five rows for ICs, Sophie's is slightly taller with six. The original System 1 used two 160x100mm Eurocards. I wouldn't be able to fit a whole System 1 on my single Eurocard, but I did want to keep open the possibility of expanding my System 1 into a full System 2/3/4/5 at some point in the future.
One approach was to leave the cassette interface off (saving 7 ICs) and put it on another card if I wanted it. This is something that was done with the Acorn System: the System 2 used the System 1's front board for the cassette interface only, accessed through the euro-connector. Unfortunately, try as I might, I couldn't find a layout to fit the rest of the ICs (along with the keyboard and display). Fitting the keyboard buttons along my card's five rows (while keeping a sane layout) was particularly troublesome.
Dropping the 74S571 chips that hold the monitor program (which could be put on the EPROM instead) didn't give me enough space to make things work. Equally, I wanted to keep the 2716 EPROM in order to have somewhere to store finished programs that didn't require loading from cassette every time.
In the meantime, I started to clean up my old board. I wanted to reuse the plastic guide strips, and cutting/desoldering all the wire was a particularly fiddly job:
I also came up with a layout that would fit all but the keyboard and display on my Eurocard:
Components labelled in Yellow are from the 6502 board, those in green are from the Cassette/Display board. The empty space near the 6502 is for the clock crystal and associated components. There's likewise space around the cassette ICs for the various resistors and capacitors needed for that part or the circuit. The holes labelled in pink are for pin headers which a separate keyboard & display board will mount. The extra board will be smaller than the normal Eurocard, and probably made from stripboard. The address selection socket will be accessible with the keyboard on, but it will have to be removed to change the EPROM. If the board gets placed in a Eurocard rack, I can simply remove the keyboard. A notable omission is the LM340 voltage regulator: regulated 5V power supplies are ten a penny these days, so it was an early sacrifice in making this layout work.
I've made a slow start on wiring the card, and progressed far enough that I felt ready to write up all the above for you to read. I'll make another post later this week to show you the progress to date.