How to easily use both real tapes and tape images by switching between the GoSDC patched and original BASIC OS ROMs.

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TonyMay
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How to easily use both real tapes and tape images by switching between the GoSDC patched and original BASIC OS ROMs.

Post by TonyMay » Sun Sep 13, 2020 9:51 pm

I have found that John Kortink's GoSDC card has enabled me to use my Acorn Electron as a fun toy, without being put off by the setup hassle of tape and floppy disks. Whilst browsing the list of games on the Acorn Electron World DVD I noticed Blockbusters and wanted to play. But only a tape image was available for this game. I remembered reading about GoSDC supporting tape images, and so a week later I had programmed an EPROM with the GoSDC patched OS ROM. I actually had a friend do the programming as I do not have a suitable programmer, but I did buy my own EPROMs from eBay. I opened up the Electron, removed the old ROM, inserted the new ROM, and was playing Blockbusters shortly after.

Sometime later I wanted to use the real tape interface, as I had some tapes with my own programs that I wanted to investigate. Unfortunately it is not possible to use the cassette interface with the patched GoSDC ROM installed and so I had to dismantle the Electron again and swap the ROMs over. I noticed that there were a couple of places a second ROM might fit inside the Electron. There was an unused space next to the OS ROM socket (IC18), and an unused space for a ROM on the Slogger Turbo board. I remembered that the Slogger Turbo-driver board does not have enough headroom for IC sockets so I focused on the Electron motherboard.

I have an issue 4 Electron motherboard that was designed with the option of fitting two 16K ROMs instead of a single 32K BASIC OS ROM. The circuit diagrams show that a chip (IC17) to handle the address decoding logic was also intended and so I came up with a design that allowed me to select either socket using a switch installed on the side of my Electron. This would allow me to easily switch between the original and GoSDC OS ROMs. A simpler design is possible if the chip control signals are routed through the switch, but I decided that since there was space for the logic, a safer design would involve the switch simply being an input to the NAND gates in the address decoder chip.

I have implemented and tested the design and it works as well as was hoped. The details of the modifications are described below. The instructions assume that a Slogger Turbo-driver board is installed and the design ensures that the Slogger Turbo-driver board operates when either of the ROMs is selected. If the Slogger Turbo-driver board was not fitted, then some additions or modifications to the instructions may have been required. The Electron motherboard was removed from the case before making the required modifications. The parts that were used are contained in the instructions.

1) Cut the track between LK8 pin 1 and IC17 pin 1. This disconnects IC17 from the Electron ULA ROM select pin, allowing IC17 to be repurposed.
2) Cut the track running up to IC17 pin 13. This disconnects IC17 from the Electron ULA A14 address pin.
3) Install IC sockets for IC17 and IC18.
4) Disconnect the Slogger Turbo-driver board red wire from LK8 pin 2 and connect it to IC17 pin 1 and IC17 pin 2. This is best done by ensuring there is enough wire at the end of red lead to bridge between the two pins and then solder to the exposed portion of the pins on the top side of the PCB. Since doing this, I realised that IC17 pin 1 and IC17 pin 2 should already be connected by the PCB. So you should only need to connect the red wire to either IC17 pin 1 or IC17 pin 2.
5) Add a 10K pull-up resistor from IC17 pin 12 to IC17 pin 14. IC17 pin 14 is the 5V supply to the chip.
6) Drill a hole in the case for a single pole single throw switch. The area to the left of the speaker should have room, assuming you purchase a suitably small switch. Cut two wires that are long enough to reach the chosen mounting point and connect one pole of the switch to IC17 pin 7 (i.e. 0 V) and connect the other pole to IC17 pin 10. This is most easily achieved on the reverse side of the PCB.
7) Cut the track between IC18 pin 27 and IC18 pin 28. This frees up the last address input on this chip so that the complete range of addresses required by the OS ROM can be selected.
8) Fit 47nF capacitors in spaces marked A next to IC17 and IC18. These are decoupling capacitors and so the exact value is not critical. 100nF (i.e. 0.1uF) will be fine.
9) Connect LK8 pin 2 to LK8 pin 3. When fitting the Slogger Turbo-driver board, some cuts should already have been made to tracks around LK8. Specifically, the track from LK8 pin 1 to LK8 pin 2 should already have been cut.
10) Connect IC2 pin 27 to IC18 pin 27. This provides the A14 address line to IC18.
11) Connect the black lead from the Slogger Turbo-driver board to LK8 pin 1. If it was not removed, then the black lead should already be connected to this point.
12) A 10K pull-up resitor should be added between IC18 pin 28 and IC18 pin 1. The VPP pin is left floating on the IC18 layout but this pin must be at 5V for correct read operation of the EPROM. For some reason I installed this pull-up resistor on the top side of the PCB (I may have just forgotten about it and did not want to remove the motherboard again).
13) Check the connections. Clean up the solder flux. Inspect the board for shorts and dull solder joints.
14) Reinstall the motherboard.
15) Install the switch.
16) Install IC17 (74LS00) and a suitably programmed EPROM into IC18.
17) Add a label near the switch to identify which position enables the second OS ROM (ROM 2). When the switch is closed, the original OS ROM will be used.

The original circuit diagram for this part of the Electron motherboard is shown in the following excerpt.
IC18-IC2-circuitDiagramOriginalClip.PNG
IC18-IC2-circuitDiagramOriginalClip.PNG (19.05 KiB) Viewed 162 times
My modifications are included in the following excerpt from the Electron circuit diagram.
IC18-IC2-circuitDiagram.PNG
IC18-IC2-circuitDiagram.PNG (23.89 KiB) Viewed 162 times
The plan view photo shows the additional switch on the top left, from which a pair of grey wires can just about be seen before they go under the motherboard. The original BASIC OS ROM cannot be seen from above as it is obscured by the Slogger Turbo-driver board.
PlanView.JPG
The side view shows the wire soldered to the IC socket for IC17 as well as proving that two ROM chips are fitted.
SideView.JPG
I hope the above helps someone save some time if they are trying to achieve similar functionality.

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