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| ACU Romchanger (Amstrad Computer User) | Hardware Montages |
Phil Craven shows you how to have two computers available at the flick of a switch IAM often asked if it is possible to convert a CPC into a different model, complete with the new model's system rom. The answer is yes. To turn a 464 into a 6128, for instance, you will need an extra 64k of memory, a DDI-1 disc drive with interface and a 6128 chip.
The first two items in that list are easy. The memory expansion is sold by Ram Electronics (0252 850085), the DDI-1 by the User Club (091-610 ****). Yer buys 'em and yer plugs 'em in. The 6128 rom, on the other hand, presents a problem. Some machines were made with the system rom fitted in a socket on the main board, in which case it is a simple matter to pull it out and change it for the 6128 version. But most were made with the chip soldered to the board. To change these would need track cutting and soldering, or the original rom desoldered and replaced. Not a job for the squeamish. To overcome this problem, I have come up with the ACU Romchanger widget which will allow you to fit the 6128 rom to your 464 or 664, or vice versa, without ever having to open your computer. You'll be able to switch between the two systems at will, and yellow and green LED indicators will keep you informed as to which system is operative. How it works If switched-in on power up, the circuit will not allow the internal system rom to be enabled, electrically replacing it with the external system rom. It all fits neatly into a small box which pushes on to the 50-way edge connector, giving easy access to the switch and reset button. Reset button? Yup. While I was about it I decided to include a button, the pressing of which will give an early morning start (EMS) type reset It is useful for those occasions when the Ctrl/Shift/Esc method doesn't work. The reset button will do the job without the need to power down. The whole thing is rounded off with a through connector for adding the disc drive and other peripherals. The CPC provides us with a line on the 50-way expansion connector, called ROMDIS with which we can disable the internal system rom. With the switch SW1 in the down position - see the circuit diagram in Figure I - ROMDIS is sent to a logical high and the internal rom is completely disabled.
The external rom, being wired up exactly as the internal one, now becomes the operating system in use. With SW1 in the up position the opposite happens. The high disabling line is fed to the external rom and the internal system is allowed to operate. It's as simple as that. There is one small thing that we have to allow for, the fact that we are not the only ones who might want to disable the system rom. Other roms send ROMDIS high when they are active -we must ensure our external rom intercepts this signal and is disabled when it happens. This we Diodes D1 and D2 are there to prevent the high line from 5v getting out to other peripherals. D3 and D4 are green and yellow LEDs to indicate which system rom is in use. The arrowheads on the left of the circuit diagram represent the computer's expansion socket connections; those on the right represent the through connector's. All 50 expansion socket lines are automatically connected for peripheral use when the through connector is fitted, but only those which are used ir» the circuit are shown. The lines to the left of the rom with the numbers 8 and 15 next to them represent the eight data lines and the 15 lines of the address bus. This is the standard way of showing multiple lines in circuit diagrams. SW2 is a push button which resets the computer by grounding the RESET line. Construction Before we start let me point out that dotted lines in the wiring diagram are connections on the underside of the board, and solid lines are topside connections. The first job is to make the board fit inside the box by cutting away all four comers and sanding enough from the two edges opposite the numbers and letters to allow the board to rest comfortably on the two central protrusions inside the box. Next cut out the square hole in the board, as indicated in Figure II, so that SW2 can poke through it. Now all the components except SW2 can be fitted to the top, non-coppered side of the board. Figure II shows the precise locations for each. Do not do any wiring at this stage. First solder all legs of the 28-pin socket to the board. It should have an equivalent indicator to the half moon shape shown in Figure II, which should be positioned as indicated. If it has a centre cross member, then solder the resistor and capacitor, any way round, flat to the underside of the board. Otherwise they will fit nicely inside the socket on the top side. Use their long legs to connect them to the socket's pins, indicated by the dotted lines. Fit SW1, which will need its corner legs broken off by repeatedly bending them with pliers so that it will fit flush against the board. Figure II shows a large and a small shape inside each of the D3 and D4 LEDs - you can see these kind of shapes in the LEDs themselves. The larger area in each LED needs to be on the left, as drawn. Slowly bend each LED's legs at right angles using pliers, just below the fat part, so that they will locate in their correct holes and poke through the top of the box. Solder them so that they stand vertically and their fronts are in line with the front of SW1. When they are soldered in, bend the left leg of D3 and solder it to pin 22 on the rom socket. The other three legs can be cut or broken off. Diodes D1 and D2 must be soldered the correct way round. One end of each is marked with a heavy black or coloured band. The equivalent band is shown in Figure II. When soldered, the bottom ends can be bent over and joined together; the top ends can be broken or cut off.
The last two components to fit are the edge and through connectors. The edge connector is the part which connects to the computer. It is important to fit it so that its main body is 8mm to 9mm from the board else it will not connect properly to the computer. When all 50 legs are soldered they must be connected to the through connector, but first cut off leg 43 - ROMDIS - from the underside of the board. It must be soldered to the board, but not carried through. Finally the through connector can be soldered to the edge connector, as in Figure IV. Pre-bend the legs and allow plenty of solder to run between the connectors. I find that using a vice is a good method of neatly bending all the legs in one go. All that's left now is to doctor the box and wire everything up. You can see that the box needs a slot in the back for the through connector, a wide slot in the front for the edge connector, some slots or holes in the top for SW1 and the LEDs, and a hole in the back for the reset button, SW2. The slot in the back, about 8mm from the bottom of the outside and 6mm wide, is the first one to do. The method I used was to drill a 6mm hole about 11.5mm up, somewhere near the middle. From that I cut the slot with a junior hacksaw blade. The assembly will now fit into the box as far as the LEDs and SW1. Mark and cut out recesses for these, but only as deep as the parts themselves -about 5mm. You could even drill 5mm holes for the LEDs to make them a bit tidier. While we are working on the main box unit, convert the screw hole at the back, the one behind the square hole on the board, into a larger hole - about 7mm diameter - ready for SW2. Since the edge connector is as wide as the box, the box lid can be modified in one of four ways. The easiest is to cut the bottom off and leave it off. Next easiest is to cut it into two pieces, leaving a gap for the connector, and screw them both on. The third is to cut the top to fit and cut a recess out of the bottom so that the bottom part has a thin strip up each side of the edge connector. This was the method I chose. The last and best way is to keep it in one piece by cutting a wide slot in the same way as was done for the through connector. This is the only way that all four corner fixing screws will work efficiently, The top edge of the slot should be 34mm from the top of the box, and the bottom edge 10mm below that. To finish the box with some style I have included a small sheet of transfer lettering in the parts list for personal embellishments. There's just the wiring left to do now. All the connections are shown in Figures II and III. Allow a little slack in each wire so that it can be moved aside if needed, but not too much so that it won't fit in the box. Both wiring diagrams show the top, uncoppered side of the board. Dotted lines mean that the wire is underneath, solid lines mean that it is on top. To avoid confusing diagrams I hav' shown some connections by arrows (which Ù Vt necessarily point in the correct direction) and some letters inside circles. Identical letters connect together - a connects to a, b connects to b, and so on.
Pin 43 in Figure II - ROMDIS - has two connecting points, the edge or computer connector, soldered to the board, and the through connector. They are marked C and T for Computer and Through. The T connection must be soldered straight on to the through connector. Don't mix them up or it won't work. Many of the wires pass under the edge connector and on top of the board; these will be obvious as you come to them. SW2 can be fitted to the box at this point, but allow several inches of wire so that the board can be taken out without needing to unscrew the switch. When that's all done, gently press the wire against the back of the board, fit the assembly into the box and screw the whole thing together. Don't put a rom in just yet. With the power off, plug the unit into the computer's expansion port, Now power down, set the switch to the left (green) and power up again. The computer should hang up. We have disabled the internal rom and replaced it with nothing. Now you can power down, fit the 464 or 6128 rom, making sure that the half moon indicator is positioned as shown in Figure II, and test everything again. This time when the green light is on you will see a different Basic version number on the screen and you have your alternative system in operation. If things don't work as described, make the following checks. Are the diodes and LEDs the right way round? Is the rom the right way round? Check that all the connections are made and to the right places. Go over the diagrams with a coloured pen when each connection is checked to make sure you've done them all. Are all the components in their correct positions as shown in the diagrams? Check that no shorts exist between close joints. If all these check out and it still doesn't work, ask a friend to check it out for you.
The unit must be the first peripheral to be attached to the 50-way edge connector - labelled Floppy Disc on the 464 and Expansion on the 664/6128 - so that the external rom can intercept all ROMDIS operations as described earlier. Because the board is not fixed solidly inside the box, use the through connector to push it on to and pull it from the computer, Avoid squeezing the wiring when inserting a rom. The unit has been used successfully on both a 464 with a 6128 rom in it, and a 6128 with a 464 rom in it, but I should point out that because 6128s and 664s have their disc roms fitted internally, they produce the ROMDIS signal internally. This means that the external rom cannot intercept the signal and cannot be disabled for the disc rom to be initialised. The 464 disc is external and will work perfectly. The two system roms contain different programs, so switching between them while the computer is switched on will put you into a different program, causing a crash. Either power down before switching or use the reset button after switching. For those who haven't yet phoned, written or read last month's letters pages, the six ceramic capacitors on the December issue DIY rom board - which are in the circuit to absorb any voltage spikes-should be connected between 5v (pin 28) and ground (pin 14) of the six rom sockets. They can be connected any way round, one capacitor to each socket. Many thanks to everyone who has phoned or written to say, "The romboard works!" We know it works, but it helps us to know that you've successfully completed the project. Good luck with the ACU Romchanger. What shall we build next? Hmmm?
ACU
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