* * * * * * * * * * * * * * * * * * * * * * C H I P < > F A S T * * * * R A M S W I T C H * * * * * * F I T T I N G * * * * I N S T R U C T I O N S * * * * * * * * * * * * * * * * * * * * * * Author: ------- Paul Lathwell Technical Adviser: --------- Mike Hilton NOTE: ----- Please read this document carefully before you undertake any modification to your computer. As you will have your computer in bits you will obviously need to print this document out, and make a sketch or print out the "MemSwitch" file. If you don't have access to a printer please read the end of this file. INTRODUCTION ------------ The following file explains how to fit a switch to your Amiga A500 to enable you to choose whether your 0.5 Mb trapdoor expansion is used as Chip RAM or Fast RAM. Why? Well: The CPU (Central Processing Unit), tells everything else what to do. The CPU is able to make use of both Chip RAM and Fast RAM. The Custom chips (used to store graphics and sound), are only able to use Chip RAM. If a program is stored in Chip RAM, it may be interrupted by the Custom chips trying to use the same memory, causing the program to slow down. Therefore if we allow the CPU to have its own memory, it should be able to run programs faster, hence Fast RAM. So why have a switch? As Custom Chips can only use Chip RAM, then obviously the more Chip RAM you can provide, the better they are able to manipulate it. This makes using the following types of software more powerful: e.g. Paint packages, Scanner software, sampling software etc... You could of course just modify the circuit board and have 1Mb of Chip RAM, but you may find some software, (usually older stuff) will not work as it requires some Fast RAM. IMPORTANT! ---------- If your warranty has yet to expire, I advise you NOT to attempt the following modification to you Amiga. This project requires you to remove the casing from your computer and modify the mother board... DISCLAIMER ---------- THE FOLLOWING INFORMATION HAS BEEN BROUGHT TOGETHER FROM VARIOUS SOURCES AND TO MY KNOWLEDGE IS CORRECT. HOWEVER, DUE TO THE NATURE OF THIS PROJECT I CANNOT AND WILL NOT ACCEPT ANY RESPONSIBILITY FOR ANY DAMAGE, OF ANY KIND , CAUSED BY INCORRECT IMFORMATION. IF YOU UNDERTAKE THE FOLLOWING MODIFICATION THEN YOU AND YOU ALONE MUST ACCEPT RESPONSIBILITY... NOTE: ---- I have made this modification to my own computer and suffered no problems. I have the following machine: A500 1.3 ROM, 6A Revision circuit board. REQUIREMENTS ------------ 1. Your Agnus Chip must be the 8372A Fatter Agnus. 2. Your expansion needs to be fast enough to keep up with the internal Chip memory. To find this out look at the last two digits printed on your expansion chips, they should be as low as possible, 08 or 10 will be fine (I have 12 which was also O.K.). If the last two digits are higher than 15, then the chips are too slow. (STOP HERE, or get a faster expansion!!!) 3. A dual position dual throw (DPDT) switch. 4. Some thin wire (thinner than speaker wire! Have you seen where they're going!!! 5. A soldering iron with a small soldering bit. 6. Thin gauge solder. (some desoldering braid is also recommended for removal of excess solder, & mistakes.) 7. Stanley, or modeling knife. (be careful now!). 8. Magnify glass. 9. Philips screw-driver. 10. An extremely steady hand. 11. Preferably someone to lend a hand. READ THIS!!! Soldering techiques: -------------------- Soldering is not particularly difficult, but can be perfected with a little practice. One of the main mistakes when soldering is to use far to much solder. The object is to make a joint that will: a: conduct b: stay attached c: not short-out with other components/tracks etc. To prevent damage to components and circuit boards try to avoid using the soldering iron more than necessary. ie use enough heat to melt the solder and make a connection. NOT enough to burn bloody great holes in your computer!!! If you do make a mistake (probably), re-heat the connection and remove the wire. To remove solder left by this action use desoldering braid: Place the braid on the solder, then heat the braid until excess solder is wicked away. Tips: ----- Tin wires prior to soldering them. This will reduce the amount of solder needed when making a connection, therefore speeding the whole process up, and allowing heat to be kept to a minimum. Tinning is achieved by heating the wire and allowing a liberal amount of solder to flow over it. A good soldered joint should appear to be bright and shiny, if it isn't it may not conduct properly and cause problems. Clean the soldering bit between joints by wiping it with a wet sponge then apply a little solder before making another joint. Precautions ----------- Disconnect the mains supply to your computer! (You don't want to die, do you?) Static electricity can destroy chips. (not the type that come with fish!) To avoid a build up of static charge, periodically ground yourself. This can be achieved touching bare metal on either a radiator, or an electrical appliance which is plugged in. (do not stick your finger in the socket!) FITTING THE SWITCH: ------------------ Removing the casing ------------------- Here we go... Firstly disconnect everything from your computer. Mouse, external disk drive, etc... OK ? Turn your computer, keyboard down and put it on something soft. NOT YOUR HEAD! (Only joking!!!). Locate the six fixing screws, three at the front, three at the back then remove them. (They may be hidden under the warranty seals). Turn the machine back over and remove the top cover. You should now be down to the metal shield, this provides protection from radio interference. With me? Before you can remove the shield you will have to remove Mr keyboard. Note the colours on the wires and their orientation to the circuit board, then carefully pull the ribbon cable from the circuit board. Now you need to remove the shield. Undo the four fixing screws, bend the metal tabs straight with some pliers and pull off the shield. Easy eh? Before you go any further it would be a good idea to familiarize yourself with the layout of things... Firstly locate JP2, this consists of three solder pads and can be found near the ROM chip. (It should be marked JP2.) Secondly find JP7A, this is located near the trapdoor expansion. Again it is made up of three solder pads. JP7A is not marked on revision 5 machines, if you are not sure you've located the correct pad do NOT continue with this modification! Here we go then... 1: Using the magnifying glass and knife carefully cut the trace between the middle and outer pad on both JP2 and JP7A. Using the magnifying glass ensure that the traces are properly cut. This is important if you want this conversion to work. If you are only converting your machine to use the expansion as Chip memory, you'll need to solder the other outer pad of JP2 to the inner pad of JP2. 2: Choose a suitable place to put the switch, making sure it will not obstruct anything else. If you don't want to start drilling holes in the casing, putting the switch inside the trap door will be quite acceptable. Once a suitable place has been located, cut three strips of wire long enough to reach from this point to JP2, and two more to reach JP7A. Now it gets tricky... 3: Tin each piece of wire as explained earlier. Take one length of wire and solder it to the first pad on JP2... Check that you've soldered the wire to the pad only, not anything else. ie next pad. Repeat this procedure for the other two pads, again checking each afterwards. If you do solder more than the pad to wire, use desoldering brade to remove the solder then try again! 4: Solder one length of wire to the middle pad of JP7A. Check. Solder the final piece of wire to the pad that used to be connected to the middle pad of JP7A via the trace. Did you get that ? 5: Consulting MemSwitch diagram, wire the switch: A: Middle pad wire of JP2, to one of the middle contacts on the switch. Check. B: Connect one of the outer wires of JP2, to one of the contact legs, next to middle wire of JP2, on the switch. Check the Join. C: Connect remaining JP2 wire, to the third contact leg on the JP2 side of switch... Check. D: Middle pad wire of JP7A to the other middle contact on the switch. Check. E: The other wire on JP7A needs to be connected, opposite the wire of JP2, that used to be connected to the middle pad of JP2 by the trace. Check. If you understood that congratulations!!! 6: Fit the switch in required position. 7: Reassemble computer in reverse order of taking it apart. 8: Use the 'Byte-Check' program (supplied) to check operation of the switch. Using the switch ---------------- BEFORE operating the switch, please ensure that your computer is switched off at the power supply. Hopefully you will have been successful making this modification. If however you do have a problem, the best course of action would be to check all connections and ensure that the appropriate traces have been cut correctly. Please remember I cannot accept any responsiblity for any damage!!! Anyone who doesn't have access to a printer can obtain a printed copy of this file, and the 'MemSwitch' file directly from me... Please send a cheque payable to 'Paul Lathwell' for £3-50 to cover printing and postage to: Paul Lathwell 3 Sweetbriar Close Gadebridge Hemel Hempstead Herts HP1 3PA. * BYE *