8-Bit Software Online Conversion

Putting two 16K ROMimages in one 32K EPROM By Hugh Williams (709) C83 asked in issue 21 if it was possible to use 32K EPROMs in the BBC to double up the number of sideways ROM images available. The answer is yes BUT a small hardware modification has to be made to the 32K EPROM to make this possible. In practice this means that you can have 32K EPROMs in your machine but you need switches on the outside of your machine to switch between each of the two 16K images. I have several arrangements of this type in my machine and they all work perfectly. This small project does require soldering and some basic electronic skills/knowledge. Neither I, nor 8-bit software will accept responsablity for any damage that may be caused to your machine as a result of following the instructions below. The first step is to chose which two images you wish to have residing in the 32K EPROM. Bear in mind that whilst both images will be stored on the same EPROM, and share the same socket on the motherboard, ONLY ONE of the two images may be present in the socket at a time. This is because we will manually decode the top address line by using a switch. The above consideration has two important bearings on choice of suitable software : Software that needs to claim private workspace on a reset is unsuitable since if it is not selected during a reset it will not get a chance to reserve workspace, and will probably crash the machine if switched in after a reset. Software of this type includes almost anything that raises the value of page after a reset e.g. filing systems such as DFS and ADFS. Software packages normally supplied on more than one EPROM are unsuitable for blowing onto one large 32K EPROM. The reason is that they can never both be present in the same socket at the same time, but for software of this type to work both need to be able to communicate with each other to function. The original plea to put two 16K AMX ROMS onto a 32K EPROM falls into this category I'm afriad. (But that doesn't mean that you can't double up some of your other ROMs to free sockets.) Toolkit and utility type ROMs are normally the safest bet. Try to choose your software carefully. For example, say you have two images that you use often, and two that you use only occasionally. You want to blow all four images onto two 32K EPROMs. If you put the two commonly used images on the same 32K EPROM you will have to switch between them each time you wish to use the other, and will never be able to use them both at the same time. However if you were sensible and paired an often used image with a rarely used image you could have both frequently images selected normally, and use them at the same time, whilst switching the other rarely used images in only when necessary. Next you have to program both images into the blank 32K EPROM. Most EPROM programmers can cope with standard 32K EPROMs. To program the 32K EPROM load the first of the two 16K ROM images from disk into the programming buffer, then program the LOWER half of the 32K EPROM with this image. Next load the second 16K image from disk and program this into the TOP half of the 32K EPROM. The above assumes that your EPROM programmer works like mine, only programming 16K of a 32K EPROM at a time. If it can only program the whole 32K in one go then you should join the two 16K images together into one 32K file prior to programming. Assuming that the programming has gone OK a hardware modification has to be made to the EPROM for you to be able to use both 16K images in your BEEB, so get your soldering iron out and warmed up. CAREFULLY bend pin 27 (second pin from the top on the right hand side as looking down onto the window side of the EPROM) until it sticks up in the air in the opposite direction to all the other pins. This is the line (A14) that selects the top or bottom 16K of the 32K EPROM, and we have to decode this manually to get out whichever image we require. Strip about 1cm of insulation from both ends of a length of thin interconnect wire about 20cm long. Solder one of the bare ends of the wire to the upturned leg of the EPROM. Take a 1K (1000) ohm resistor, chop both wires coming out of it off about 1cm from the main body of the resistor. Wrap the other bare end of the interconnect wire around the stiff metal wire extending from either end of the resistor. I will refer to the stiff wire coming out of the end of the resistor that you have wrapped the interconnect wire around as the south wire from now on, the stiff wire from the other end of the resistor will be the north wire. Turn your BEEB off and unplug it from the mains. Do the same with any peripherals connected to your BEEB. Remove the fixing screws, take the top cover off, unscrew and remove the keyboard. Take the south wire of the resistor and solder it to the center (common) contact of a single pole toggle switch (the simplest sort of toggle switch). Take another length of interconnect wire and solder one end of this to either of the two outer contacts of the toggle switch. Connect/solder the other end of this interconnect to any negative or ground rail inside the computer. e.g. Pin 8 (bottom left) of IC26 (just to the left of the 6502). Take a third and final length of interconnect wire and solder one end to the north wire of the resistor, and the other to any positive 5 volt rail inside the computer. e.g. Pin 16 (top right) of IC26. Now plug the 32K EPROM onto any free ROM socket in your BEEB. You MUST now check that everything is OK. In particular check that no bare wire ends touch any other components in your computer. An electrical short could damage your compuer. Fix the switch to the inside of the case in a place where it will not touch any other componets. Better still drill a small hole in the side of the case and fix the switch so that it protrudes through to the outside. After checking thoroughly fix the keyboard back in place, and fix the lid back on plug it in and switch on. One of the two images should respond to a *HELP, and if you flick the switch to the other position, press BREAK once and type *HELP. Hey presto! the other image should be there. Don't forget that you can only use one of the two 16K images at a time. Schematic of Wiring : ______________________________ +5v | | 1 K ohm resistor | From pin ___________| 27 of 32K | EPROM / switch / | | 0v ______________________________ Gnd.