Cheetah 64K RAM Pack

Sinclair User

Stephen Adams

CHEAP IS NOT NASTY IN MEMORY PACKS Stephen Adams examines the many ways of expanding the storage of the ZX-81. There are so many ZX-81 RAM packs around these days that it is worth looking at what they can offer in the way of more facilities. First, a little explanation of the ZX-81 memory map and ROM routines would help in understanding the limitations of the ZX-81. Sinclair engineers, when they designed the ZX-81, did not expect that anyone would need more than 16K of RAM. So they took some short cuts in the design which made it cheaper and easier to access the RAM and ROM. One of the them was to restrict the upper 32K of the memory map to working the screen and nothing else. The other was that the ROM, which is only 8K long, was allowed to repeat itself throughout the memory map unless the RAM was working in that area. Those decisions allowed them to decode only the top two ADDRESS lines - A14/A15 - to determine whether RAM/ROM or RAM with DISPLAY was on at any one time, the address line A15 deciding whether the display was on or not and the display being on only in the top 32K of memory and thus dividing the memory map in half. The A14 address line divided each 32K section into two quarters, the bottom half of which was ROM and the top half RAM. As they were the only decoding done inside the ZX-81 the 1K of RAM repeated itself all the way through the 16K RAM section. When the 16K RAM pack is added, however, it is switched-off by the use of the RAMCS line on the edge connector and the RAM chips in the RAM pack do all the extra decoding necessary to divide the 16K section into individual bytes. To expand the memory above 16K required some special decoding in the RAM pack so that it did not interfere with the display, which needs a repeat of the first 16K of RAM at 48K and above in the memory map. It also required that the ROM be restricted to the first 8K of the memory ma . That was done by using the ROMCS line in the same way as the RAMCS was used on the 16K RAM pack. Memotech was the first company to produce a 56K RAM pack. Most of them are now called 64K packs but you can use only 56K and that allows you to use 48K for Basic and 8K RAM where the ROM used to be, between 8K and 16K, for machine code. There are some restrictions on using the 48K as you cannot run machine code in it and you have to be careful that the display file does not cross the 32K border. Memotech now has 16K and 32K RAM packs in the same boxes as its 64K ones, which are often used by dealers as a second choice to the Sinclair 16K RAM. Memotech has a good reputation for service. The 32K pack allows you to use your 16K RAM pack as well to achieve 48K. The boxes are made from extruded aluminium and are used as a heatsink for the internal +5V regulator. RAM wobble problems should be familiar to all readers and Memotech is no exception but the company provides a Velcro strip which binds the packs to the ZX-81 and other packs. The instructions are clear and concise in a booklet accompanying each pack but they are expensive. The Sinclair 16K RAM pack has received both complaints and praise. The complaints are from users who have had to experience RAM pack wobble, which occurs when using the Sinclair keyboard. The design of the RAM pack is such that it is not connected firmly to the ZX-81 and the top of the RAM pack rests against the top of the ZX-81 case. Every time the keyboard is used that lifts the RAM pack from the table and shakes it. The edge connector eventually becomes so loose that any slight movement will disconnect it from the ZX-81, corrupting the ZX-81 memory. Several methods have been produced to stop the wobble - flexible cables so that the RAM pack can be laid flat on the table from DK'tronics and the RAM-LOK from Adapt Electronics which bolts together the ZX-81 and RAM. The best method, though, is to buy a RAM pack which has been designed to eliminate the wobble. They consist mostly of not allowing the circuit board to be connected to the casing of the RAM pack, so that the board can move up and down inside the case with the movement of the ZX-81. A reliable stiff edge connector is also required. Some, like that from Cheetah, have also been designed to wrap round the back of the ZX-81 so that there is little movement between the two casings. Some manufacturers are also using +5V.on1y RAMs so that they do not have to provide a power supply from the +9V supply which can vary depending on the equipment used. Kayde, Camel and Econotech 16K RAMs are good examples of the first type, the first two being cased. The ZX-Panda and the Cheetah are good examples of the second type. The ZX-Panda can also be used to give 32K by adding a small PCB containing 16K more RAM inside the case. The special Audio Computers RAM pack is in the category where the design has nor been so successful. The RAM pack was specially-shaped to conform with the back of the ZX-81 but it still requires sticky pads and a special plastic insert to prompt it to work. Some of the RAM packs provide a LED - a little red light - but that only provides an indication that there is power to the RAM pack and not whether it is working correctly. The best choice is obviously a non-wobble RAM pack which is inexpensive. If you can afford it and do not want to use pre-programmed ROMs or other devices, opt for a cheap 64K RAM pack. They take up just as much power as the 16K and will also work with the Sinclair printer - see the table for a list of facilities of the various RAM packs. Basicare provides a different kind of RAM; you must first buy a base module to use its RAM, called 3 Persona. It is in 16K and 64K packs for the ZX-81 up to a maximum of 512K. That is divided into banks and you will require other modules to have the banks talk to each other. The maximum memory available at one time is 32K. It is the only system which allows you to run big programs but at the moment there is very little software to run it. Yet another type of RAM pack is available and that is the battery-backed RAM - DROM, which allows the user to do things which normally would have to be run in from tape - instantly. They can provide storage for routines to side-scroll the screen in any direction, assemblers, new operating systems to replace the Sinclair ROM or even your own favourite Basic program. A multitude of DROM packs is produced by Camel Products. Two of them work on the ZX-81, the Memic 81 and the Cramic. The Memic 81 contains one or two static 2K by one-byte chips giving 2K or 4K of RAM backed-up by a battery when the power is off to preserve its memory. That occupies any position in 8K-16K area of the memory map. DROM devices allow you to write a machine code or a Basic program and transfer it into the 8K-16K area of the memory map. Machine code can be run from there, saving valuable RAM space, but Basic programs must be uploaded to the Basic area before running the program. That means that any Basic program is limited to 8K unless the Cramic is used which is 16K long. The routines for the transfers are included in the notes with every pack and consist of machine code routines which must be run by using the USR command. The Cramic is a special case as the 16K can be switched in or out by a software switch and so a Basic program can be loaded from tape into the Cramic and sealed-off from the ZX-81. Powering-off the ZX-81 then has no effect, as the RAM module has its own battery to keep it operating. The program can be restored by powering-up the ZX-81 and running a machine code program to step back in the Cramic where you ceased. That leads to my first criticism of the notes; nowhere is it mentioned that you should save the machine code program on tape, in case something goes wrong. The code required is very short but as it needs to be put into memory before using Cramic it would have been better to load it from tape. That routine could be stored in the Memic 81, of course, which is outside the Basic area and the routine loaded into the Basic area from there. The advantage of DROM over ROM is that it can be changed. The other DROM is available only in kit from from Hunter Electronics and consists of board which plugs into the back of the ZX-81 and provides 2K of battery-backed RAM, with space for three more chips. If you wish, some of them can be turned into ROM sockets to take 2K or 4K EPROMs. The RAM pack race has now become so cut-throat that a few firms which were doubtful have withdrawn. The firms remaining are giving the customers what they want, depending on price. Cheap is not necessarily nasty any more. So look for the bargains - they are there to be found. DROMS Memic 81, 4K, £34.45 Cramic, 16K, £91.95 Basicare, 2K, £39.50 Hunter, 2K, £19.95 SUPPLIERS Basicare Microsystems Ltd, [redacted]. Cambridge Microelectronics (Camel), [redacted]. Cheetah Marketing Ltd, [redacted]. JRS (Econotech), [redacted]. DK'tronics, [redacted]. Kayde, [redacted].

Sinclair User

NON-WOBBLE RAM PACK Cheetah Marketing has produced a 16K RAM pack for the ZX-81 which stops any RAM pack wobble without using glue, tape or screws. The front has been contoured to fit the curved back of the ZX-81 and fits it like a glove. Vigorous tests were carried out, such as thumping the keyboard heavily, but it refused to crash. The cost is £19.75 inc. VAT and postage. Cheetah also makes a 64K RAM pack for £44.75. Printer owners may be pleased to know that it also works well with the Sinclair printer. Cheetah Marketing is at [redacted].