80-Bus News

Reprinted from     Nascom – Thames Valley User Group Newsletter No. 5     with permission

If you like me are one of those people who purchased a 48K (RAM B) memory card and then kicked yourself for not waiting until a 64K version became available and now wish you had the full 64K to run your disk system etc., all is not lost. Here are the modifications required to turn those eight (spare) 24 pin dil sockets on your Nascom 2 into that extra 16K.

I will introduce you to an amazing little beasty called the HM6116-4 manufactured by Hitachi, available a year ago for the princely sum of £30.00 each – have I frightened you – but now available for about £4.00 each in 10 up quantities. Each of these devices is 2K x 8 bits (16K) so you will need eight chips, one for each socket!!!! This will cost about £40.00 inc VAT, sounds a lot, but when you consider that to update by that 16K you will have to sell your 48K and buy a new 64K (anyone seen any second hand 64K cards), plus the cost of the extra RAM, I suspect it will cost about this amount anyway.

As I said earlier the HM6116 is an amazing device – the fastest version will run at over 8MHz and the low power version consumes about 0.1uA (I do mean micro-amps) in standby mode. I used the standard version (cheapest) and out of a batch of 20 samples they all exceeded their quoted performance by an enormous margin, particularly in terms of their standby mode current consumption. For the standard version a standby current of 20uA is quoted, which for eight chips is 160uA, which would be Ok for battery backup. However, of the samples I have checked, the worst device had a standby current of only 5uA, only three were above 1uA and typical values were between 0.3 and 0.7uA (almost two orders of magnitude inside the spec.). So if we were to power them from one mempack battery (3.6V, 150mA/hours), we would be able to operate them for about 15,000 hours on one recharge, assuming a total consumption of about 10uA for the eight chips. That’s approaching two years, in practice one year would be about right taking self discharge of the battery into account, and if we were to float charge the batteries at 1mA (milli-Amp) for four hours each week – I’m sure your Nascom is on for more than that time per week (if not, there’s a little mod) – then the batteries will remain virtually fully charged for the life of your Nascom.

Now battery backed up RAM opens up many interesting possibilities, firstly it can be used like ordinary RAM, as workspace, to hold programs, for CP/M or whatever, secondly it can be used as loadable EPROM, and if a ‘write disable’ were incorporated the data in the RAM would be as safe as if it were in EPROM, but can still be changed virtually instantaneously when required. Other advantages of battery backup and write disable are when developing programs, where they can be safely tucked away whilst the program is tested, or you go away for a tea break and the wife decides to pull out the mains plug so she can use the Hoover. Or, if like me you are plagued by mains dropouts and have to save programs after virtually every modification just in case you loose the lot – very time consuming on tape, although I’m now disk based, it’s still a nuisance. I am convinced that battery backed up RAM is preferable to normal RAM, and am now seriously considering fitting 64K of CMOS RAM. I noticed an advert the other day offering 32K of CMOS RAM on a card suitable for NASBUS/80BUS.

So after this extended introduction I will proceed to enlighten those interested parties on how it is done. It’s based on using the eight EPROM/RAM sockets on the Nascom 2 pcb. (Anyone still using the EPROM sockets?? If so, my next article will deal with converting them into 2732 sockets, I’ve 64K of ‘em on one card in my system.)

The eight sockets will accommodate most ‘BYTE WIDE’ 24 pin devices and the HM6116 falls into that category. For straight forward RAM expansion without write protect and battery backup, fitting is very similar to fitting MK4116’s in these positions except that a higher order addressing is required to access all the RAM available. The only additional circuitry is to provide RAMDIS on to the Nascom 2 to disable this new RAM if you are using external RAM or ROM at the address chosen to locate your new RAM. It is suggested that this stage is done first anyway to check that RAM is operating correctly before introducing the complexities of power down, battery backup or write disable.