16k CMOS Memory Extension for the Nascom 2 Main Board

Paul Anderson

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 the 64k (Ram C) became available and now wish you had the full 64K to run your disc system etc., all is not lost. Here are the modifications required to turn those 8 (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 Hitatchi, available a year ago for the princely sum of £30 (yes THIRTY POUNDS) each – have I frightened you? – but now available for around £4 (yes FOUR POUNDS) each in 10-up quantities. Each of these devices is 2K x 8 bits (16K), so you will need 8 – one for each socket!!! This will cost you about £40 including VAT. This may sound a lot but when you consider that to upgrade by that 16K you will have to sell your 48K card and buy a new 64K (anyone seen any secondhand 64K card!!), 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 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 Power Down (standby). For the standard version, 20uA is quoted which, for 8 chips, means 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 3 were above 1uA) and the typical values were between 0.3uA and 0.7uA (almost two orders of magnitude inside spec. and better than that quoted for the low power version). So IF VE VERE TO POWER ZEM from one Mempack battery (3.6V 150mA hours), 01 VE VOULD BE ABLE to operate them for around 15000 hours on one recharge, assuming a total consumption of less than 10uA for your 8 chips, thats approaching 2 YEARS. In practice, 1 year is about right allowing for the self-discharge of these batteries. If VE VERE to float charge the batteries at about 1 milli Amp for 4 hours a week (I’m sure your Nascom is on for more than that each week, if not there’s a little mod…), then the batteries will remain virtually fully charged for the life of your Nascom. (Old Nascoms never die, they just get mod’ed–Ed)

Now, battery backed up RAM opens up many interesting possibilities. Firstly, it can be used just like ordinary RAM, as workspace and to hold programs, etc, secondly, it can be used as “Loadable EPROM” and if a Write Disable is incorporated, the content is as safe as if it were in EPROM, but can still be changed virtually instantaneously when required. Also, when developing programs, they can be stored in the RAM safely whilst the program is tested, or you can go away for a tea break and and not have to worry about mains dropouts or the wife unplugging your machine to use the Hoover. I am convinced it is preferable to using just normal RAM and I am seriously considering fitting 64K of Cmos RAM.