Micro­power

  

Volume 2, Number 3 – July 1982











Page 19 of 37











So, after this extended introduction, I will proceed to enlighten those interested parties on how it is done. It 15 based on using up the 8 EPROM/RAM sockets on the Nascom 2 main board. The 8 sockets will accomodate most byte wide, 24 pin devices and the HMefies fall into this category, For straight forward RAM expansion without battery back-up and Write Disable, fitting is very simlar to fitting 41185 in these positions except that a higher order addressing is required to access all the available memory. The only additional circuitry 15 to provide RAM-Dis onto 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. [t is suggested that this stage be done first anyway. to check that the RAM 15 operating correctly, before introducing the complexities of the power down and Write Disables. Refer to the link block diagram in your manual (you have got a manual, haven’t you’’) and connect pin 8 to pin 4 (chip select), pin & to pin 2 (output enable to read enable), pin 5 to pin 1 (write enable) and finally, loop pin 7 (address line A10) along each pin 7 and pick up address line A13 off the data bus (best done from pin 12 of IC 47 (N2MD)), This has the effect of addressing through all the lower IKs of each chip and then through the upper iKs of each chip (the upper and lower halves of each chip are seperated by 8K) and is the simplest way to do it. Finally, to make the whole thing go, connect the RAM-gates to the blocks of addresses required eg. DOGD to F000 would give you 16K of RAM starting at D000 and finishing at FFFF so, 1f you are using a CP/M type system, then set your 48K card to go from 0000 ta CFFF and you will have 64K on-line by connecting either RAM-G1 or RAM-G2 to block address CooO + DOOD + E/F000 strapped together (the last 8K block 15 normally used to enable the BASIC ROM). To provide RAM-Dis onto the p.c.b. requires two ICs, one type 74LS32 (quad OR) and one type 74LS04 (hex inverter) and these devices will be used to provide the Write Disable control as well (you mght as well go the whole hog’). In my system, these are mounted piggy back, on top of two other conveniently situated 14 pin ICs close to the link block LKS1, to which most of the connections will be made. Remove IC 18 (74LS06), bend up pin 13 and then replace the IC. For the remainder of the connections 1t is best to refer to circuit diagram 1 alongside the Nascom 2 Memory circuit diagram, top left corner, where all the origina) circuitry will be found. I would suggest that you now test, very carefully, the operation of your new RAM, both with EPROM cards and other periphera: cards occupying the same address space in both enable and disable in Case there is any interaction. My own system has been operating for many months and I have had no problems. NOTE that the linking is altered so that the Write Enable is linked to the Write Disable switch.

Now for the tricky bit, battery back up covection. It 15 not difficult, just time consuming and some care is necessary to ensure that the correct connections have been made. The idea is to isolate all of the +5V connections to pin 24 of each chip and also, to pin 3 of the associated link blocks. This can be done by cutting about five tracks and adding a few wire links. I would have included details but 1t now very difficult to gain access te the underside of my N2 card and I have no record of what I did – however, I would be pleased to reinvent the wheel if anyone wishes to take the plunge. Otherwise, lifting IC legs


This is an OCR’d version of the scanned page and likely contains recognition errors.











Page 19 of 37