INMC News

   

Christmas 1979, Issue 5
Page 12 of 38

Basic Chips On NAS­COM 1’s

So there you are with your brand new (and expensive) MK36271 BASIC ROM, and wondering where to fit it. There don’t seem to be any holes for it. When the memory board was designed it wasn’t envisaged that anyone would want to fit an 8K ROM to it, so no holes were provided. Never mind, a little bit of surgery, and all will soon be working.

Take a careful look at fig. 1, and carefully cut the tracks as as shown. Then connect wire jumpers as follows:


IC27   Solder side

  1. IC26 pin 19 to IC27 pin 20.
  2. IC24 pin 2 to IC27 pin 19.
  3. IC24 pin 3 to IC27 pin 18.
  4. IC25 pin 7 to IC27 pin 21.
  5. Decode pads 11 and 12 together to pad P5.
  6. Check it!
  7. Plug the board back in the Nascom and try it. Lo! You should have BASIC.

Now this mod. means that you have lost the use of the other 3 sockets, so for those more adventurous souls, you can get them back again. The tracks to pins 19 and 21 were the +12 and




−5 volt supplies to the EPROM sockets so these must be restored to make ICs 28, 29 and 30 operable. Two jumpers from the points where the tracks were cut to pins 19 and 21 on IC28 will restore the juice. But this will also put juice back on IC27, as these pins are linked on the top side of the pcb. Some deft knife work is required to cut these two tracks between ICs 27 and 28 sockets (and get it right ’cos you’ll never repair a mistake!!!). Now the way we have mod’ed the chip select on the Basic ROM means that it will come on regardless, so a switch is called for to disable the ROM when not required. So make it a double pole switch, and use it for

selecting the decaodes as well. Note that alternative arrangements are shown, for any other decode apart from ‘E’ and ‘F’, and decode for ‘F’. Why? Well some idiot will want to run Big BASIC and Super Tiny BASIC on one board. Apart from that our version of Naspen is an early one, and runs from F800 to FFFF. Naspen usually resides between B800 and BFFF.

Can we make one thing quite clear, these mods have been tried and work, but the INMC can not take responsibility for any failures, or chewed up pcbs as a consequence of trying them. If you don’t think you are capable of attempting these mods, DON’T TRY. Seek the help of someone competant, then you can blame him if he chews up half the tracks on the pcb.

STOP PRESS; 100nF capacitor required across pins 24 and 12 of the BASIC ROM.

Page 12 of 38