80-Bus News

Reinsert IC46 while making sure that pins 3 and 13 remain out of their respective holes. Connect the wire from pin 3 (decoder address A0) to pin 12 of workspace link block (LKB9). (Connects to A11). Connect the wire from pin 13 (decoder address A1) to pin 9 of workspace link block (LKB9). (Connects to A12). TEST. On LKB1 connect pin 5 to pin 3. (Connects all pin 21s to +5V). TEST. Connect LKB8 pin 5 to LKB9 pin 10. (Connects all /OE pins to the /XROM signal). TEST.

Now we come to setting up the decoding. What you do now is a matter of taste. What I did was: Ensure LSW1 switch 8 up, switch 7 down. On LKS1 connect 4 to 6 and on to TP13 (0V in bottom right corner) – this permanently enables the decoder IC46. Connect any two desired 4k blocks from their respective pins on LKS1 to pin 7 (/XROM). This gives a memory map of:

0000-07FF   Nas-Sys
0800-0FFF   Video+Workspace
1000-BFFF   Ram B(?)
C000-DFFF   4 x 2716 EPROMs
E000-FFFF   Basic.

Adding an external switch between IC6 pin 13 (or lower end of R56) and Ground allows either bank of four 2716s to be switched in to the C000-DFFF address range. If the full 16K is wanted on line together, then IC6 could be connected to A13 rather than a switch. In either case it might be advisable to lift the leg of IC6 from its socket and make the connection direct. If you don’t, your system will crash if you absent mindedly close switch LSW1/7.

The modification for 2732 EPROMs is very similar. Instead of strapping pin 5 to pin 3 on LKB1 (to put 5V on the pins 21), extend the under-board wiring of the pins 21 to pin 8 of the Basic ROM to pick up A11. The other differences are concerned with the address decoding. Connect pin 3 of IC46 to LKB9 pin 9 (to pick up A12). I suggest that pin 13 of IC46 is connected to another switch. The two switches now let you select one of four 8k banks to occupy the selected 8k block of EPROM memory. Obviously the switches could be replaced by address lines as above. (In extremis this would give 32k of extra ROM – makes the N2 a little bit like the BBC, all ROM and no RAM!).

The more adventurous among you might like to replace the switches by a 74LS74 and some additional logic, and have software controlled selection of the EPROM banks via an IO port .... even more like the BBC!