80-Bus News

If you do not like the thought of cutting up the tracks of your board until you are sure that these modifications will work, you can get round this by putting the ICs which require circuit changes to be made into new sockets; they should be the type with the pins oriented so that they can be plugged directly into the original IC sockets (you may have some difficulty finding this type). The new sockets have those pins which require modifications bent out so that they are accessible for soldering; they are then inserted into the original sockets and the rerouting of the wiring made to the bent-out pins. This technique can be used for a permanent modification and leads to quite a neat job but do not forget that the 5 volt line should be decoupled at each memory chip.

So that is all you have to do to produce a full 64k conversion job. Adding another 64k follows along the same lines, with the /CS for that block being taken from P7. The 74LS32 OR gate needs to be added to ensure that the flip-flop reset pin goes high when either block 1 or block 2 is selected. However, adding this additional gate led to another problem; on my board, the rising edge of the clock occurs almost simultaneously with the generation of the /RAS signal so that SWMx is produced within a few nanoseconds after /RAS. Even though the RAM specification states that the address should remain stable for 20 nS following /RAS, the unmodified board and the 64k version appear to work happily. However, the delay introduced by the OR-gate (even using the higher speed 74S32) causes the flip-flop reset to go high after the clock transition, which upsets the correct phasing of SWMx and /CAS. The solution is to delay the clock signal applied to the flip-flop by inserting four 74LS04 inverting buffers coupled in series between the clock signal from the bus and the flip-clop clock pin, which introduces a delay of about 30 nSec; this appears to be sufficient. I believe that similar modification to the original board was suggested in an early issue of the INMC to delay the generation of SWMx and /CAS, but I was unable to find a reference to this.

We now have to select which block, or part of a block (which we will call a page, following the usual custom) is coupled into the memory map. If you are content to couple in either one block or the other, you only have to toggle one bit derived from the PIO, so that when P6 is up P7 is down and vice versa. Provided you have NAS­SYS overlaying the RAM you can keep control by using the O command. However, greater flexibility is achieved if you are able to couple in any 16k page from either block. One way of doing this is shown in Fig. 4; the 74LS75 4-bit latch is partly decoded as port 3 – this fits in with the partial decoding of the other ports used on the N2 main board; you can, of course, fully decode it to be any other number between 9 and 255, but then /I0EXT must be generated too. The four Q outputs from the latch are ANDed with the four 16k decodes which appear at the points labeled 0 to 3 on the RAM board, and the outputs ORed to produce a /CS for the first block of RAMs on P6. The /Q outputs are treated similarly and fed to P7. This ensures that there can be no contention between the two RAM blocks. Since /CS acts as the output buffer control line, it must only activate one block at a time; this is ensured using the 74LS32 to create logic-low AND gates with the inputs P6 and P7 to give two /CS lines, one for each block. Control of the paging can then be done by writing to Port 3, either statically by using the NAS­SYS O command or as part of a program by using the out instruction. O 3 0 selects the whole of block 1, and similarly O 3 F selects block 2, while O 3 3, for example, maps pages 1 and 2 of block 2 and pages 3 and 4 of block 1 into the memory map.