80-Bus News

decoupling capacitor of 0.luF on the board, as it really is necessary with power leads this length. If you thread the power leads down through the veroboard, then this will make them far less prone to coming off due to metal fatigue, after they have been moved about a bit (this is a point worth remembering whenever you are connecting flying leads to vero-board). Connect the +5V lead to bus line 75-78, and the GND lead to bus line 67 (which SHOULD be a ground, if your motherboard is wired properly). Alternatively, connect the power leads to the power test points on the CPU board.

The circuit should be fairly self-explanatory.

I also include a circuit for Nascom 2 owners to page out Nas-Sys, as no-one seems to have worked this out. Nas-Sys is paged in after RESET, and when port 0 bit 2 goes low.

During the development of RP/M V2.3 an interesting feature of the 1797 was investigated. In order to determine if there is a disk card present in the system, RP/M outputs a value to the track register of the 1797 and then reads the value back. If it is not correct then RP/M assumes there is no disk card and gives the “No disk” message.

On page 20 of the Western Digital technical note there is a comment in microscopic print that the data cannot be read back correctly for 4 microseconds. In fact RP/M 2.1 waited 5 microseconds and operated correctly. However the delay needed depends on several factors and careful tests have revealed that the following number of NOPs (for example) are needed between the OUT and IN instructions for the correct value to be read back reliably. This value corresponds to the number of microseconds since each NOP uses 4 T states and the processor operates at 4 MHz.