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.
TECHNICAL NOTE ABOUT 1797 DISK CONTROLLER CHIP
By Richard Beal
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.