This article covers hardware modifications to a standard(??)
Nascom 2 to provide the fallowing functions. They don’t sound
much but it does provide single bit logical control either by
switch or port control (and enables compatibility with EPROM/RAM A
card where fitted).
Switch in CP/M Boot ROM.
Switch out Monitor ROM.
Move Video/Work RAM.
Automatic Reset on “switchover’
This avoids over-writing due to a ‘wandering’ CPU and
avoids the necessity on doing a Reset on each
Automatic switching of restart address.
This allows restarts at F000H plus any other Dil switch
selected address of your choice.
These modifications have been found useful in fast changing
from one system to another without the risk of corrupting the
contents of memory so enabling the Nascom to get at CP/M!!
The modifications outlined (which are open to variation)
also cover re-arrangement of the use of the workspace RAM and
the HASIC ROM positions for these using EPROM cards. These
enable a complete CP/M system on N2 & RAM cards whilst still
retaining the use of the normal RAM / EPROM sockets for more
useful applications than JUST CP/M.
CP/M / Nascom Switching.
Fig. 1 shows the physical wiring arrangements. The 74LS157
is a Quad 2-line-to-1-line data selector and provides 4
‘change-over’ switch functions (operating active High). These
Disable Monitor ROM.
Enable CP/M ROM.
Move Video & Work space.
Change Reset address.
In 1 & 2 the Chip Enables are high when not selected, 4 is
Fig. 2 shows the very simple track modifications to enable
Reset address changing. It involves cutting two tracks (the one
on the upper PCB side is superfluous anyway – being a ground
track which is bypassed several times on other parts of the
PCB). The other cut isolates the ground connection to the 4