From this we can see that the weird order of the lines in the Nascom
display was done with malice aforethought! (I’ve always assumed it was due to
some quirk of the hardware implementation). By rotating the contents of the
PROM by one location, and preloading IC68 with 12 instead of 11, the N2 can
have a conventional video ram where the screen starts at the first address,
and ends at the last address. If required, a non-scrolling top line can be
implemented with a zero software overhead in this environment. Why did they do
it the way they did? – I suppose an N1 circuit diagram may answer that.
However, returning to the matter in hand, it is a very easy matter to get
all 16 lines out of the character generator. All that is necessary is to lift
pin 1 of IC53. (If this is all you do, then this pin should be tied to +5v,
either direct, or via a 1k resistor). This will prevent the row address
counter being cleared on line 14, and it will now cycle round all the 16 lines
of the character generator. There are two side-effects to this modification.
i) The active display height will increase by 32 TV lines, which may require a
picture height adjustment on your monitor or TV. ii) The frame rate will
change. There will now be a total of 22x16 TV lines (=352) instead of the
previous 308. (The 625 line standard actually requires 312.5). This will give
a frame rate of 44Hz. When I made this modification to my N2 it did not
disturb the frame hold on my monitor – you may not be so lucky.
NOTE The only effects of this modification will be an increase in screen
height and a possible loss of vertical hold. The extra lines of the character
generator can only appear in the correct place. Other symptoms may appear if
you have disturbed dry joints, or other ICs.
Getting back into Sync
To get back to a 50Hz frame rate some more modifications are necessary.
Somehow we need to take out about 40 TV lines from a frame. We can remove an
integral number of 16 lines by changing the address intially loaded into IC68
at the start of every frame. However the effect of this is to move the display
upwards on the screen. (The screen is unblanked sooner after the vertical sync
pulse). It is also a simple matter to remove some lines from the end of the
display, all you have to do is program a new N2V PROM(!). Ignoring the latter
suggestion as it is impractical for most people, lift IC68/1 and connect it to
IC68/8. Lift IC68/10 and connect it to IC68/16. This now arranges for the
counter to start at a count of 13, rather than 11, and will thus remove 32 TV
Lines from the start of the frame.
This leaves us with a further 8 lines to get rid of if we want to hit a
50Hz frame rate. We can do this by including the row address counter IC53 in
the ‘frame reset’ sequence, and preloading it to 8. Unfortunately the ’LS161
has a synchronous load although the clear is asynchronous, and the
modification involves more than a simple strap. So, unless you’re a
perfectionist, I suggest you stop here.
(With the ’LS161 the load occurs synchronously with the clock. i.e. The
load input has to be taken low, and held low until the clock input makes a
low-to-high transition, at which point the data on the parallel inputs will be
loaded into the counter. The clear input is asynchronous, which means that as
soon as the clear input is taken low the counter is cleared immediately
irrespective of the state of the input clock.)