The suggestion made to Richard was to try adding diodes to do this, but
the same effect can be acheived by using an open-collector AND gate (e.g.
74LS09), or even an open-collector buffer (e.g. 7417). The input(s) to the
gate(s) are connected to the drive select lines that do not provide a ready
line, and the output(s) are connected to the Ready line (see fig 3). So, as
soon as a drive select line connected to one of these added gates goes low,
the Ready line is also pulled low. N.B. The gate used must be an open-collector
gate, otherwise it will interfere with the 8″ drive when it attempts
to drive the ready line. In the approach suggested to Richard the diodes
provide the required isolation, but as they are passive devices there may be
problems with voltage drops across various components (as he discovered). The
74LS09 approach avoids this.
How to find your feet on a Nascom 2
A letter from Mr Mathison of West Germany has brought the N2 out again.
He’s been chasing the feet of the little men in the N2 character generator,
put, despite following the suggestions published recently, has had no success. –
I’ll start by describing the action of the N2 video circuitry before running
through the solutions. That way any of you who attempt the modifications
should have some idea of what you’re up to. You will need a copy of the N2
circuit diagram if you want to follow the description below.
How it works
A 1MHz character clock emerges from IC49/13 (IC43 pin 13). This is
divided down in the six-bit counter formed by IC51/IC52. The outputs of the
six-bit counter form the address of a character within a display line, and
address the video RAM via multiplexors IC62/IC63. The address is also decoded
by IC55/IC60 to provide the blanking signal that frames the active 48
characters of the display, out of the total of the 64 characters that make up
a line. (Remember that the Nascom video RAM has 16 unused characters between
the end of one display line and the start of the next one). The output of
IC52/13 triggers the monostable IC57 to provide the horizontal sync pulse, and
also clocks the four-bit counter IC53, whose outputs form the ‘row address’ to
the character generator. The character generator ROM address lines are also
decoded by IC44c which, when the 625 line option is selected, resets the row
address counter to zero everytime row 14 is reached. Thus only rows 0-13 of
the character generator are displayed. Every time IC53/11 returns to 0, the
five-bit counter formed by IC68/IC13b is clocked. The outputs of IC68 address
the video ram via multiplexor IC64 and form the ‘character line’ address.
Finally the IC68/1C13b counter is decoded in the N2V PROM (IC59). There are
two outputs from this PROM, D1 provides the vertical blanking signal, and the
D0 output is used to trigger the vertical sync signal (from IC57) and preset
the five-bit counter IC68/IC13b to 11. (Check the hardwired inputs on IC68 to
confirm this). The contents of the N2V PROM can be found in the Nascom 2
documentation, and from the listing you should be able to deduce the
following, starting at the point where IC68/IC13b is reset to 11…
|Display is blanked.
|Display is unblanked. Display lines go 15,0,1,2,3…13,14.
|Display is blanked again
|Display is blanked. (Counter wraps round to 0).
|Vertical sync triggered & counter immediately reset to 11.