80-Bus News


November–December 1983, Volume 2, Issue 6

Page 46 of 67

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…

11-14Display is blanked.
15-30Display is unblanked. Display lines go 15,0,1,2,3…13,14.
34Display is blanked again
0Display is blanked. (Counter wraps round to 0).
1Vertical sync triggered & counter immediately reset to 11.

Page 46 of 67