80-Bus News

at any TV circuit diagrams recently, but with transistorised sets this point can be found relatively easily. With a modern set full of ICs you may find that the point you’re after lies within an IC. You need to inject the composite video signal (of the appropriate amplitude and polarity) at a point just before the video and sync signals are separated.

(4) Always Switch Off And Allow Time For High Voltages To Discharge Before Making Any Change To Your Circuits.

(5) If you must make any internal adjustments on a live set always use only one hand, keeping the other well out of the way (like in your pocket). Don’t Grope Round The Back Of The Set. If you need to see the screen while you make the adjustment sit behind the set and use a mirror. Always Keep An Eye On The Hand Making The Adjustments. As well as high voltages there are usually HOT resistors in the set. If you touch one of those inadvertantly who knows what your reflex action may make you come in contact with, and you may end up with far more than a burnt hand.

CP/M Users Group

Some time ago I mentioned the CP/M Users Group. The CP/M Users Group (UK) publishes a magazine (quarterly?) that varies in content. It normally contains news, reviews and comment. In the last issue (March 1984) over half the magazine was a listing of the new UK and SIG/M disks that have recently been added to the library. Currently a years subscription costs #7.50. The CP/M Users Group can be found at __ ____ ____, Hawley, Dartford, Kent ___ ___.

Using a Nascom as a counter-timer.

Next on the pile is a rather confused letter from someone in Southampton who had better remain nameless. (Anyway we reckon the name was a pseudonym.) He is wanting to use the interrupt line on a Nascom to measure the duration and frequency of a pulse stream. For some obscure reason (not stated) he wants to use interrupt mode 1. This effectively executes a ‘RST 38’ in response to an interrupt, vectoring to address 38H. However Nas-Sys has its “RDEL” routine at that address which is not much use in measuring anything! As a result he has got tied up in knots trying to get a RAM based version of Nas-Sys going. He is trying to do this so that he can patch the RST 38 location to jump to his own routine. I’ll ignore the mire he has got into with his attempt to get Nas-Sys into RAM and point out the large errors he has made and a possible approach to the problem.

1) The interrupt problem.

Use Mode 2. There is nothing mysterious about it, it just needs a little thought as there are two levels of indirection to go through before you arrive at your service routine which can be anywhere in memory. If you must use mode 1 why not use NMI instead of INT? In response to an NMI, Nas-Sys 3 vectors via a JP stored in RAM at 0C7D. (i.e. At address 66 – the NMI execution address – Nas-Sys has a JP 0C7D. It intialises 0C7D to a JP <register display routine>.) Thus anyone wanting to use the NMI for another purpose can change the JP stored in the workspace area so that control passes to their routine, rather than back into Nas-Sys. (If necessary you connect your interrupt signal via a 2-input OR gate (e.g. 74LS32) to the NMI input. By connecting the other input of the OR gate to a spare bit on the PIO you can have a ‘maskable’ NMI).

2) Why there is no point in doing it anyway.

To go back to his original requirement. “To input a stream of variable length pulses direct into the Z80 pin 16 /INT, and thereafter analyse their lengths & groupings by program”. He has started off by making a fundamental