80-Bus News

Next a minor bug but slightly more serious. Old habits die hard, and in some C/80 source files I entered a few hexadecimal constants with a trailing ‘H’ (e.g. 0xAH rather than the correct 0xA). The compiler accepted these with out comment and interpreted the ‘H’ as part of the Hex number so I ended up with an incorrectly evaluated constant. Finally my latest encounter has been with an unforgiveable bug from Microsoft in an 8086 cross assembler. Being relatively inexperienced in 8086-assembly language programming my initial programs contained some glaring errors like loading a segment register with an immediate 16-bit value. (Although you can load normal registers with immediate data, the segment registers can only be loaded with data from another register, or from memory.) The Microsoft assembler accepted my illegal instructions without complaint, and proceeded to generate an opcode for a completely different instruction. It was only by manually dis-assembling the opcodes on the assembly listing that I found the error. (Luckily, as I was just starting to familiarise myself with the 8086, the program was only some 25 lines long and it didn’t long to find the error.)

One final comment on this topic – if you do find a bug Report It, not by complaining to your friends, but to the Author. Document the bug thoroughly, for example by writing a four or five line program to illustrate it, and send the print-outs in with your comments. Don’t assume somebody else must have already reported it. You might find that your report vanishes into a black hole, or, as I did with the Software Toolworks, get a polite note back and find the bugs corrected in the next release of the software (C/80 version 3.0).

Do-it-yourself Electrocution.​(c.f. Richard Beal in the last issue!)

Next a request from Mr E.Jones for an article covering the general principles of converting a domestic TV set to a monitor, and interfacing a Nascom to it. These sort of articles tend to be few and far between, and I assume that the reason for this is that no magazine editor wishes to be sued by the relatives of a late hobbyist who attempted to follow the article. voltages inside a tv set or Monitor are lethal. If you do not know what you are doing LEAVE THE BACK ON THE SET. If you think you know what you are doing I still advise you to leave the back on the set. With the proliferation of home computers and video recorders that has occured in recent years it should be possible to pick up a cheap black-and-white monitor, or to buy a TV set that already has a video input socket on the rear. For those who want to progress further I offer the following suggestions/​observations.

(1) Before doing anything buy a copy of the service sheet or service manual for the set in question.

(2) Look at the power supply section of the diagram. Many sets directly rectify the mains input and then use the filtered DC supply either directly, (old Valved sets), or via a switch-mode step-down power supply (newer integrated sets). Certainly in the first case, and possibly in the second, you will find that one side of the TV chassis is connected directly to the Neutral of the mains supply, (or directly to the LINE side if the plug has been wired incorrectly). In this case Do Not Proceed Further unless you can isolate the TV supply from the mains, or provide a suitable barrier at the video interface into the TV. (A small black-and-white portable offering mains/​battery operation will almost certainly have a mains transformer providing the required isolation.)

(3) The signal present at the VIDEO pin on the Nascom 2 circuit board is a convential composite video signal containing a mixture of Video and the horizontal and vertical sync pulses. This is similar to the signal present at the output of the detector following the IF stages in the TV. I haven’t looked