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ComputerAnswers Column 14


COMPUTER ANSWERS FEBRUARY 1986



Copyright 1985-2011 Simon N Goodwin

FREE MAC SOFTWARE

I recently purchased an Apple Macintosh computer and am searching for sources of public domain software. I would be most appreciative if you could direct me to any possible sources of public domain software, such as Macintosh User Groups, in the UK.
Ronald A Marshall, Alameda, California, USA.

There are not many Macs in the UK so much of the free software here has made its way from the States. The British Apple Users' Group, BASUG, has tried to set up a separate organisation for Mac users but found that the numbers did not justify it. There are some local Mac groups; these meet under the BASUG banner.

Mac software is available from the BASUG library at a fairly nominal price of £8.50 per disk; the group magazine, Hardcore, also contains information of interest to Mac users. Write to BASUG at PO Box 177, St Albans, Herts, AL2 2EG, United Kingdom. Their 'phone number is 0727 73990.



AMSTRAD COLOUR

I wish to link the RGB output of my Amstrad CPC464 computer to the 'SCART/Peri-TV' socket of my Mitsubishi CT201 colour television. I have tried to make a lead up myself with only limited success. I am only able to obtain the eight colours of the Red, Green and Blue signals at TTL levels. I am stumped as to how to get the other colours that can be displayed by the Amstrad colour monitor. I own the monochrome monitor, hence the desire for a colour display.
P.J Wilson, Lordswood, Southampton.

The SCART interface, as you have discovered, is designed for digital signals rather than the analogue levels generated by your Amstrad. In other words the interface only allows you to turn each fundamental colour (Red, Green or Blue) on or off, with no intermediate stages, and this restricts you to eight colours (including combinations).

The interface has no provision for an 'intensity' signal so you can't use it to obtain the other Amstrad colours - a pity, since they're some of the more attractive ones! You'll have to use a modulator (available from Amstrad, but much cheaper to assemble yourself if you can get the parts) and convert the Amstrad's composite video output into a signal that can be fed into the TV's aerial socket.

2011 update: SCART implementation has settled down since and I'm not very happy with this reply. I now suspect this could have been solved without a modulator, using at most half a dozen resistors - the non-standard CPC RGB levels otherwise overload the 1v p/p analogue SCART input; CPC retro pages suggest adding batteries to ensure the TV input interface defaults to TTL input, but again 8 rather than 27 colours are to be expected unless you tailor the connection for the particular TV sensitivity and input impedance. I'd start with 470 ohms in series with R, G, B lines from CPC to analogue SCART, and add 100 ohm pull downs from the SCART inputs to ground if shades are too bright on that TV. Can you do better? Simon



COMMODORE CONNECTIONS

I have a Commodore 64, C2N tape and a 1541 disk drive, but I do not know which printer goes with the Commodore 64.

I cannot find out how to disable the RUN/STOP key on the 64, so all my Basic programs can be broken into.

What is the name and address of the Commodore 64 Computer Users' Club?
P.S Chahal, Eastleigh, Hants.

The Commodore 64 comes with a non-standard serial interface (the six-pin DIN sockets, also used by the disk system) but it is not difficult to convert this to a standard RS-232 interface. Commodore make a converter with a typically meaningless name - the VIC-10-11A - and similar devices, including Centronics interfaces, are available from other suppliers.

If you want to avoid the expense of a new interface you can use any Commodore Home Computer Printer (i.e. a printer with Commodore's name on it) since they all support the serial interface, which is used by the VIC-20 and Commodore 128 as well as the 64. Some of these printers are selling at very low prices, partly because of the non-standard interface - you can pick up a re-badged Juki daisywheel printer for well under half price if you want a Commodore interface.

The vector for the RUN/STOP key is held in memory at addresses 808 and 809. You can disable the key with the command POKE 809,255. POKE back the original value if you want to make the key work again - you can find that value, of course, by using PEEK before you POKE 255.

IGPUG, the Commodore Users' Club, is a big organisation which caters for users of all CBM computers. The address is 30, Brancaster Road, Newbury Park, Ilford, IG2 7EP; telephone 01 597 1229.



TANDY DISK PROBLEMS

My TRS-80 Model 1 with one Tandy Minidisk has worked well for years, until I added a second drive - a Teac FD-55E. Either make of drive functions perfectly in position 0, but if I make the Teac drive 0 and the Tandy drive 1, I hit trouble. I have kept the Tandy in drive 1 because I do not know where the terminating resistors on the signal lines are - they appear to be hard-wired, whereas those on the Teac are easily removed.

On setting up everything works well, but next day it is another story. Calling drive 0 is OK, but access to drive 1 gives 'Directory read error' or other messages. In other words drive 1 is quite useless.

When I re-format the disk in drive 1 all is well again - all commands work perfectly. I have tried two new disks from Tandy and two Wabash disks and all give the same results. I feel the connections are at fault, but where?
Name and address omitted by request.

This is one of those problems which seems to fly in the face of reason - there's nothing obviously wrong with the system you describe, yet it fails intermittently. I can only point out some weaknesses of the Tandy machine (I use one of its close cousins to write this column) and suggest possible operational causes of the problem.

The error you describe indicates that the TRSDOS software is unable to read information from the drive. This software is loaded into the TRS-80 keyboard memory. It communicates with the disk controller, in the Expansion Interface, which communicates with the drives. It is most likely that the problem lies in the cables to or from the interface.

The most common problem area on the TRS-80 (besides tape-loading errors!) is the connection between the keyboard unit and the Expansion Interface. The cable carries just about every signal used by the TRS-80, so faulty connections here often have bizare effects.

The edge-connector is not plated, so it corrodes quickly; the signals are not buffered so they are prone to interference, especially if the connections are dirty. Clean the connections by unplugging the cable and rubbing the circuit board with a typewriter eraser. Don't be tempted to use sandpaper or anything similarly abrasive. Replace the connector firmly and see if this cures your problem.

If this doesn't help you might like to treat the connectors on the disk drive cable the same way; these are less likely to be a problem but again they are not plated - or at least, not on my aged Tandy drive.

Another possibility is a fault in the cable between your Expansion Interface and the disks. You must be using different parts of the cable when you have two drives connected. Try swapping the connectors to the drives and see if the problem moves or disappears. Check that the press-fit connectors are firmly attached to the cable. If possible, borrow another cable and see if the problem persists. You are most unlikely to run into problems with terminating resistors unless you have two sets, in which case your disk controller will probably go up in smoke - literally! See if the fault is caused by switching on of the system. Try turning it on and off several times. Allow ten seconds with the machine off between attempts, to let the power-supplies settle. You should be sure to turn on the supply to all of the components of the system at once; use a multi-way adapter to distribute power from a single socket.

Finally, but perhaps most pertinently: be absolutely certain that you never turn the machine on or off with disks in the drives or near the system (on top of the monitor, for example) - this is very likely to produce the symptoms you describe.



ADVANCE/RETARD

I have an Advance 86B, made by Ferranti, which has a new BIOS installed to enable a memory expansion card to be inserted, taking the total RAM to 512K. I should like to install an OEM internal hard disk upgrade kit intended for the IBM PC, but I am quite unable to find out whether I could expect its controller to work. The supplier seems to be only interested in fitting an external hard disk system at twice the price of the internal one.

Can you suggest how I can get the necessary technical information? If you know the right address for Ferranti that I could write to, that would help. I need to know whether there is any reason to suppose that the controller would not work (I would be willing to take some risk, since I could always sell the upgrade kit second-hand), whether there is reasonable spare capacity in the internal power supply and what changes (if any) I should make in DIP switch settings.
Alan Curtis, Oxford.

First some general advice on compatibility problems - try before you buy! If you are planning to make a major purchase you should take your computer to the shop and ensure that the add-on works before you part with any cash. In this case it may be that the hard disk kit is too intricate to make such a policy feasible.

Ferranti are not willing to help Advance users since they say that they only made the machine under licence for Advance Technology - they deflect any inquiries to Advance, who can be contacted at 8A Hornsey St, Holloway, London N7 8HR. If you need faster response you can call Mr Gadhia at Advance, on 01 609 0061. Most software questions are answered in the 'Advance Programmers Reference Manual', which costs £25 from the above address.

In answer to your specific questions, I have no reason to believe that the disk system you mention would not work - the Quest Firefly and Xebec 9710 drives certainly work fine with the Advance. The 86B, unlike the model A, has a 250 watt power-supply which should be able to shoulder the load of all but the most greedy disk. You shouldn't need to alter any DIP switch settings, but I can't be categorical about this without knowing what system you intend to install.



POCKET COMPUTING

Has PCW ever printed any articles or programs connected with the Sharp PC-1500 or its peripherals? Where could I obtain advanced details of the 'Microsoft' Basic and the commands PEEK, POKE and CALL? Do you have any idea what each pin on the interface connector is?

I am toying with the idea of adding a micro disk drive, or even the Sinclair microdrive, and was wondering if you knew somebody who had experience connecting this type of peripheral to the PC-1500. Obviously the last query will not be a straight plug-in affair, but any help to increase the storage capacity and speed up data transfer would be appreciated.
Clive Westwood, West Croydon, Surrey.

We reviewed the PC-1500 way back in June 1982; I don't recall that we have published any information since then. Any book on Microsoft Basic for the Tandy, Apple or IBM PC should tell you more about your interpreter, although you have not actually got a Microsoft interpreter - just a program that Sharp claims is 'Microsoft-style'.

The PEEK function expects to be followed by a number in brackets - this is an address and may range from 0 to 65535; the function returns a value between 0 and 255 which corresponds to the contents of the specified memory address. POKE expects two numbers, separated by a comma; the first is an address, as above, and the second is a value between 0 and 255 to be stored at that address.

The BASIC interpreter is stored in ROM between addresses 49152 and 65535; RAM starts at address 16384 and extends up towards 32767. Addresses between 0 and 16383 and 32768 and 49151 are not used on a standard machine.

The CALL command lets you invoke machine code at a given address - the address should follow the command. The snag is that Sharp don't even tell you what the processor is on the PC-1500, let alone the instruction-set, so machine-code programming is pretty well impossible (unless a reader can come up with the goods, c/o Computer Answers).

Sharp are similarly inscrutable about the pin-out of the edge connector, and without that you can't do much interfacing. In any case it is very difficult to connect a Sinclair Microdrive to another computer. The drive itself is very simple but it relies upon sophisticated software and some fairly fiendish hardware in the controller ULA, which is not part of the drive.



68000 WORKSHOP MANUALS

I write regarding information on the operating systems and user-interfaces of 68000-based micros. Is there any information commercially available about the operating systems used in such machines? I would be particularly interested in descriptions of Gem, Windows, the Mac Operating System, Intuition and so on, together with details of system-calls and assembly language programming.
Peter Vogt, Rutland, Leics.

I dealt with the problem of Atari ST (Gem) documentation last month - details for software developers cost £400 from Atari, and they're pretty heavy going - you might be able to get some Gem documentation from Digital Research in the States, but don't expect bedtime reading. A number of technical books on the Atari are being prepared as I write, but I have no specific details to hand.

The Amiga looks as if it is going to be rather better documented, if Commodore's publication list can be believed. The user-interface here is called Intuition and the underlying operating system is Amigados. 'Intuition: the Amiga User Interface' is the snappy title of Commodore part number 327-267-01. There are three guides to Amigados: the 'Technical Reference' (327-266-01) 'Developers Manual' (327-265-01) and 'User's Manual' (327-264-01). The dedicated can also peruse the 'ROM Kernel Manual' (327-271-01) - if Commodore can be persuaded to part with them. Prices have not been decided as I write.

Addison-Wesley are about to publish the definitive guide to the Apple Mac. 'Inside Macintosh' should be available by the time you read this, at a definitive price of £25-30.

About the only 68000-based operating system you can read about right now is the much-maligned QDOS, used on the QL and new machines planned by Sinclair and others. The best books on QDOS - which is actually quite a nice system - are the QL Advanced User Guide, published by Adder, and the Sinclair QDOS Companion from Sunshine Books.

I don't know of anyone using Microsoft Windows on a 68000-based system. Books on most Microsoft products usually arrive quite quickly, once the software turns up...


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