Users who are not sure about electrical connections should not proceed but should contact their local dealer for advice.
My Archimedes 310 is something of an oddity. Instead of using a normal monitor, my machine is connected to a familiar household object - the television set.
This is a standard Sony 14in Trinitron colour model, which happens to be working as an RGB monitor. By pressing the video/TV button on the television set, it switches between normal television programs and computer output.
The picture is not quite as crisp as a standard monitor, which has a smaller dot-size, but the colours are slightly brighter. Even in 132-column modes, like mode 16, it is quite possible to read text. Only special-purpose modes, such as those supplied with Impression are too big to fit on the screen. In other respects, it is the same as using a standard RGB monitor.
This is all achieved via the Scart socket on your television set. But, sadly, it isn't quite that simple. If you try to use a standard Archimedes lead to connect to the Scart socket on a television, you will normally get a blank picture, even with the television switched to video input. The special lead needed is the same as a standard lead, with the addition of a connection from pin five, at the Archimedes end, to pin 16 of the Scart plug (see figure one).
The Archimedes A310 requires an additional change, in order to obtain the required switching signal in pin five. This is achieved by adding links between two easily-reched points.
There is a standard way to send audio, video and RGB-video signals between TVs and other equipment - this is known as the Scart standard. The archimedes RGB and synchronisation signals conform to this standard and an ordinary monitor is connected in this way.
However, the problem with using the Scart socket on a television is that, unlike monitors, most television sets need a signal between one and three volts on pin 16 of the Scart connector. This signal switches them from the composite video mode, used by most video recorders, to the RGB mode used by the Archimedes output.
Scart pin 16 is not connected in standard Archimedes monitor leads and, to make matters worse, the 13mA current required on this pin is not available from any of the pins on the built-in Archimedes video socket.
So, to supply the signal you have to use pin five of the video socket, which is not normally in use. However, the signal from pin five on an A3000 is not strong enough and this is why links are needed. Link 25 inside the A3000, or link 11 in the A310, allows pin five to be connected to various synchronisation signals.
Take the case of the A3000 first. In most machines this link will not be closed, but in the A3000 it is easy to do so - just ensure that the jumper is over both pins. If you have already moved things round inside your A3000, you may need to move the link back again.
The A310 presents a slightly different situation. Closing the same link will not produce a stroing enough signal, as all the output signals are at very low levels - half those of the A3000 or 400 series. This means that you have to join the link from a source that provides 5V to the centre of link 11 by soldering a piece of wire between the two. A good example of the former is the end of resistor R44, nearest to the back of the machine. Line 11 is then connected to pin five of the video socket.
For both the A3000 and the A310, a resistor of 270 Ohms is used to limit the current in the lead to the appropriate level (see figure one). You may prefer to remove the disc drive in the A3000 to gain access to the link, but it isn't essential. Access is easier in the A310.
So what about the lead itself? Some makes of standard Archimedes lead have spare cores in the cable, which may be uwed to make the additional connection in the lead. However, this is certainly not true of Acorn leads, and it may well be necessary to make your own.
All you need to make the lead can be obtained from Maplin, whose current catalogue has a description of the Scart standard. The average costs for the simple Archimedes RGB lead is around £3.23.
|Fig 3 - Mono Video To Scart|
Alternatively, you may be able to contact a dealer who can supply a lead and will also modify uyour machine if you wish.
The leads should be as follows. An RGB lead should be made with screened cable - for example, the Maplin XR26D six-core cable. Video-only leads should use 75 Ohm coaxial cable - for example, the Maplin XR88V miniature Coax. Finally, the audio leads should use screened cable.
Scart plugs are made-up by soldering or crimping the pins on to the writes first, folding the metal tabs around the wire ends and then clipping them into the plastic core, before assembling the plug. A point to note is that pin 21 on the scart plug is a bit of a fallacy. This pin is actually wedged between the plastic core and the metal shield around the plug - it sounds strange but it works. Cables will function without the screen, but this may lead to radio interference and static-damage to your computer or television - you have been warned!
Once you are ready to use your lead, just plug it in and select video input on your television. On some sets, you just press a button - on others, you have to select a particular channel.
Some sets provide no switching options and the Scary connector does provide pin eight - this connection should allow the computer to takeover the television, ignoring the channel or any other selections. This needs a supply of 9.5V or more - voltage which is hard to come by in an A3000. However, you could be lucky - the aforementioned Sony switches at just 7V. If you are determined, you may get a current of 9V from the serial chip, since very little current is needed at this pin. However, this may not be wise and, anyway, not everyone has the serial chip fitted. It can also be generated from the 5V supply by a simple voltage-doubler circuit similar to that used by the serial chip. Another option is to use a 9V battery.
Finally, let's not forget about the Beeb. It is equally possible to connect a television set to a BBC micro and figure two shows the connections As you can see, several resistors need to be put into the Scart lead to reduce the high output of the BBC to an appropriate level. This is easily done with the Scart plug, with the resistors connected directly to the pins of the plug. It may be wise to put some sleeving around them to stop unwanted connections.
I have been using my television in this way for many years without any ill effects. If you try it, do bear in ming that many commercial leads do not contain enough resistors. This can pose a hazard to some televisions, and impose an unnecessary load on the Beeb's power supply.
Some TV sets do not supply all of the required RGB Scart connections. A small number of sets use the Scart connector exclusively for sound.
In this case, it will be of no help to oyu (other than to connect the sound to) and you will have to use an RGB-to-UHF adaptor, available from most dealers. this will, however, give a lower picture quality than that obtained from a video or RGB link./
Some sets only have video input and output from their Scart connectors, and no RGB connection. In this case, the much simpler connection is shown in figure three.
This will give no colour from the Archimedes and no colour from the BBC unless the connection at link 39 is made. this link is quite hard to reach and has to be soldered; it is important to get someone with soldering skill to make it.
Any dealer experience in upgrades or repairs to BBC machines should know how to do this. The colours are less sharp than those obtained using an RGB.
The Scart connector can also be used to send audio signals to the television. This means that you have more than one write going to your Scart plug and figure four shjows how to connect the headphone socket on an Archimedes, or the audio-output from a BBC, to the Scart.
BBC users have more of a problem - audio output is only available from the connection at PL16 at the front-left of the machine. This can be brought out to either the Econet DIN socket, or to a new jack socket in the unused hole in the back. (This is found next to the label 'RST SW' on the main board.)
Both of the connections from the Scart plug, as shown in the diagram, should be connected to the BBC output, perhaps via a single cable. Again, your dealer should be able to advise on any relevant connections.
Reprinted from BBC Acorn User, December 1991