Section 2: Upgrades and Expansion.


2.1) What are the memory limits of the various Acorn machines?

For the most part the 8 bit machines were limited to a maximum of around 256k of memory. Although various expansion systems including second processors effectively meant the upper limit was about 1 Meg. (Although I have been told of a second processor with 4 Meg of memory in it. ) For the early 32 bit machines the upper limit currently is 16 Meg of memory. Not all of the range of machines are capable of this however and the list looks something like this :-

   A3xx   -   8 MB maximum through third party, 1 MB official limit.
   A4xx   -   8 MB maximum through third party, 4 MB official limit.
   A4xx/I -   8 MB maximum through third party, 4 MB official limit.
   A3000  -   8 MB maximum through third party, 2 MB official limit.
   A540   -  16 MB maximum.
   A5000  -   8 MB maximum through third party, 4 MB official limit.
   A4     -   4 MB official limit.
   A30x0  -   4 MB official limit.
   A4000  -   4 MB official limit.
   A5000  -   8 MB official limit. (alpha variant of the A5000)
   A7000  - 128 MB + memory on mother board. (Tops out at 132 MB and requires a
                   128 MB SIMM to do it.)

Newer RiscPC machines have an official upper limit of 256 Mb, plus 2 MB of VRAM, on all models. However this does require you using 128 MB SIMMS which are currently not very common...

Return to the index.

2.2) What is a second processor and what second processors are there?

A second processor was the generic name for a range of parasite processors that could be linked to Acorn's 8 bit machines via what was called the `Tube` interface. Basically the host machine became dedicated to handling the Input and Output while the second processor would do the higher level functions (like running your programs). The second processor ran asynchronously to the host processor allowing incredible increases in execution speed for programs. A wide range of processors were supported this way allowing Acorn's eight bit range of machines to remain viable and useful for much longer than their technology would suggest.

The Second Processors that existed are :-

Return to the index.

2.3) Can PC VGA & Multisync Monitors be added to an Acorn machine?

It depends on what monitor you have, and what Acorn machine you have. There are two main types of PC VGA monitors out there...

  1. Fixed Frequency
    These monitors will only display video signals with certain line and refresh rates. They will typically only display CGA / EGA / VGA modes.
  2. Multi Frequency
    These monitors will display any video signal within a certain range, typically 30-50Khz line rate and 50-80Hz refresh rate.

Type 1 almost always require separate syncs as the monitor uses the polarity of the syncs (mainly positive going negative or mainly negative going positive) to determine what the line and refresh rate should be.

Type 2 vary. Some require separate syncs (vertical and horizontal) and others will work with composite syncs (vertical and horizontal EOR together).

As PC monitors typically start at a line rate of 30Khz compared to the TV broadcast modes (mode 12 etc.) that have a line rate of 15Khz, VIDC has to do more work to obtain a 30Khz line rate. This means that you computer will slow down slightly if you use a 30Khz+ line rate monitor. If you have an ARM 3 fitted such slow downs will probably be negligible.

Now, depending on what type of Archimedes you have depends on what type of monitor you can use.

However to connect an older machine (A3000,A300,400 series) to a VGA monitor, you will need a means of connecting the standard 15-pin VGA plug to the 9-pin socket in the computer. A number of companies sell 'converters', but all this boils down to is a lead with the following configuration :-

Text diagram of VGA to Arc 9 pin conversion

In conclusion, apart from the A5000 and newer machines, no computer as standard can drive either a fixed frequency or multi-frequency PC monitor in all of the Archimedes modes satisfactorily. However VIDC enhancer boards can be bought to upgrade an Archimedes series machine to handle the needed timing and signals.

Return to the index.

2.4) Are there any Acorn cards for IBM PC or compatible machines?

Yes. Three cards in total :-

However, to the best of my knowledge none of these cards are commercially available.
Return to the index.

2.5) What is a VIDC enhancer? Will I need one for my new multisync monitor?

A VIDC enhancer is basically a clock change for your VIDC. Most Arcs (bar the A540 and newer machines) have 24 MHz VIDC chips installed in them. A VIDC enhancer increases this to 36 MHz allowing much higher resolution screen modes to be displayed on your Arc. (800x600x16 or SVGA standard becomes available.) You do not need one to use a Multisync monitor - the standard VIDC handles that just fine. However having a VIDC enhancer is only really useful if you do have a Multi-sync monitor.

A "build-it-yourself" VIDC enhancer is available from http://mic2.hensa.ac.uk/local/riscos/drivers/autovidc.arc?&fn=autovidc.arc and Maplin Electonics supply 9-15pin converters for around £6.00

Note a VIDC enhancer is unnecessary and incompatible with the RiscPC range of machines.

Return to the index.

2.6) What configuration of serial cable should I use for modem work?

Here follows a diagram of the necessary connections for common terminal programs to work properly. They are as far as I know the informal standard agreed upon by commercial comms software developers for the Arc.

Pins 1, 4, and 8 must be connected together inside the 9 pin plug. This is to avoid the well known serial port chip bugs. The modem's DCD (Data Carrier Detect) signal has been re-routed to the Arc's RI (Ring Indicator) most modems broadcast a software RING signal anyway, and even then it's not really necessary to detect it for the modem to answer the call.

Text diagram of 9 to 25 pin cable

Of course you can connect the Modem pin 20 to any one of pins 1, 4, or 8 on the Archimedes plug, as they are all connected together anyway.

Chocks Away Extra Missions (the flight simulator from 4th Dimension) suggests that the serial cable be wired as above except that pins 1-4-6 are connected together and the modem's CTS (pin 5) be connected to the Arc's pin 8 (ie the connections at pins 6 and 8 be swapped over at the Arc's end). This has been tried and it also seems to work fine.

However newer Arc's like the A5000 have come out (and indeed the occasional A310) with a `corrected` serial port. This newer serial port operates as it should and is directly compatible with standard PC cables. Older comms software about do not take this in account and assume that you have a cable patched in the manner described above. If you do not use such a patched cable on these `fixed` serial ports this software will generally fail to work completely. (Usually hardware flow control fails.)

With the advent of the Risc PC a standard PC cable is advised.

Return to the index.

2.7) How can I get unfiltered sound from an Acorn machine?

All Acorn machines are equipped with a sound filter designed to remove high frequency harmonics from the sound output. However this does cause a muffled feel to the sound as on some machines the filter is a little too excessive and it filters out valid frequencies. Also the filter is optimised for 20.833 kHz output and has less desirable results when the output rate is changed. Accordingly people who do audio work often want to bypass the filter.

On all machines bar the A3000 there is the Internal Auxiliary Audio Connector (usually called link LK3), which can be easily plugged into to provide the unfiltered output. This connector has 10 pins on it and is usually found near the headphone socket on the motherboard. The pins are :-

        1       Unfiltered Left
        2       Ground
        3       Filtered Left
        4       Ground
        5       Auxiliary Input
        6       Ground
        7       Filtered Right
        8       Ground
        9       Unfiltered Right
        10      Ground
Simply hook into the Unfiltered outputs.

On an A3000 you need two 10uF 16V ALEC capacitors. Look for chip LM324 (IC39) and hook the capacitors like this :-

        Pin 1 --> --|+ |--- Unfiltered Left
        Pin 2 --> --|+ |--- Unfiltered Right
The Risc PC & A400 machines have a connector similar to the A5000.

There are several caveats to this procedure. Opening your machine may void your warranty and most definitely should not be attempted if you are unsure of the procedure. Do not unplug/plug the unfiltered audio output while the machine is turned on, by bypassing the filter you also bypass the normal protective circuitry for the audio output.

Finally you will hear higher harmonics present in the audio signal so you will need to connect the signal to a filter of some kind to reduce this extra noise.

Return to the index.

2.8) Can I connect a SCART monitor to my Acorn machine?

If you have an older Acorn machine with a nine pin video socket, then yes and here is the wiring diagram :-

Text diagram of the pinouts for SCART and Arc 9 pin connectors

A SCART connector is also known as a Euroconnector or a Peri-Television connector.

                Arc             SCART

Case            | ------------- 21              Case
Red             1 ------------- 15              Red
Green           2 ------------- 11              Green
Blue            3 ------------- 7               Blue
CSYNC           4 ------------- 20              Composite video input
Ground (0V)     6 -+---------+- 13              Red ground
Ground (0V)     7 -+         +- 9               Green ground
Ground (0V)     8 -+         +- 5               Blue ground
Ground (0V)     9 -+         +- 13              CVBS video ground
Ideally each ground wire should be linked to a separate Arc pin. Also, depending on your SCART monitor, pin 16 may need a +5V input to it. Unfortunately the Arc 9 pin socket does not provide a +5V output so this will have to be sourced from somewhere else.

If you have a newer Acorn machine, with the 15 pin high density video socket then you need this kind of wiring :-

Text diagram of the VGA pinouts for a SCART connection

Notice the two resistors. Also notice that the HSync output (pin 13) of the 15-way plug has to be connected to the ID[0] input (pin 11) of the same plug. (Be aware I have no direct confirmation that this wiring works.)

As is usual care must be taken when doing this procedure. Older Acorn machine did not have their VIDC chips fully buffered and unplugging/plugging cables from the video socket while the machine is turned on can cause damage to the video circuitry.

Return to the index.

2.9) How do I make a Null modem cable?

For starters you will need soldering skills and the necessary components. Namely cable, connectors (9 pin female D-type), a soldering iron, solder and the will to use them. All of these items, bar the will, can be found down at the local electronic components store. Assuming you have them all then you will need to decide what kind of machines you are hooking together.

There are three cases and I need to define a few terms.

This means that if you are connecting your Acorn machine to a non Acorn machine then generally treating the foreign machine as a RiscPC, in terms of serial handling, will work. There are exceptions, Macintoshes in particular have had non-standard serial ports and may require further research before you can create a cable for them.

The cases are :-

Note that most PC compatible machines have 25 pin D type male ports for their second COM port. You have two options in this case - either re-wire the cable for the 25 pin port or you can buy a 9 to 25 pin converter plug. Either solution works well. Here are the relevant pins for the 25 pin port :-

  Pin No.  Function
  -------  --------
     8       DCD
     3       RX
     2       TX
     20      DTR
     7     GND (0v)
     4       RTS
     5       CTS
Return to the index.

2.10) What are StrongARM dipswitch settings?

The StrongARM card has a set of dipswitches on it that control the clock speed of the processor. These can be altered but there are some important caveats to mention first :-

Without cutting the tracks, the dip switches can be used to slow the processor down from 202MHz to 88MHz if required. Each switch sets one bit making sixteen possible speed settings. This table summarises those speed settings :-

   Setting  Switch  Speed (MHz)
   ----------------------------
      0      uuuu       88.3
      1      uuuD       95.6
      2      uuDu       99.4
      3      uuDD      106.7
      4      uDuu      143.5
      5      uDuD      150.9
      6      uDDu      161.9
      7      uDDD      169.3
      8      Duuu      191.3
      9      DuuD      202.4
     10      DuDu      213.4
     11      DuDD      228.1
     12      DDuu      242.8
     13      DDuD      257.6
     14      DDDu      276.0
     15      DDDD      287.0

   u = up, D = down, MSB to the left.
(Thanks go to Dr. Nicko van Someren for this information.)
Return to the index.

Return to Index page
Comments and Feedback