(HOW TO SPEED UP YOUR PCW)

By

EMMANUEL LARDO

I. To speed up the PCW (tested with PCW10 and PCW 8256-8512), you will need:

-A 12Mhz Oscillator

-A 50 to 65Mhz Oscillator

-A 74AC04 I.C. (+ socket)

-A 74F04 I.C. (+ socket)

-A 330ohms 1/4watt resistor

-A 330nf bipolar capacitor

-A 6 to 8Mhz Z80 CPU (+ socket if necessary)

-A VGA female connector + cable.

NB. It is recommended that the good quality Gold plated I.C.sockets be used where possible as these stop the oxidation that sometimes appears on the low cost sockets, due to heat and etc.

II. Explanations.

The 12Mhz oscillator is to replace the 11Mhz oscillator for the Amstrad printer and boot I.C. (40026), because at 11Mhz this I.C. is apparently too slow to send the boot code in time with a faster main clock (the inconvenience of this manipulation is a possibility of a malfunction in the Amstrad printer data transmission? Never tested because I use // printer only).

The 74AC04 is a faster replacement to original 74HCU04 which is unable to run faster than 40Mhz.: You have to remove the original I.C., the original 32Mhz oscillator, and also capacitors, inductance and resistors connected to it. (Note: you can also use a 74ACT04).

The main clock is normally 32Mhz, divided by 8 by the gate array to generate the CPU clock, by 640000 to give video Vertical sync and by 2048 to give video horizontal sync. So if you change the main oscillator you also alter the video timing. With 64Mhz, you obtain 31Khz Horizontal Sync but with a 100 Hz refresh rate (Vertical Sync). Some VGA-SVGA displays support MultiSync and a few can even handle Horizontal Sync as low as 24khz (thus with 50Mhz main clock). Do remember that not all VGA displays can do that and can be damaged if the sync is too high or too low. Anyway, if you hear an unusual noise from your monitor, cut immediately, power down and never try again with the same monitor. It is better to use a monochrome monitor, for the pixels display is perfect, but even with a colour VGA, the precision is far better than with any TV-like video display.

In fact, the Amstrad gate array can run faster than 65Mhz but I do not counsel it, because for example the CPS serial interface uses a SIO clocked with the CPU clock. So at more than 4Mhz, you must replace it with a 6Mhz SIO. It can run faster, but not faster than 8.125Mhz, and it is also the case for many old computer I.C.'s. The second reason is that at 65Mhz the Vertical Sync is a little more than 100Hz and this is nearly the limit for many old VGA monitors.

The main problem with some older NMOS I.C.'s is that at higher speed, the power dissipation become too high. This is the case with Amstrad 40026 but much more crucial with the old 150 nanosecond 41257 nibble mode memory modules. So you must provide a ventilator or/and a piece of metal with special thermal grease to help them to stay cold and alive.

As sync signals are mixed on the PCW, and most VGA displays need separate ones, the 330nf capacitor and the 74F04 serves as a simple separation filter for the Vertical Sync. It is not necessary to eliminate the PCW Vertical Sync from the VGA Horizontal Sync. Another possible problem with some VGA displays is that the polarities of the PCW sync signals are inverted regarding VGA ones, but most VGA accepts normal and reversed polarity for sync signals. If it is not the case with your monitor, you just have to add another 74F04 in order to reverse the polarity of Vertical and Horizontal sync before connecting to VGA.

You must use only 74F04 to manipulate PCW video signals, especially with a main clock of 65Mhz, as it does not work correctly with HC, HCT, LS, AC, ACT types, (You can also try with S, AS, ALS types). I do not know why, but at 65Mhz, if you use HC, AC or ACT types, you will have only 25 lines displayed, but with a Vertical Sync of 124Hz.

It can be seen as cruel, but if your CPU is soldered into the motherboard, the only elegant way to realise this update is to sacrifice it by cutting high each pin, throw the body of the animal away, de-solder each little metal leg and finally clear the solder from every hole with a toothpick made of wood. Then solder a high quality 40 pin socket at the scene of the murder. You will see, it is just a question of habit...

When all this surgery is done, all the machines respond much faster, even the internal CP/M clock (but as you know, time is just a question of convenience), and you now have escaped from the risk of the Amstrad TV-screen users myopia. Note that if you are a game-addict you can adapt this update in order to stay able to switch (preferably before start-up) between normal and fast clock, but don't forget to maintain the possibility to re-connect the normal Amstrad video output to a TV compatible monitor.

III. Connections.

(PCW8512, PCW10)

-Replace I.C. 74hcuo4 (I.C.103, PCW10 I.C.5) by 74AC04.

-Remove L101, c126, c127, c125, (PCW10 L1, c8, c9, c10, r34), and the 32Mhz oscillator.

-The oscillator in, is then connected to the gate array pin 61. You have to connect a cable to the only free hole between it and I.C.103 pin 4. (On the PcW10 it is gate array 72, but there is no hole thus it is better to connect the oscillator in to I.C.5 pin 5)

-It is easier to find a 4 pin oscillator with ground, vcc and osc out. Connect the oscillator out pin to gate array pin 61 (or I.C.5 pin 5 on the PcW10). Don't forget to connect the power pins of the oscillator. (You can also use a two-pin oscillator, but you have then to use another 74AC04 and filters to realise a TTL compatible oscillating signal.)

-Replace X701 (XL3 on the PcW10) with a 2 pin 12Mhz oscillator.

-Replace the CPU.

-Connect video out (pin 3 of cp101 (cp11 on the PcW10) to one input of the 74F04. Then connect the out, to another in.

Connect a 330ohm resistor (used to attenuate the signal) to the output and connect the other side of the resistor to VGA 'Green' (pin 2).

- Connect sync out to two different inputs of 74F04.

- Then connect one output to another input of 74F04. The output is then Horizontal Sync and must be connected to VGA H (pin 13).

-At the second output, connect one side of 330uf bipolar capacitor (other side grounded). Then connect this out, to another in of F04. The out is then Vertical Sync and has to be connected to VGA V (pin 14) -pins 5, 6, 7, 8 and 10 of VGA must be grounded. Leave the other pins free.

Using a Sprinter board with PCWBOOST.

On the Sprinter board itself, you have to remove two capacitors, one connected to 74HCT00 (10 nf) and one to cpu /iorqst signal. You have, of course, also to replace the NMOS 8Mhz Z80 by a CMOS one able to run at 12Mhz or more (the quickest Z80 version runs at 20Mhz).

Timing is crucial to synchronize precisely the Sprinter with the Amstrad gate array. The Sprinter uses the inverted Osc signal normally present at the expansion port, divided by four by a 74F74 (so 8Mhz at normal speed). So with "PCW BOOST", you also have to provide this inverted osc signal (on the PCW 8512, pin 1 of HCU04 must be connected to osc in. On the PCW10, it is pin 5)

The Sprinter is able to run with a 48Mhz PCW main clock at most. To make it work, your PCW motherboard replacement for the 74HCU04 must be a ACT04 (not a AC04). It is also imperative to replace 74F74 by a 74ACT74 on the Sprinter itself. (I further replaced HCT32 by ACT32, HCT174 by F174, the two LS133 by S133, but it seem not to be really necessary).

It is important to notice that I own a Sprinter version 2 with 70ns SRAM and no extended memory, so I do not know if it works with any other Sprinter versions.

What is the gain:

I used two simple benchmark tests to compare:

Firstly I measured the time to load a DTP programme:

PCW standard: 1 minute 16 seconds.

PCW standard plus Sprinter: 43 seconds.

PCW Boost 50 Mhz: 43 seconds.

PCW Boost 65 Mhz: 41 seconds.

This it was not a really good test because of the disc timing bottleneck, although it does show that the Sprinter substantially accelerates the PCW, and that it is comparable with a PCW with a 50Mhz main clock (so a little more than 6Mhz for the CPU clock).

To understand how the Sprinter can accelerate the PCW, it is important to notice that the Amstrad gate array inserts a huge amount of 'wait states' to have the time to output video data, this behaviour significantly slows down the raw CPU speed. The Sprinter CPU plus SRAM seems to be able to bypass it (maybe by transferring CP/M and buffers into SRAM at start-up in order to continue to work, and at higher speed, even when the standard memory on the motherboard cannot be accessed because of the video data to be displayed).

The second test is simply the time to copy CP/M from one user area in virtual memory to another, with crc verification:

PCW standard: 12 seconds.

PCW standard plus Sprinter: 7 seconds.

PCW BOOST 50 Mhz: 8 seconds.

PCW BOOST 65 Mhz: 6 seconds.

PCW BOOST 48 Mhz plus Sprinter: 5 seconds !

So the best set-up to speed-up your PCW is to combine the Sprinter and an accelerated main PCW clock. It is a pity that the Sprinter seems not to be able to run faster than 48Mhz (so with a Sprinter CPU clock of 12Mhz, video V sync= 75Hz, H sync= 23437,5Hz - so with the need of an old multisync monitor: for example a "NEC multisync gs"). This combination leads to a PCW nearly three times faster than the original one, without the need of dangerously pushing the I.C.'s to their upper speed limits. The disadvantages comparing with "PCW Boost" only are the costs, more things to solder, and no VGA-like compatibility.

Note: This add-on has been tested for hours without any problems. It is best to improve reliability by using a ventilator and a correct PC power supply, instead of the Amstrad's not very efficient one.

PLEASE NOTE. All these modifications are done entirely at your own risk. If you get it wrong then you can only blame yourself.