1) 3.5" discs are a fraction of the cost of 3" discs and are easier to
buy.
2) 3" drives are no longer made and even second-hand ones are becoming harder
to find and more expensive. Drives wear out in time like any other mechanical
device and you could be left with valuable data on 3" discs which can't be read.
3) Files on 3.5" discs can be more easily transferred to PC and other formats.
It makes equally good sense to fit 3.5" standard drives to other Amstrad PCWs such as the PcW9512+, PcW9256 and PcW10, because these machines use a non-standard 3.5" 720K drive with different connections. The same drives are also used on some of the earlier Amstrad PCs, and were made by Citizen. As far as I know they are not available new and are belt-driven. Fitting a replacement belt is covered in the Repairs section of this site.
PCWs drives can be fitted with either 720K or a 1.44M drives, but will only format to 720K. Unless you have a 720K drive by you, it is better to use a 1.44M drive because they are easier to get and are usually cheaper.
NOTE. A 720K drive is mandatory when adding a drive to an Amstrad PC1512 or PC1640 and some other early Amstrad PCs with 5.25" drives.
To fit a standard 3.5" drive to a PCW a power supply, a conversion cable and a ready signal are required. There are also other practical considerations, such as whether the drive is to be fitted internally or externally, or as an A: or a B: drive. Only broad principles will be discussed here. I hope to make proposals covering specific machine available soon.
Power supply connection
The power board of the PCW delivers enough power for the machine as designed,
plus a little more to allow for some peripherals but if an extra drive or two
is added it may need some help. The screen of the standard PCW tends to distort
a little at times, but if the distortion becomes excessive it is a sure sign
that the power supply needs some help.
3" drives have a 4-way power connection. The two inner pins are to ground, the outside pins are 5 volts and 12 volts. The motors use 12 volts, the 5 volt supply is to drive the electronics. Standard 3.5" drives also have a 4-pin connection which is the same as for the 3" drive, but only uses 5 volts for all functions. The connector can be used directly, as the 12 volt pin is not connected and is ignored by the 3.5" drive but as 3.5" drives use only 5 volts, the power supply may need to be modified or a separate power supply used. Failure to address this could lead to failure of the power board.
One solution is to use a simple electronic circuit at each 3.5" drive based on the 7805 voltage regulator chip to convert the 12 volt supply of the computer to 5 volts and add it to the existing 5 volt supply. Alternatively a 5 volt power supply of the type that plugs into a mains socket can be used as a simple option but it should deliver around 1 amp and be voltage regulated.
Data connection
The 3" drives have a 26-pin data connection while the standard 3.5" drives have
a 34-pin connection, so they obviously cannot be connected simply. Although
the Citizen 3.5" drive has a 26-pin connection it still cannot be connected
simply by a 26-way cable as the pinouts are different.
However, all floppy drives use a similar method of communicating with the computer, based on the Shugart system, and signals on the pins are much the same, although pin positions and numbers differ. Thus a cable can be made which will enable the computer to communicate with a different type of drive, although it may sometimes need a little ingenuity.
The ready signal problem and solutions
On 3" drives pin 26 of the data connection carries a ready signal which is necessary
to enable the computer to recognise and use the drive. This signal is not available
on the equivalent pin (34) of most 3.5" standard drives (although it used to
be) and so a suitable signal has to be devised to replace it. Some drives generate
a suitable signal on the PC board which can be put on pin 34 by soldering a
connecting wire but this operation demands great skill and is likely to wreck
the drive. It also makes the drive into a special, which will lead to problems
when the drive has to be replaced in the future. A better solution is to provide
a small electronic circuit which converts signals from other pins into a suitable
ready signal and puts it on pin 34. I haven't cracked this yet although I'm
still trying and would welcome help.
My solution is to connect the signal from INDEX to pin 26/34 or rather the cable which connects them. This works with some reservations. It can only be used when an A: and a B: drive is present and when using Locoscript both drives must have discs in or Locoscript hangs. It is therefore OK for adding a B: drive to a PCW8256 for example, but will not work when replacing the 3" A: drive of a PCW9512 with a 3.5" drive. Also it will not work with DISCKIT, as when you remove the discs from the drives as directed, DISCKIT insists that there are still discs in the drives. I get over this by connecting a 5 volt supply also to the 26/34 cable strand via a small push-button switch. When DISCKIT asks for the discs to be removed, press the button and any key. This method works very well for me and seems to work with any 3.5" standard drive. I hope that it will help you.
Stop Press
I seem to have discovered a way of using the above method with a single drive
machine such as a PCW9512. Just connect pin 4 (Drive select 0) to ground. This
seems to make the machine believe that two drives are fitted although there
is only one. In Locoscript both A: and B: drives are shown on the management
screen with identical files. You can work from either- it doesn't seem to matter.
It doesn't seem to affect the working of the machine in any other way. Spooky
or what? only wish I understood it myself.
Ron King, Andover, England
Email address... ron@king27.freeserve.co.uk.