A Drive More Modern

By Robert Sprowson

Originally published in EUG #28

Having given Gus a headstart with a disk drive power supply recently, I thought I might set the proverbial ball rolling amongst EUG members with this letter.

A few issues back, I asked Gus how the Elk makes use of a 3.5" disk drive. i.e. was it with an 8271 controller as with the BBC B or the later 1770 like a Master 128?

He interested me sufficiently enough to go out and buy one. As regard the old problem of adding a more modern drive to a BBC series machine, I shall attempt to pass on my experiences so far:

Fortunately the hardware designers of disk drives thought in advance when designing the new 3.5" drives and made them pin for pin compatible. All that you need to do is build a lead, and this can be done by:

  1. Cutting the card edge off a 5.25" drive and crimping on a new 34way IDC plug. Alternatively, if you wish to use the 5.25" drive again, then you can start from scratch, requiring 2 x 34way IDC connectors and a length of ribbon cable. It should not be longer than a metre to prevent data corruption due to electrical noise.
  2. If you wish to use both 5.25" and 3.5" drives at the same time, you need only crimp on a second connector daisy-chain fashion somewhere along the length of existing cable. Then, move the option links on one of the drives so that both drives don't try and read the disk at the same time. See the chart enclosed on how to do this - noting that the resistor pack will need to be unplugged on the drive described as only one pack is allowed per run of cable.
  3. If you wish to use both 5.25" and 3.5" drives at the same time but don't want to risk opening up the drive case, then you'll need to carefully cut the ribbon cable on the two drive select lines (pins 10 and 12) and cross them over as shown - see diagram enclosed. This will mean that both drives can be selected (as far as the drive is concerned) as a drive zero and two, but to software will appear as 0 and 2 then 1 and 3. The cables that run around inside the case of a PC use this idea of twisting the wires and so a cable from a PC will do fine (and these are available ready assembled from PC suppliers).
  4. The next problem is timing. The older 8271 disk controller that most use has problems accessing some 3.5" drives and will report a drive fault when loading large files. Try *FX255,207,0 then press BREAK.
    or *FX255,223,0 then press BREAK
    or *FX255,239,0 then press BREAK

    These alter the drive step speeds, etc ... and may fix the problem.
  5. Failing that, you'll need to get a 1770 upgrade card (the 1770 is the disk controller supplied in the Master) which will definitely cure the problem. These cards appear in BBC disk magazine "For sale" columns occasionally so you may get lucky.

I hope that this has answered any queries you may have had. Now, if you're after a cheap brand new SONY 3.5" drive, I got mine from CL Computers for £15.95 + VAT + P&P, you can telephone them on 01274 664110 and they take plastic over the 'phone.

Note that mine got bumped in the post and the spring that opens the metal cover on the disk got knocked out. Fortunately this just clips back in place. The option link at the back of the Sony drive is for DRIVE0 and DRIVE1 selection, and the minicom plug has the same pin outs as a disk drive power plug, namely 5V,0V,0V,12V.

Robert Sprowson, EUG #28