To construct this external floppy drive cable for your +3 you will require:
- Approximately 14″ of 0.05″ pitch ribbon cable.
- One 34 way IDC card edge connector.
- One 34 way IDC ribbon cable socket.
- plastic keying pins for connectors if not included.
- One 1N4148 signal diode.
- Heatshrink tubing (must fit over the body of the diode).
Modern PC floppy drives do not provide a drive ready signal which the Spectrum requires. The diode is used to hold the Spectrum's READY line low whenever the external drive is selected. This is better than simply shorting the line to ground as it still allows the the system to detect that the internal drive is empty and therefore doesn't cause the +3 Loader to hang if there is no disk in drive A:. In other words, you can still load tapes from the menu loader without having to remove the cable or manually toggle a switch.
Attaching the diode reqires a small amount of slightly fiddly soldering, so an alternative solderless solution is also described.
Guide to floppy drive connectors
A selection of different 34 way female IDC connectors are shown below. These have been reclaimed off old cables to illustrate the types so please excuse the discolouration, scratches, etc. on some of them.
Connector with polarising bump
Drive connector with bump and pin 3 blank
Black version of above
Motherboard connector with pin 5 blank
Edge connector with key in second slot
Side view of drive connector
Some connectors have a polarising bump on the side which fits into a corresponding slot on the drive's male connector. The bump is adjacent to pin 17.
An additional method of polarisation is provided by blanking off holes in the female connector and having the corresponding pin missing on the male header. This keying is sometimes moulded into the plastic body of the female connector but can also be achieved by fitting small plastic plugs into the hole.
The connector which mates with the floppy drive should have pin 3 blanked off. Note that some connectors will have the pin 5 position blanked instead; these are keyed to mate with the floppy header on a motherboard to prevent people fitting PC twist floppy cables back to front and therefore won't mate with the floppy drive.
The connector on your 3½″ floppy drive should have a polarising slot adjacent to pin 17, and pin 3 should be missing. On some drives both sides of header may be notched, in which case extra care must be taken to insert the cable in the correct orientation.
Polarisation and keying of the card edge connector type is achieved by fitting small plastic keys between pins in the card slot. For floppy drives these are usually in the second slot (i.e. between pins 3/4 and 5/6). The pinout of the Spectrum +3 external drive connector has the slot at the other end but a standard drive connector can be used by crimping the connector on the other way around as described later.
For a Spectrum +3 eternal floppy drive cable it is possible to start with a suitably long PC floppy drive cable, if you have one spare, and cut it down so that you have a length of ribbon cable with a single floppy drive connector. This saves you having to purchase and crimp the floppy drive connector, just be aware of the issue with connectors keyed to only mate with motherboard headers so that you don't assemble a cable and find it won't fit.
I strongly recommend using connectors with the proper polarising and keying features when constructing a cable. I would personally consider it mandatory for anyone supplying cables to end users.
Assembling the cable
An Insulation Displacement Connector (IDC) is terminated onto a ribbon cable by little blades cutting through the insulation and gripping the stranded wire inside. The blades are staggered to connect the two rows of 0.1″ pitch holes to the 0.05″ pitch cable.
You must ensure that the wires in the ribbon are aligned properly with the blades before crimping the cable down. The back part of the connector may have notches to help with this. The wire highlighted red should be placed in line with the blade for pin 1 on the connector.
The connector may have a small arrow moulded into the plastic to indicate which hole is pin 1.
It doesn’t matter which side of the connector the cable is inserted from so long as the red wire is at the correct end. You may prefer to have the cable enter from the other side depending on where you plan to place your floppy drive when in use.
Proper crimping tools are available quite cheaply to attach IDC sockets to ribbon cable but if like me you don’t have one, a small vice can be abused to press them together quite effectively. The important thing is to apply pressure evenly across the width of the connector so that the back goes on straight, otherwise the plastic tabs may break.
Drive connector crimped to ribbon
Cable mated with floppy disk drive
At the other end of the cable the ribbon should be cut as shown. The two wires left longer than the rest are the 12th and 34th conductors.
If you are unable to perform the soldering described below, then a second connector fitted at the floppy drive end of the cable can be used to make connections to these wires instead as described at the bottom of this page. In which case the cable can be cut off straight.
The IDC edge connector goes flush with the end of the ribbon with the two wires protruding. The cable should be oriented as shown so that it exits the connector at the top when mated with the Spectrum.
Amstrad rotated the pinout of the Spectrum +3’s external floppy connector by 180° meaning that the polarising slot is at the wrong end. The polarising key should therefore be placed at the other end of the connector, between pins 29/30 and 31/32. If your IDC edge connector already has the standard floppy cable polarising key it can simply be rotated through 180° before being crimped onto the cable as shown here.
Both connectors attached to the ribbon cable.
Cut and strip the 12th and 34th wires left protruding in the previous step and twist and tin the ends as shown. Aim to have the wires sitting on the centreline of the connector back with a gap large enough for the body of a 1N4148 signal diode. Make the 34th conductor short and the 12th conductor longer as shown so that there is room for the heatshrink tubing to be slid on in the next step.
Cut the cathode (black stripe) leg of the diode short (approximately 5mm) and carefully solder it to the longer middle wire. Cut a peice of heatshrink tubing and slide it over the diode and down over the wire. This needs to be long enough to cover the diode and both soldered joints when completed.
Cut the anode (red) leg of the diode as before and solder to the shorter end wire.
Slide the heatshrink tubing over the diode and joints, then very carefully heat it with hot air or a flame until it shrinks. Finally fold the wires back over the back of the connector and secure with a small drop of superglue.
A completed +3 external floppy drive cable. Two right angle folds can neatly perform the half twist required between the Spectum and a floppy disk drive placed to the right hand side of the computer.
If you are not confident soldering or don't have soldering equipment the diode can be connected by means of a second IDC socket attached to the cable. The Diode can be folded and pushed into the socket as shown. Electrical tape should be wrapped around the connector to insulate the leads and prevent the diode from being pulled out.