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COMPUTER BITS BY FRANC ZABKAR Installing a PC-compatible floppy disc drive in an Amiga 500 This simple circuit allows a PC-compatible 1.44Mb floppy disc drive to be used in an Amiga 500 computer. The new drive can take the place of the original Commodore unit which was very expensive. Replacing or upgrading the floppy disc drive in an Amiga 500 has traditionally been something of a headache. Unfortunate­ly, a PC-compatible 1.44Mb drive is not a plug-in replacement, while the original Commodore unit is costly and no longer easy to obtain. The good news is that only a few simple modifications are required to make the PC-compatible unit work in the Amiga 500. A PC-compatible 1.44Mb drive can now be picked up for $50 or less, whereas the Commodore drive costs about $150. If the floppy drive in your Amiga 500 has died, you can save about $100 by substituting a PC-compatible unit. Note, however, that you will not be able to use the greater capacity of the PC-compatible drive, unless a special software driver is installed. Instead, discs will still be formatted to the 880Kb Amiga standard (does anyone know where to obtain a suitable driver so that the full 1.44Mb capacity can be used?). CHANGE output; on the Amiga, it is the READY output. (2). Pin 2 on the PC is the LOW/HIGH DENSITY input; on the Amiga it is the DISKCHANGE output. Drive differences The 34-way interface cable is made as follows: (1). At the drive end of the cable, separate leads 10, 11 & 12 as a threelead wide strip and twist this strip, PC fashion, through 180°. This step effectively transposes leads 10 & 12 The main interfacing differences between the PC and Amiga floppy disc drives (FDDs) involve pins 34 and 2. These differenc­es are as follows: (1). Pin 34 on the PC is the DISK22  Silicon Chip In addition, the PC’s outputs are open collector and the corresponding inputs to the drive are pulled high at the drive itself. Furthermore, PC-compatible FDDs are set up as Drive 1 rather than Drive 0 as on the Amiga. Basically, all we have to do to get the PC-compatible floppy drive to work in the Amiga is make an appropriate 34-way interface cable and add a simple logic circuit. This logic cir­cuit is based on a 7438 TTL quad NAND gate (only two gates used) and is shown, along with the interface cable, in Fig.1. The logic circuit is needed to simulate the READY signal that Amiga requires but which the PC floppy drive does not provide. Several pull-up resistors (2.2kΩ) are also required for the open collector outputs (see Fig.1). Making the cable when the headers are attached and serves to designate the FDD as Drive 0; ie, pin 10 on the Amiga header goes to pin 12 on the drive header and vice versa (note: the pin 11 connections are unaffected). Note that lead 1 of the cable is designated by a red colour stripe. (2). Install a 34-way female IDC header at the drive end (pin 1 to the lead with the red stripe). (3). At the Amiga 500 end, snap on a similar IDC header about 50mm from the end of the cable. This 50mm-long free end is used to make the connections to the logic circuitry. (4). At the drive end, carefully separate and cut lead 2 and peel it back until its end is just over half-way down the cable. Similarly, at the Amiga end but working from the drive side of the header (important), separate and cut lead 34 and peel it back until it reaches the end of lead 2. (5). Strip the two lead ends, slip some heatshrink tubing over one of them, and solder the two leads together (ie, solder lead 2 to lead 34). Push the heatshrink tubing over the join and care­fully shrink it down with a hot-air gun. (6). Assemble the logic circuit on a small piece of Veroboard or similar and connect this to the 50mm of trailing cable at the Amiga end. You only need to connect leads 2, 8, 10, 26, 28, 30 & 34 to the logic circuit – the remaining leads can be cut off flush with the end of the header. (7). Connect a power cable consisting of two leads (+5V, GND) to the logic board. Terminate the other end of this cable in a 3-pin header (+5V, GND, key) somewhere on the Amiga PC board (choose your own spot). Fig.1: the interface circuit consists of a couple of NAND gates plus a modified drive connector cable. Alternatively, you can hardwire the supply leads to the appropriate terminals inside the Amiga. Mechanical arrangement The mechanical arrangement is reasonably straightforward. Note that you will have to remove the plastic facia from the front of the drive and that you may need to pack the standoffs with washers to raise the drive to the correct height. Depending on the arrangement, it may also be necessary to cut a hole in the side of the Amiga’s cover to gain access to the FDD’s disc eject lever. This lever may also have to be extended by some suitable means. $7.95 + $3 p&p Finally, note that some FDDs keep spinning for a second or two after the Amiga’s disc activity LED has extinguished. For this reason, don’t change discs until you hear the disc motor come to a stop. Alter­ natively, re-route the FDD’s own disc activity LED to a visible SC position on the front panel. Especially For Model Railway Enthusiasts THE PROJECTS: LED Flasher; Railpower Walkaround Throttle; SteamSound Simulator; Diesel Sound Generator; Fluorescent Light Simulator; IR Remote Controlled Throttle; Track Tester; Single Chip Sound Recorder; Three Simple Projects (Train Controller, Traffic Lights Simulator & Points Controller); Level Crossing Detector; Sound & Lights For Level Crossings; Diesel Sound Simulator. Order direct from “Silicon Chip” PRICE: $7.95 (plus $3 for postage). Order by phoning (02) 9979 5644 & quoting your credit card number; or fax the details to (02) 9979 6503; or mail your order with cheque or credit card details to Silicon Chip Publications, PO Box 139, Collaroy, NSW 2097. April 1997  23