1541/1571 WRITE-PROTECT BYPASS MODIFICATION latest updates and corrections: 6-11-2011 There is a way to safely disable the write protect circuit in the 1541 and 1571 disk drives. This modification would enable writing to or formatting a disk that was write protected or one that was never punched to begin with, such as the backside of a standard 5.25" floppy (sometimes called a "flippy"). Although it's unnecessary to punch the other side of a 1571 disk used in 1571 mode with a C128, many users run 1571 drives in 1541 emulation mode with a C64, so disk "flips" are sometimes used to access the reverse side. An unmodified drive senses whether a disk is write protected by shining an invisible (infrared or IR) light from an LED through the left (notched) edge of the disk to a sensor facing the LED. With no disk in the drive or a if a notched disk is inserted, the write protect sensor (WPS) "sees" the light from the LED. When an unnotched disk (or one with a write protect tab) is inserted, the light to the sensor is blocked. This represents the two logic states of the sensor: on and off. No disk or a notched disk produces a logic low or zero volts out of the sensor and an unpunched disk or one with the write-protect notch covered generates a logic high. The sensor is a phototransistor with its output (in most 1541 drives) tied to connector P6 pin 12 (orange wire) and ground on pin 13 (green wire or, in some older drives, violet). P6-12 goes to the input of an invertor IC UA1-5, a 74LS14. That buffer IC outputs an inverted logic level on pin 6, and that is used to "drive" the input of controller IC UC1-6 and VIA IC UC2-14 in most versions of the 1541. Since it isn't the same in all drives, it's important to know how to tap into the correct circuit for the drive you have. The explanation given here will be for the most common versions of 1541, and the following text will use that as a basis for instruction. Specific install information will also be provided for different models such as the SX drive, the 1541C, 1541-II, etc. The most obvious way to disable the write protect circuitry is to short out the two wires going to the WP sensor (P6-12 shorted to P6-13 in a 1541) inside the drive. That method will work, but has the disadvantage of also disabling the disk change sense used by the drive to update the BAM when another disk is inserted. If the disk change update is disabled along with the write protect, the possibility exists of corrupting a disk when writing to it. The drive, thinking it is still looking at the first disk, may not update the BAM information before writing new information. At best, you will get a "disk ID mismatch" error, assuming the disks ID numbers (the two digit alpha-numeric information just after the diskname in the header) are different. If they are the same, you can potentially corrupt a disk. There are a few workarounds for that problem. One is to initialize the drive each time you change disks. JiffyDOS makes that easy: @I and . Another is a drive reset via a hacked reset switch to preserve the program in computer memory. With either of those methods, you must remember to do it each time a disk is swapped. Note what happens when you insert a disk into a 1541C or 1571... the drive spindle motor runs. That "trigger" comes from the WPS circuit. Ideally, any modification to a drive write protect circuit should still allow disk change updating. With the cutting of one PC board trace and the addition of three components (diode, resistor, and capacitor), a drive can be modified to allow writing to a protected disk without sacrificing disk change sensing. The buffer IC output line (UA1-6 in most 1541 drives) is opened by cutting a trace (conductive PC foil) on the PC board. A 10uF 16 volt electrolytic capacitor is installed across the cut trace ends with the negative capacitor terminal going to UA1-6 and the positive lead to the cut trace (actually to the next solder point on the board). Next, a 10K ohm 1/4 watt resistor is connected between IC UA1-6 and the +5 volt source (UA1-14). Lastly, a diode is connected between the positive terminal of the installed capacitor and the +5 volt supply, with the cathode lead (indicated with a line or bar at one end) of the diode going to +5v (UA1-14). To easily disable the modification and return the drive to a normal write protect mode, a SPST (single pole single throw) switch can be wired across the capacitor and installed somewhere on the drive case. With the switch closed, drive operation is returned to normal. When the switch is open, the write protect only is defeated. These added components do not upset normal circuit functioning. How it works: With the modification installed, the logic low signal out of the WPS circuit becomes a logic low pulse... just long enough to trigger a re-read of the BAM during the next drive access. The WPS line goes logic high again as the capacitor charges up, allowing the drive to write with the sensor blocked. The 10K resistor functions as a "pull up", necessary because TTL outputs don't normally go above about 3.3VDC on their output lines. The diode is a protection device for the downstream chips. It keeps the DC level from the capacitor to the controller and interface chips from going above 5 volts. The resistor, by the way, can be any value from 2.2K ohms to 10K... the value and wattage is not critical. The diode can be any standard silicon type. I used a 1N914 for my drives, but a 1N2004 or equivalent power supply diode will work as well. The capacitor is a small electrolytic, 10uF at 16 volts. The added circuit looks like this for all drives. The disable switch is connected across the capacitor. _________________pin 14 (+5VDC) | | / |C| <---cathode end resistor \ | | <----diode / |A| <---anode end |\ | | | \ | cut | ---| /0-----|---X---|--------> to VIA 6522 pin 14 |/ | | and motor control IC existing |--][---| <--- capacitor buffer IC | - + | '--o/o--' <--- switch Installing the modification: Most versions of the 1541 use a standard TTL 74LS14 hex invertor at board location UA1 for the WPS buffer. Pin 6 of that chip is opened for the modification. Newer drives include board numbers 1540049-1, 1540050, 251748, 251777, and 251830 (see NOTE 1). The WPS buffer IC is UC1 in the very early 1541 long board (includes number 154008-02), and pin 8 is opened for the mod (see NOTE 2). The later 1541C short board 251854 (sometimes called a 1542) uses UB1 as the WPS buffer and pin 8 is opened for the mod (see NOTE 3). A 1541-II uses a 74LS04 (same IC pinout as 74LS14) for the buffer at board location U9 and pin 8 of the chip is opened for the mod (see NOTE 4). The drive in the SX uses chip UE2 on the FDD board for the buffer and pin 2 is opened for the mod (see NOTE 5). Lastly, the 1571 drive WPS buffer is U17 and pin 6 is cut for the mod (see NOTE 6). Obviously a 1581 drive needs no modification since the write protect tab on the disks can be easily opened and closed by the user. ********************************************************** NOTE 1: For these newer drives, the buffer IC is located at the left rear of the PC board nearest the serial port connectors. The trace off pin 6 is cut and the added components are soldered to the bottom of the board. The chip pinout and board trace appears as follows when viewed from below: cut /-------------------X------0 <--- 1 2 3 4 5 6 7 solder pad near pins 1 & 2 of IC UC1 14 13 12 11 10 9 8 *********************************************************** NOTE 2: I don't own a very early version of the 1541, but the buffer IC 74LS14 is at board location UC1 (nearest connector P6), and the PC trace to be cut comes from pin 8 of the IC. That line goes to UF5 pin 12 and to UCD4 pin 14. *********************************************************** NOTE 3. This late version short board 1541 has a metal shield on the bottom of the PC board. There is no need to remove the board since all of the modifications can be done on the top. Board trace (conductive foil) from UB1 pin 8 runs under the IC, comes out between pins 6 and 7, and goes to a solder pad. Cut the trace near that pad and solder the components to the IC legs and the foil pad as indicated. The view from above is as follows: 14 13 12 11 10 9 /8 | > | | 1 2 3 4 5 6 | 7 | X <-- cut \ 0 solder pad *********************************************************** NOTE 4: The 1541-II mod requires removing the mechanical deck from the drive. The trace (conductive foil) to cut goes from a solder pad under IC U11 to another pad under IC U10 on the bottom of the board. The exposed trace is cut (no chips need to be removed but the board does to access that trace) and the added components are soldered on the top of the board directly to the chip pins. The components need to be mounted close to the board so they don't touch the underside of the mechanical drive when it is reinstalled. *********************************************************** NOTE 5: The SX drive controller FDD board will have to be removed from the case to access the buffer chip UE2. Cut the line from IC pin 2 and install the components on the top side of the board. Make sure they don't contact any other parts or wires when the board is reinstalled. *********************************************************** NOTE 6: A 1571 WPS mod requires removal of the mechanical drive to access IC U17. The mod can be installed on the top of the board without removing it from the case. Use a 22uF capacitor instead of the 10uF specified above. Note that the PC board trace starts at pin 6 of IC U17, goes under (but doesn't connect to) transistor Q7 and on to a solder pad on the board between resistor R18 and ferrite bead FB6. Viewed from above, the trace to be cut appears as follows: 14 13 12 11 10 9 8 | | > U17 74LS14 | | | 1 2 3 4 5 6 7 | X <--- cut | | | | 0 <--- pad ********************************************************** To install this modification in a C128DCR, it's necessary to use a different capacitor (47uF instead of 10uF as in other disk drives) because this drive has a pull-up resistor the other drives don't have and which makes a stronger pulse necessary for reliable operation in protect-bypass mode. One board trace needs to be cut to separate the output of IC U114 pin 10 from the rest of the circuitry. The added components are wired across this "separation". U114 pin 10 wiring goes across the board to a solder point just above IC U112, and that spot is used as a connection point. The negative end of the added capacitor and the 10K resistor connect to that point. The positive end of the capacitor and the anode lead of a diode go to pin 39 of IC U106. The cathode lead (marked with a bar on the diode case) and the other end of the 10K resistor go to a convenient +5VDC connection at IC U112 pin 14. The SPST switch wires go to each side of the added capacitor. ********************************************************** I mounted the bypass switch on the front panel of all the drives I modified. I prefer to use the lower half of the case to keep the switch wires from interfering with removal of the case top during cleaning. However, if other drive mods have already been done such as installing a drive reset switch, you may not have that option. Switch locations really depend on how you use your equipment, desktop space, etc. A reset switch on the rear of a drive makes it harder to accidently "bump" it, but is not easily accessible if your drives are recessed in an enclosed space or are on a shelf away from the workspace. I believe a switch to select normal or bypassed write protection should always be clearly visible on the front of the drive so you always know the WP status when you insert a disk. Please let me know if you find any errors or omissions here. If there is any interest in modifying the Oceanics/Excelerator or Amtech/Bluechip drives, email me and I'll see what I can do. I have no schematics for those drives but I should be able to "reverse engineer" the circuit in my drives. It's a bit of work, but if someone needs that information, I'll give it a go. Ray Carlsen CET Carlsen Electronics... a leader in trailing-edge technology.