1541/1571 WRITE-PROTECT MODIFICATION
latest updates and corrections: 2-12-08
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
**********************************************************
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 I can
get a digital camera, I'll take some shots of the boards I've modified.
Anything is better than ASCII. 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.
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