1571 WRITE-PROTECT BYPASS MODIFICATION
latest updates and corrections: 3-16-2013
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
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 most obvious way to disable the write protect circuitry is to short
out the two wires going to the WP sensor 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, will 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 and every 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 is opened by cutting a trace (conductive PC
foil) on the PC board. A 22uF 16 volt electrolytic capacitor is installed
across the cut trace ends. Next, a 10K ohm 1/4 watt resistor and a diode
connected to a 5V source complete the modification.
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. It's important that the diode and
capacitor are installed with the proper polarity!
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, 22uF at 16
volts. The negative end of the capacitor goes to pin 6 of IC U17 and the
positive end goes to the other end of the cut trace. The added circuit looks
like this for all drives. The disable switch is connected across the capacitor.
_________________pin 14 (+5VDC)
/ |K| <---cathode end
resistor \ | | <----diode
/ |A| <---anode end
|\ | |
| \ | cut |
---| /0-----|---X---|--------> to downstream circuit
|/ | |
existing |--][---| <--- capacitor
buffer IC | - + |
'--o/o--' <--- switch (optional)
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. 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
PLEASE let me know if you find any errors or omissions here. I appreciate
comments and questions!
Ray Carlsen CET
Carlsen Electronics... a leader in trailing-edge technology.