I think this is beginning to make sense.
1451A CN6 1541B/C CN3 1541-II CN4
------------------ --------------- ----------------
WP LED POWER(optional)
1 GND 1 GND 1 GND
2 150 OHM to 5V 2 150 OHM to 5V 2 150 OHM to 5V
MOTOR ACTIVITY LED
8 5V 6 5V 6 12V
9 MTR 9 MTR 7 MTR
WP SENSOR
12 WP 12 WP 3 WP
13 GND 13 GND 4 GND
TRACK 0 SENSOR POWER
4 150 OHM to 5V
5 GND
8 150 OHM to 5V
Probably Not Used
5 GND
For the benefit of those who do not have a 1541 drive to look at:
There is a small circuit board mounted on the Newtronics drive mechanism.
The stepper motor is connected to the Commodore system board through this
circuit board.
On my 1541 Newtronics drive, there is a mounting for a track 0 sensor and
there is a tab on the head assembly that would trigger the sensor if it
were there.
Nicholas reports that on his 1541C drive there is a track 0 sensor that
gets it's power from the Commodore system board, but connects the output
signal to the stepper motor board.
This makes sense because none of the 1541 ROMs and none of the Commodore
system boards provide support for a track 0 signal. If the track 0 signal
is connected to the stepper motor board, then it can turn off the stepper
motor at track zero without any active support from the Commodore System.
It only needs a power supply, which the 250442 (1541A) and 250446 (1541A2)
system boards supply. On the other 1541s, just removeing the track 0 sensor
and disconnecting it from the stepper moter board should convert it to a
drive without track 0 sensing.
To summarize:
1541/1541A drive without track 0 sensor and CN6 connector
1541B/1541C drive with track 0 sensor and CN3 connector
1541-II drive without track 0 sensor and CN4 connector
It appears then that the same Newtronics D500 drive mechanism is used on
all 1541 drives, only with different connectors and the track 0 sensor
removed on some.
This would also mean that the drive mechanisms should be compatible
across all the 1541s if you change the connector.
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