Firing up the pdt11
cz at alembic.crystel.com
Mon Oct 12 18:15:20 CDT 2020
Well, the drive is working. Better.
Summary: The PDT11/150 needs to have resistors on the harness to the
floppy drives to limit the current for the sector LED. Without it the
LED will be "off" and you will get a quick 8 stepper motor clicks
without any click to load the heads. Looks like the PDT uses the sector
count signal to determine if a disk is in the drive, and if it doesn't
see that it just fails with a quick braaap!
And after trying a 47 ohm resistor (marginal) and a 22 ohm resistor (no
luck at all) it turns out you want to use around 67 ohms of resistance
for best results on the sector led. I didn't have any 67 ohm resistors,
but I did have two 140 ohm resistors, so putting those in parallel gave
Drive PD1: now boots, reads disks, and seems kind of happy. I still need
to clean up the whole board and put this harness in the drive, but at
least it's running with two floppies again.
On the bad side (and something to think about): My memory must be in
error: I can't format RX01 floppies with the PDT11. I recall that the
PDT would literally rewrite the sector information along with the actual
data track on each write (why) but this is not enough to format or init
a blank disk apparently.
Which is odd, because I have a few Elephant Memory systems floppies that
still work and obviously *are* formatted for RX01/PDT. But I never owned
an RX02 so how did I get these formatted in the first place?
On to the RX02 again I guess. I'm going to put this pdt11/150 back
together and start thinking about how to format floppies. I've heard
that the RX02 can format an RX01, but can it? Certainly not with an
RXV11 controller, I'll need to either find an RXV21 or get a Unibus
pdp11 working so I can use this RX21 controller someone graciously lent
me (or find my Quniverter and try that, but that would be
Which could be interesting: I have two Unibus pdp11's: My 11/05 (which I
think only has 24kw of core memory anymore) and my 11/24 (which I have
never gotten to work right and doesn't have any memory boards). Quick
question to end this thread: Can an 11/24 come up to ODT without any
memory? I've never seen anything on the serial ports.
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