Firing up the pdt11

Chris Zach cz at
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 
me 70.

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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