Dec RQDX: What kind of chips on it?

Chris Zach cz at
Tue Jan 5 09:15:22 CST 2021

> It's surprising that the RQDX3 wasn't made format-compatible with the older controllers.

It makes sense: The 1/2 used interleaving, which would have required 
extra code in the 3 to support. The 1/2 figured out the disk geometry 
based on ROM lookups, the 3 stored the disk geometry on the disk. That 
meant you didn't have to worry about ROMs and technically you could use 
pretty much any MFM disk with less than a certain number of heads and 

What I realized mentally is that from the OS/driver/register level they 
are identical. So while the physical disk format is different, there 
should be no reason I can't hook up an RQDX2 to the DSM disk, fire up 
PDPGUI, select the drive as a DU: type, suck the data into a file over a 
serial connection, and boot it on SIMH without any issues.

> As for floppies, the RX50 format is the same for all PDP-11/VAX systems.

True, and I can download P/OS, dig out the PC with a 1.2mb floppy, and 
make the floppies on these Micro-RSX11M floppies I have in a box but man 
it's 30 floppies of *stuff*.

Technically I see there is a disk writer utility for the Pro/POS 
somewhere so I *could* download that to the Pro, download the disk 
images, and copy them one at a time. Or maybe I could use this Dave G 
MFM emulator to emulate an RX50, then just present the disks over and 
over till install is done. Hm....


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