DEC RQDX1 Hard Drive Compatibility

[ThisPostContainsComputers]

Hello,

In my quest to get some sort of high capacity storage for my PDP 11/23, I have turned towards the later MFM based controllers. As of right now, the RL drives are on the backburner for having too many issues to get running right now (sand will do that to you, I guess). Instead, I got my hands on a M8639-YA RQDX1 combination controller card. This way I can put some of the old MFM hard drives are I have laying around to work, plus I can also add a floppy drive or two.

Now, from what I have read, the RQDX1 is not nearly as capable as its RQDX3 successor. In addition to only being able to access about 33MB on a drive, it also only seems to support the ST506 and ST412. These drives are not exactly common, and outside of the ones I have embedded in other computers, I don't have any on hand. I was wondering if it was possible to use other models of drives with the RQDX1. Specifically, I was eyeing the ST251 because I have a bunch of them and they seem to be somewhat reliable.

There are mentions online about placing RQDX2 ROMs in RQDX1 boards in order to increase supported drive geometries. Not that it would help much, as the ST251 isn't on the RQDX2's compatibility list. Someone also mentioned something about modifying the ROMs directly. On that front, I have been able to successfully disassemble the RQDX ROMs that I have found online, but I haven't taken the time to actually reverse engineer them yet. Finally, I have heard something about a possible patch to the ZRQCH0, but I also haven't been able to find much information about this one online.

Thank You

 

[mcguire]

The ST251 was rebadged and sold by DEC as the RD32, and the ST225 was sold as the RD31. The RQDX3 supports these, but I don't remember offhand if the RQDX2 or RQDX1 does, I'm pretty sure they don't. To my knowlege there were no modifications to the drives other than stickers, and I've used "generic" ST225s and ST251s on RQDX3s several times without difficulty.

-Dave McGuire, LSSM

 

[ThisPostContainsComputers]

That is about what I read too. I do technically have a RQDX3 on hand, but it would need to be "liberated" from my MicroVax II. This isn't ideal for obvious reasons, but for the short term I might as well use it as the MicroVax II does not boot anyways.

I think this weekend I am going to start the fun process of reverse engineering the firmware. From what I have read, the RQDX1 stores all of the supported geometries in a table somewhere in ROM. I can't imagine it would be too difficult to change, but there are always complications. I don't know if this is the best way of going about this, but this is where I am going to start for the little knowledge I have about RQDX controllers.