Thanks
As I understand it, DEC has a 3 component solution for 8" floppy disks:
1) RXV11 / RXV211 Qbus interface with some of the controller functionality.
2) A board inside the RX01 or RX02 enclosure with the rest of the controller functionality (This is different to the non-DEC world where the floppy controller eg 9224 / WD17xx / WD27xx / WD37C65 directly connects to the Shugart 800 type interface on the drive mechanism)
3) A DEC 8" Drive mechanism either RX01 or RX02 with analogue and motor driver boards.
Items 2 & 3 are inside the standard RX01 / RX02 chassis.
The MSCP RQDX3 works more like a standard controller talking directly to the 5.25" or 3.5" floppy drive via a pin swapping breakout board. I was hoping to modify the T11 code to support FM and use a tweaked version of the MSCP driver to tell the OS RT11 / RSX how big the disk was, similar to the tweaks for different MFM hard drives.
The Qbus I/O ports and bit definitions are well defined for MSCP RQDX3, RXV11, or RXV211.
What ends up on the disk media is also relatively well defined.
Mention was made of non-DEC RXV(2)11 Qbus cards that directly support standard non-DEC drives, please suggest some makes / models.
BQT,
What do I need to do to put a RXVn11 Qbus card into a Micro PDP 11/73 together with the existing RQDX3 controller. I read that there was an incompatability between the RXVn11 cards and Micro-PDP11/73? - Backplane changes / card order / special config considerations please.
Peter
If we limit ourself to Qbus here (there are obviously also controllers for Unibus as well as Omnibus), then yes, what you describe is accurate.
The RQDX3 have a more normal floppy interface connector, so indeed, from that point you should be able to hook up various things. I don't know exactly what the RQDX3 would do if you hooked up anything but an RX50 or RX33 though, as those are the only ones defined to work. In general, MSCP always have a function to tell the capacity of a drive, and the floppy port is also exposed as a MSCP disk. But at the same time, the OSes do know about this floppy specifically, and it is not just managed like any other MSCP disk in all aspects.
So I do worry about things like what capacity it would be assumed to have at the OS level, because I don't know if they get it over MSCP, or if it is just fixed/known.
The MSCP protocol is rather complex, so writing your own firmware to do this on the RQDX3 would be a very complicated task. The I/O registers are dead simple, but that is partly because almost all communication between the CPU and the controller is not happening over those registers, but through shared buffers in memory that the controller access through DMA.
The RXV11/21 on the other hands is a simple, straight forward controller with a few I/O registers, and all operations are done through those. Nothing much to say really.
As for third party, compatible controllers, there were various different ones. A quick look on bitsavers turned up this one:
http://www.bitsavers.org/pdf/plessey...floppyCtlr.pdf for example. RX01/RX02 compatible, use any shugart interface drive.
Finally, the issue with the RXV21 in an 11/73. It's simple, actually. The problem stems from the fact that the RXV21 is a DMA device, but it is one of a few that only do 18-bit DMA addressing on the Qbus. The RXV11 do not have this problem as it don't do any DMA to start with. But the solution is just to use a bounce buffer, and RSX do this already. No hardware changes or anything is neccesary. It's just that DMA from the floppy need to go into the first 256Kbyte of memory. So the OS need to make sure this happens, and then it can copy the data over to the final destination, when reading. Or, when writing, first copy the data down somewhere in the first 256K, and then do the I/O operation.
I would expect that both RT-11 and RSTS/E also already handle this. But if you write your own software, you need to be aware of this, or bad things might happen.
Not sure where CHD got the rest of his information from. Apart from the 22-bit DMA issue, the 11/73 isn't different than an 11/23, which obviously should work with any revision RXV21. That there were different revisions is nothing new. That just meant that they cleaned the design up. The earlier revisions should also work, as long as any neccesary ECO have been applied, which I would expect to be the case.
