Dec RQDX: What kind of chips on it?

Paul Koning paulkoning at
Tue Jan 5 11:36:06 CST 2021

> On Jan 5, 2021, at 11:00 AM, Warner Losh <imp at> wrote:
> On Tue, Jan 5, 2021, 7:20 AM Paul Koning via cctalk <cctalk at> wrote:
> ...
>> As for floppies, the RX50 format is the same for all PDP-11/VAX systems.  Unlike hard drives, Pro floppies and RQDX3 floppies are identical.  The geometry is handled in the host for the Pro and in the controller for the RQDX case, but it's the same geometry.  And by the way, a PC floppy drive can handle RX50 floppies without any trouble, if you set it to the correct geometry -- in particular, you have to tell it there are 10 sectors per track rather than the PC default of 9 sectors.
> Reading this is true. There are several drivers for this for DOS, linux and FreeBSD (though the latter may be out of tree). Writing on PC and reading on a Rainbow is possible, but it is unreliable because the 1.2M drives aren't that good at writing lower the lower density data rates.

I don't know about FreeBSD, but you don't need special drivers on DOS or Linux.  In DOS, it's just some int 13 magic.  In Linux, it's an ioctl to set the floppy layout.

I did this a long time ago for my "flx" utility (RSTS file access on a PC).  The initial version was for DOS, and that code should still work.  I added Linux support later.  There are two versions of this code: one in C (flx 2.6) for DOS and Linux; that one on Linux needs to have the layout set via a Linux utility first.  The other is flx 3.0 which is written in Python; it handles everything directly.  That version also includes a separate utility that just does interleave or de-interleave of 800 block disks and container files, so you can create a SIMH compatible disk image from a floppy, or a Xhomer interleaved layout image from a regular (logical order) image.  That simply uses the disk access code from FLX with the file system pieces left out, so it isn't specific to RSTS.

In my experience (admittedly only on a few PCs I have had that include a 5.25 inch floppy drive) both reading and writing work with no trouble at all.  For my Pro development I've always used floppies as the transfer medium, which means creating bootable RSTS disks on the PC.  Works fine.


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