Combination and Orientation of RQDX boards in one PDP-11/23 PLUS

Pete Turnbull pete at
Sun Jun 21 20:24:48 CDT 2009

On 21/06/2009 20:08, SPC wrote:

> As far as I can imagine it appears that they were used in one PDP QBUS
> system in the order, from up to down in the order of the previous list, with
> one ribbon cable from the RQDX3 to the 7513 J1 (?) connector, and another
> one from the J2 (?) to the RQDX distribution board. Exists another connector
> (J3 ?) in the 7513 that appears to point to one external connector that I
> saw in one BA23.

That would make sense; the M7513 is an RQDXE extender, used to allow you 
to connect an external set of drives as well as the internal ones 
handled by the distribution board.  Unless you want some drives in the 
host enclosure and one or more in an external one, you don't need it.

> Finally, the RQDX comes with MFM cables but too with one standard floppy PC
> cable. I have some doubts with this last matter:
> * Is used this crossed floppy cable in DEC systems too (as in PeeCees) or
> it's used other kind of cables ?

No.  PCs are the only things that use the crossed cable.  Other systems 
would normally not have a cross, but would have different jumper 
settings on the floppy drives for the drive selects.

> * I have a couple of RX33. Must I use TWO cables (one per floppy unit) ?

No, you can use a simple bus arrangement with two connectors in parallel 
-- but set the DS jumpers on the two floppies so one is DS1 and the 
other is DS2 (that's assuming you count DS1...DS4.  If the links are 
labelled DS0...DS3 then set the first to DS0 and the second to DS1). 
Don't use a PC cable with a twist; it doesn't do the right thing.

You need quite a late version of firmware in the RQDX3 to support RX33s, 

> * Same question with the MFM disks

If you're using the M9058 distribution board, the easiest way to do it 
is to use one 34-way cable and one 20-way cable for each fixed disk (ie 
for two fixed disks, two pairs of cables).

> * What is the orientation of the RQDX ? I imagine it with the connectors
> side turn down because turning it up would be a problem with the previous
> board in the BA11-SB

Do you mean the RQDX-E?  Just don't use it unless you have to.  The 
M9058 distribution board is meant for a BA123, and has its own slot at 
the bottom of the backplane where it's not in the way.  It only takes 
power from the backplane, no signals, so it can go way below anything 
else without having to worry about grant continuity or bus termination.

Look at the PDF files at
and specifically at M9058_layout.pdf  because that will show you what 
needs connected.

Regard an RX50 as being equivalent to a pair of RX33s, at least as far 
as the interface connections are concerned, so it (or the 2 x RS33) 
connect to J11, and your first fixed disk as RD1 so it connects to J1 
and J5.  Set the drive select on any fixed disk in this setup to DS3.

J12 connects to the RQDX3 via 50-way ribbon cable, and J9 connects to 
the front panel switches.  You probably don't have those, but if you 
look at the diagram in RQDXn_LEDs_switches.pdf you'll see that it would 
be easy to construct a substitute, and if need be, just use a couple of 
pullup resistors to ensure the drive is ready and write enabled.

If you want to see more, fetch the "microPDP-11 Maintenance Manual" from 
the net, and look at the chapter on storage devices.

Pete						Peter Turnbull
						Network Manager
						University of York

More information about the cctalk mailing list