Q-BUS primer?

Pete Turnbull pete at dunnington.plus.com
Thu Jan 31 05:43:08 CST 2008

On 31/01/2008 03:12, Dave Dunfield wrote:

> At this point, all I'm really looking for is a good "starting point".
> Can anyone recomment a good document/resources for a Q-bus newbie?

There are more descriptive documents, but as a reference to identify 
backplanes and related things, you might want to look at


and possibly some of the other files in that directory 

One thing that can be confusing is the xx-bit terminology.  All the 
PDP-11 processors are 16-bit (data-wise) but there are several different 
address ranges: 16-bit, limited to 64KB (32KWords); 18-bit, limited to 
256KB; and 22-bit, with the full 4MB range.  Similarly, some backplanes, 
memory cards and bus terminators only support 16 or 18 address lines. 
However, I/O devices generally use the I/O page, which is always the top 
page in the memory map, and uses a special signal for access regardless 
of how many memory lines the system has.  The gotcha is that some early 
DMA devices only have 18 address lines for accessing the buffers in 
system memory.

> M3106       4-line async
DHV11, needs a breakout cable for the four lines.

> M7555       Winchester and floppy disk controller
That's an RQDX3, so you might want to check the ROM revisions on it.  It 
is supposed to be used in a chassis that has a built-in board for all 
the cable connections to the drives, or with a breakout board such as an 
M9058, but you can make your own breakout cable or board (there's a PCB 
layout for a board and a document describing the connections on my 
website, should you ever need them).  I'd guess this one came from your 
MicroVAX and probably has fairly up-to-date ROMs.

> M7606       MicroVAX II KA630
> M7608   x2  2/4 MB RAM (boards are fully populated)
These go together.  You can't use that MS630 memory on an ordinary QBus 
(PDP-11) machine, as it connects to the CPU by the over-the-top connectors.

> M7946   x2  RX01 floppy disk controller
For an RX01 dual 8" floppy, of course, but it can also be used with an 
RX02 dual floppy if you set some switches inside the RX02.  However, you 
can't use it with standard (eg SA800-style) floppy drives.

> M8047       RAM, Async, ROMs
That's an old MXV11-A and probably came with the 11/03.  It's not 
terribly useful for the 11/23 unless you only use older peripherals, and 
it won't work with the MicroVAX.

> M8186       11/23 CPU
That's a basic 11/23 processor, and might be either 18-bit if it's an 
early one, or 22-bit otherwise (the common ones are 22-bit).  They were 
often used as upgrades to older 11/03 systems.  The CPU chip is the 
40-pin ceramic carrier with two chips on it, furthest from the edge of 
the board.  If it doesn't have the MMU chip, a plain-looking 40-pin 
ceramic chip at the edge of the board, it will only access 64KB.  It may 
have an optional FPU, which is a 40-pin ceramic carrier with two smaller 
chips on it, like the CPU.

> M9400YA     120-ohm terminators with refresh & floppy boot
That's 18-bit only, so will only really be useful with the 11/03, unless 
your 11/23 is an early one that's 18-bit, or was only set up as an 
18-bit system.

> M9400YE     Headers and 250 Ohm resistors
That's usually used as a bus extender for a second chassis.  Is/was 
there another card (M9401) with it?


Pete						Peter Turnbull
						Network Manager
						University of York

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