[TUHS] pdp11 question

Pete Turnbull pete at dunnington.plus.com
Sun Nov 14 22:32:46 AEST 2010

Warner Losh wrote:
>  On 11/13/2010 18:03, Larry McVoy wrote:
>> Back in the day there was something called a microvax and I think there
>> was a micropdp - it was a tall slim thing.  Might google that.
> The MicroPDP11 was in more or less the same form factor as the MicroVAX 
> I and II (also marketed as VaxStation I and II).  It was also known as 
> something like the PDP 11/73.  A lower-end version was the Digital PRO 
> 350 and 360.

The MicroVAX and MicroVAX II postdate the microPDP-11 range (1985 and 
1982 resp.), but do indeed use the same boxes: BA23 floorstanding 
(sometimes called the space heater, because of its shape) and the larger 
BA123 with casters (sometimes called the hostess trolley because of its 
shape).  Anything else is not a microPDP-11, and any PDP-11 sold in one 
of those boxes is a microPDP-11 (as opposed to an 11T23, 11V23, etc). 
It's true that the BA23 chassis can be taken out of the floor cabinet 
and rack mounted but that's fairly unusual (I've only seen one that way, 
and it had been removed from its original floor case).

It's also true that you can convert a MicroVAX to a PDP-11 by swapping 
memory and CPU - but there are some VAXstations that you can't convert.

In general, the QBus machines are physically smaller and less 
power-hungry than Unibus systems and that's especially true of the 
microPDP-11s.  Of course it's certainly possible to have a small Unibus 
system or a big QBus one!

The first microPDP-11 was the microPDP-11/23 which is a variant of an 
11/23-plus in a BA23; it was followed by the microPDP-11/73, usually in 
a BA23, and soon after by the micro-PDP-11/83 (same processor, faster 
clock and different memory) in either a BA23 or BA123.  The 
microPDP-11/53 integrates the memory onto the CPU card and is cheaper 
but also slower.  Later came the 11/93, which is faster -- and that, 
along with it's Unibus cousin the 11/94, was the last PDP-11 made. 
There's also an 11/84 Unibus machine to match the 11/83, and although 
its always a rackmount machine it uses the same CPU and memory as the 
11/83 -- but a different CPU box with a different panel and a 
Unibus-to-Qbus converter.

The 11/73 that Greg's photos show is slightly unusual; it uses a BA-11 
chassis like earlier rackmount QBus machines.  You quite often find 
11/73s in that form as upgrades to what was previously an 11/03 or 11/23 
system.  The drive above the CPU box in Greg's system isn't original, 
and that rack was part of a system with a pair of RL02s.  If it was 
originally sold as an 11/73 it would be called an 11/73S (but I don't 
think it is, because 11/73S systems had a black decal on the front to 
say so), or if it was the result of a CPU upgrade, it would originally 
have been an 11T03 or 11T23 system - probably the latter.

You could get the 11/73 CPU card in two versions -- KDJ11-A is a 
dual-height card with just the processor and MMU; KDJ11-B is quad-height 
and incorporates serial ports, LTC, etc.  Similarly there are dual- and 
quad-height 11/23 cards called KDF11-A and KDF11-B.  The microPDP-11 
series always used the quad KDx11-B cards.

Rackmount PDP-11s usually have larger (physically) drives like 
RL01/RL02, RK06/7, RM0etc and/or RX01/2 floppies whereas the microPDP 
series normally have physically smaller RD or RZ series winchesters and 
RX50/33 5.25" drives and perhaps a TK or TZ series tape in a 5.25" form 

You can run 7th Edition on an 11/23 and up (I have an original 7th 
Edition machine which is an 11T23).  On anything less than 11/73 (like 
an 11/34, 11/23, etc) it has some limitations and needs some software 
tweaks (and an RL driver was not a standard piece of the code).  Mine 
has 256KB memory, two RL02s (10MB each) and an RX02 (dual 8" floppy), 
and even something as simple as "ls" is slow and accompanied by quite a 
lot of very audible disk access.  It would be better with more memory.

You can run BSD2.11 on an 11/73 and up (I've got that running on an 
11/83 in a BA23 box with 2MB of memory and a 150MB RD54 winchester). 
That runs quite well, and it's on my local Ethernet network.

The PRO-325 and -350 are desktop machines and aren't really PDP-11s, 
though they do have an F11 CPU chip (same chipset as 11/23 and 11/24, 
different board). I think you'd have a hard job running Unix on them as 
they have a lot of custom hardware and no QBus or Unibus.  Bitmapped 
graphics console, no DMA on any I/O devices, and a weird (for DEC) 
interrupt system.  They normally ran P/OS which is a highly modified 
version of DEC's RSX-11 operating system.  The later PRO-380 used a J11 
processor (as in 11/73,83,84,53 etc).

Pete						Peter Turnbull
						Network Manager
						University of York

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