Real PDP11 model question

From: Jerome H. Fine <>
Date: Sun Sep 19 18:38:08 2004

>Ron Hudson wrote:

> Were there any PDP11's as small as the DECmate III?

Jerome Fine replies:

Please define "small"!

In terms of actual physical size, probably the PRO325, PRO350
and PRO380 systems are about the same size as a DECmate III

But if you want a system which is even smaller, then place a Qbus
backplane INSIDE a VT100 terminal which then becomes a VT103
computer. Even if you count the Qbus backplane as the PDP-11
portion, overall it is still smaller than the DECmate III or the PRO
series. AND since the space occupied by the Qbus backplane
in a VT103 is NORMALLY empty, then the actual EXTRA physical
size of the PDP-11 portion might be considered to be ZERO.

By the way, the Qbus backplane in a VT103 can be either 2 * 4
or 4 * 4. I have seen BOTH, although ONLY one of the former.
The 4 * 4 Qbus backplane can hold either 4 quad Qbus boards or
8 dual Qbus boards or any combination. NOTE, however, that
the power supply is VERY inadequate - the 5 volt supply has ONLY
16 amps for EVERYTHING - including running the VT100 portion
of the system.

Even so, as a test case, at one point I had the following boards:
KDJ11-BB or M8190-YB or a quad board
Cristlin quad 4 MBytes of memory
M8029 RX02 controller or a dual board
3rd party MFM controller RQD11-B for an RD53 or a dual board
with MSCP emulation and onboard bootstrap
DHV11 8 port serial quad board

In order to use all 4 MBytes, 4 extra wires were used to modify
the DEC backplane to handle all 22 bits. The standard Qbus
DEC supplied backplane in a VT103 handles only 18 bits. The
extension to 22 bits was also needed to use the Sigma RQD11-B
MFM controller, or at least that was what seemed to have been
necessary to solve the problem when the standard 18 bit DEC
backplane was used.

Since I did not run the system for more than 15 minutes, I don't
know if the 16 amp limit on the 5 volts was adequate. I placed
the RD53 under the monitor inside the VT103. When I wanted
to run for a longer period, I removed the DHV11 and ran the
RD53 from an external power supply in order to reduce the load
on the internal power supply as much as possible.

Many other configurations are also possible with a VT103,
including the use of a TU-58 drive from DEC - which is
NOT recommended since RT-11 takes too long just to boot.

In your opinion, since the space inside a VT100 occupied by
the Qbus backplane to turn the VT100 into a VT103 (plus
a few other needed parts) is normally empty, does a VT103
qualify as a PDP-11 of ZERO size - or is the Qbus backplane
the actual size of the PDP-11? From my point of view, the
VT100 portion of the VT103 should not be considered to
be part of the PDP-11 since some terminal is essential to run
any PDP-11 like system, even if it is a separate monitor and
keyboard which is used by the PRO350.

Any other questions?

Sincerely yours,

Jerome Fine
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Received on Sun Sep 19 2004 - 18:38:08 BST

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