Backplanes

Overview

In order build a Q-bus system, you need a Q-bus backplane. There are two basic varieties; 18-bit and 22-bit. An 18-bit bus will limit the system to 256 KB of memory. This will not be a problem if you want to build a PDP-11/03, or want to run an operating that can live within the 256 KB limit. An 18-bit bus, however, can be easily converted to 22-bits, by performing some minor wiring. This will allow your system to address the maximum 4MB of memory.

Quad-Width Backplanes

The following table is a breakdown of available quad-width backplanes. These backplanes will accept dual-width and quad-width modules.

H9270 H9270-Q H9273 H9275-A H9276 H9278-A DDV11-B
Width Quad Quad Quad Quad Quad Quad Hex
Slots 4 x 4 4 x 4 4 x 9 4 x 9 4 x 9 4 x 8 6 x 9
AB/CD Configuration Q-bus/Q-bus Q-bus/Q-bus Q-bus/CD Q-bus/Q-bus Q-bus/CD Mixed1 Q-bus/Q-bus/EF
Q-bus Addressing 18-bits2 22-bits 18-bits3 22-bits 22-bits 22-bits 18-bits2
Termination (ohms) no no no 120 no optional4 no
Card Guides yes yes yes yes yes yes optional5
AC Loads 4.4 4.4 2.6 10.0 10.0 10.0 6.4
Typical PDP Use 11/03, 11/23S OEM6 11/03-L, 11/23 OEM 11/23+ Micro/PDP-11 OEM

1 Slots 1-3 are Q-bus/CD. Slots 4-8 are Q-bus/Q-bus.
2 Backplane can be converted to 22-bits by wire wrapping jumpers for BDAL18-21.
3 Backplane could be converted to 22-bits by soldering jumpers for BDAL18-21.
4 Backplane is equipped with sockets for pluggable resistor networks.
5 Using the H0341 card guide assembly, or an H909-C (BA11-L) mounting box.
6 This part number is used liberally, with an H9270 being called an H9270-Q, on production systems, after wire wrapping lines DAL18-21 on the backplane. The real H9270-Q has a set of jumpers on the backplane to enable and disable DAL18-21, and was only sold for OEM use.

Dual-Width Backplanes

The following is a breakdown of available dual-width backplanes. These backplanes will only accept dual width modules.

H9281-AA H9281-AB H9281-AC H9281-BA H9281-BB H9281-BC H9281-QA H9281-QB H9281-QC
Width Dual Dual Dual Dual Dual Dual Dual Dual Dual
Slots 2 x 4 2 x 8 2 x 12 2 x 4 2 x 8 2 x 12 2 x 4 2 x 8 2 x 12
AB Configuration Q-bus Q-bus Q-bus Q-bus Q-bus Q-bus Q-bus Q-bus Q-bus
Q-bus Addressing 18-bits1 18-bits1 18-bits1 18-bits1 18-bits1 18-bits1 22-bits 22-bits 22-bits
Termination (ohms) no 120 120 no 120 120 no 120 120
Card Guides no no no yes yes yes no no no
AC Loads 1.1 1.8 2.9 1.1 1.8 2.9 1.1 1.8 2.9
Typical PDP Use OEM OEM OEM OEM OEM OEM OEM OEM OEM

1 Backplane can be converted to 22-bits by wire wrapping jumpers for BDAL18-21.

Backplane Termination

One thing to consider is many systems will need bus termination to reduce signal reflections on the bus. The processor is typically installed in the first slot and provides the termination for that end of the bus. The basic rules for terminating a single box system are as follows:

Termination of the far end of the bus can be either built into the backplane, or provided by another module equipped with appropriate termination. If you decide to use a bus that has no termination, then one of the modules in the table below can provide the necessary termination. The module would normally be installed as the last module in the system and match the number of address and data lines your system will have. For information on bus termination practices, DEC MicroNote 29 and MicroNote 35 discuss acceptable scenarios.

Modules with Termination
Module Number M8012 M8016-YB M8016-YC M9400-YA M9400-YB M9400-YE M9400-YH M9400-YJ M9404-YA M9405-YA M9405-YB
Model number BDV11 KPV11-B KPV11-C REV11-A TEV11 REV11-E REV11-H REV11-J
Module Size Quad Dual Dual Dual Dual Daul Dual Dual Dual Dual Dual
Q-bus Termination 22-bits1 18-bits 18-bits 18-bits 18-bits 18-bits 18-bits 18-bits 22-bits 22-bits 22-bits
Termination (ohms) 120 120 250 120 120 250 120 250 240 120 240

1 Requires CS Rev. E or later, ECO M8012-ML005 installed. Earlier revisions are 18-bits.

References

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