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:
- A system with a 240 ohm processor can accommodate up to 20 AC loads, without far end termination.
- A system with a 240 ohm processor can accommodate up to 35 AC loads, with 240 ohm termination on the far end. The far end termination may also be 120 ohms, but this is considered to be less optimal.
- A system with a 120 ohm processor can accommodate up to 35 AC loads, without far end termination.
- A system with a 120 ohm processor can accommodate up to 45 AC loads, with 120 ohm termination on the far end.
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
- LSI-11 Systems Service Manual, 5th Edition, January 1985 (EK-LSIFS-SV-005)
- Microcomputer Products Handbook, 1985 (EB-26078-41)
- PDP-11/23B Mounting Box User's Guide, 1st Edition, January 1982 (KE-23BMB-UG-001)
Additional Resources
- MicroNote 5 - Q22 Compatible Options
- MicroNote 29 - Q-bus Expansion Concepts
- MicroNote 35 - Backplane Expansion/Termination
