Module Ordering
The order in which modules are placed in the Q-bus backplane can have a impact on the overall system performance. As a rule-of-thumb the ordering should be as follows:
- Processor
- Memory
- High speed devices and devices that cannot handle much bus latency
- Low speed devices and devices that can handle bus latency
This also depends on whether the device monitors the bus. Devices that do monitor the bus should be placed ahead of devices that do not.
My system is ordered as follows:
- KDJ11-AB processor - must always be in the first slot.
- CMV-504 memory - should be close to the processor.
- DEQNA Ethernet controller - high speed device that supports block mode DMA transfers. Monitors the Q-bus for other pending DMA requests and suspends its transfers when interrupted by a lower priority device. This module cannot quickly recover from bus latency conditions, therefore, it needs a high priority slot.
- Emulex UC07 disk controller - high speed device that supports block mode DMA transfers. Monitors the Q-bus for other pending DMA requests and suspends its transfers when interrupted by a lower priority device. Provides adaptive DMA to ensure the CPU is not locked out for long periods of time.
- Emulex QT13 tape coupler - slower than the UC07, supports block mode DMA transfers. Monitors the Q-bus for other pending DMA requests and suspends its transfers when interrupted by a lower priority device. Provides adaptive DMA to ensure the CPU is not locked out for long periods of time.
- MXV11-BF multifunction module - Low speed device that does not monitor the bus and does not support DMA transfers. Relies on interrupts for transmitted and received data.
- DHQ11 asynchronous multiplexer - Low speed device that does not monitor the bus. Performs DMA transfers from the bus for transmitted data and relies on interrupts for received data.
References
- KDJ11-A CPU Module User's Guide, 1st Edition, May 1984 (EK-KDJ1A-UG-001)
- MVX11-B Multi-Function Option Module User Guide, 1st Edition, 1983 (EK-MXV1B-UG-001)
- DEQNA Ethernet User's Guide, 1st Edition, August 1984 (EK-DEQNA-UG-001)
- DHQ11 User Guide, 2nd Edition, July 1987 (EK-DHQ11-UG-002)
- UC06, UC07 and UC08 Intelligent Host Adapter Technical Manual, Revision H, November 1990, Emulex Corporation (UC0751001-00)
- QT13 Tape Coupler Technical Manual, Revision J, August 1990, Emulex Corporation (QT1351001-01)
Additional Resources
- DEC Micronote 41 - Device Ordering Chart for Q-bus Systems
