RH11 MASSBUS controller

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The RH11 MASSBUS controller allowed the interconnection of MASSBUS devices such as the RP04 to systems with a UNIBUS, primarily PDP-11's.

The RH11 has the capability to operate in 18-bit mode; in this mode (used in the UC15 UNICHANNEL-15 on the PDP-15), the PA and PB UNIBUS lines are used for data bits 16 and 17.


The RH11 contains 4 registers, plus a share of a fifth; they are

  • RHCS1 - Control and Status 1 (shared)
  • RHWC - Word Count
  • RHBA - Bus Address
  • RHCS2 - Control and Status 2
  • RHDB - Data Buffer (for maintenance)

As is standard for the MASSBUS, all the other device registers are in the device.


The RH11 consisted of a double system unit backplane into which plugged a number of cards:

Two of them hex-sized:

  • M7294 - DBC - Data Buffer and Control
  • M7295 - BCT - Bus Control

Two dual-sized cards containing controller logic:

  • M7296 - CSR - Control and Status
  • M7297 - PAC - Parity Generation and Checking

Three dual-height M5904 MASSBUS transceiver modules.

Optionally one or two (see below) single-height cards:

  • M688 - UNIBUS Power Fail Driver

The RH11 backplane also contained three SPC slots in otherwise-unused slots.


The RH11 contains connectors and circuitry for two separate UNIBUSes; the second UNIBUS is primarily used on systems with multi-port memory, such as the PDP-11/45. If no CPU is connected to UNIBUS B, an M9300 terminator at the start of the bus can be configured to do NPR grants. A jumper allows the RH11 to do block transfers on UNIBUS B without going through an arbitration cycle.