RH20 Massbus Controller Unit Description

This document describes the RH20 Massbus Controller, a high-speed interface facilitating data transfer between up to eight Massbus-compatible storage devices (such as disk-pack and tape drives) and eight data channels located within a system's MBox. It supports both synchronous data transfers and asynchronous operations for accessing device registers.

The RH20 consists of three double-height hexboards housed in the system I/O cabinet, connecting via the EBus (Execution Bus) for communication with the EBox, the Massbus for connecting to storage devices, and the CBus (Channel Bus) for high-speed data transfer with the MBox channels. Each RH20 has a unique device code and physical number, enabling parallel control of up to eight drives, though only one drive can perform data transfer at any given time.

The functional description covers the RH20's system interfaces, its instruction set, and various registers and commands. The EBus handles control data and status using CONO, DATAO, CONI, and DATAI commands, and services interrupts via PI SERVED/PI ADR IN. The Massbus provides independent control and data buses for interaction with storage devices, detailing signals for device selection, data transfer, and error reporting. The synchronous CBus manages high-speed data flow between the RH20 and MBox channels, with specific timing cycles for select, request, wait, and data transfer.

Key RH20 commands include:

• CONO: Loads miscellaneous control data and initiates various controller functions.
• CONI: Reads control and error status bits from the RH20.
• DATAO: Writes data to internal RH20 registers (like SBAR, STCR, IVIR, WR, DCR) and external device registers.
• DATAI: Reads data from specified internal or external registers.

The document also details drive commands (non-data and data transfer types) and channel commands (DATA TRANSFER, JUMP, HALT) used by the MBox to manage data transfers from memory. The logic description provides an in-depth look at the internal operations, using flowcharts and timing diagrams to explain asynchronous and synchronous data transfer control, command file processing (including "backup" commands to minimize latency), and diagnostic features for testing controller functionality, as well as the mechanisms for normal transfer termination and error handling.