PCL20 Parallel Communications Link Users Guide

This "PCL20 Parallel Communications Link User's Guide" from Digital Equipment Corporation (July 1980) provides comprehensive information for the installation, operation, programming, and maintenance of the PCL20 communications system.

The PCL20 is a time-division, multiplexed, parallel communications link designed to connect up to 16 DECSYSTEM-2020s and/or PDP-11s on a single TDM bus. A DECSYSTEM-2020 can interface with up to four TDM buses, enabling a network of up to 64 systems. The system comprises a PCL11-B parallel communications link interface, a BA11-K mounting box, and a Unibus Adapter (UBA).

Key aspects covered in the document include:

• System Overview: Describes the PCL20 as a time-division multiplexed link transferring parallel 16-bit words. It includes odd parity and CRC-16 error detection, supports a maximum bus length of 90 meters (300 feet), and details data transfer rates that vary with bus length and the number of units.
• Installation: Provides detailed procedures for new installations and adding units to existing systems, covering site considerations, physical placement, cabling (using BC20K and BC20P-xx), grounding, and initial setup of TDM bus addresses and timeslice adjustments.
• Checkout Procedures: Outlines standalone diagnostic tests (MAINDEC-10-DSPCA) and network exerciser tests (MAINDEC-10-DSPCB) to ensure proper functioning and customer acceptance.
• Operation and Programming: Explains the PCL11-B's functional areas (transmitter, receiver, master section) and its interaction with the Unibus adapter. It details the TDM bus operation, time phase signals, address generation (via decrementing counter or address silo), data transfer protocols, error checking mechanisms, and procedures for channel opening, closing, message rejection, and truncation. Comprehensive bit assignments for various transmitter and receiver registers (e.g., TCR, TSR, RCR, RSR) and their roles are also described.
• UNIBUS Adapter (UBA): Details the UBA's function as a KS10 I/O controller, managing NPR data transfers between Unibus devices and KS10 memory, I/O register data transfers, and interrupt requests, including virtual-to-physical address translation via a paging RAM. It also covers various external I/O instructions.
• Maintenance: Focuses on fault isolation through diagnostic software and circuit board replacement, highlighting that individual units can be removed or powered down without affecting the entire TDM bus network.

The guide is a technical manual for users needing to implement, configure, and maintain this parallel communication system within a DEC environment from the early 1980s.