MAINDEC-11 ReadAlph

MAINDEC 11, the Read Alphanumeric Test program, is a utility designed to test the performance of the PDP-1 photo-electric paper tape reader during its alphanumeric operation. (It is distinct from MAINDEC 10, which tests binary operation.)

The program utilizes two tapes: the MAINDEC 11 program tape loaded into memory, and a closed loop test tape. This test tape contains alternating lines of eight 1's (octal 377) and eight 0's (octal 000). As the reader continuously reads this loop, each bit is checked against expected data stored in core memory.

MAINDEC 11 identifies two primary types of reader malfunctions: "picking up bits" (reading a 1 where a 0 is expected) and "dropping bits" (reading a 0 where a 1 is expected). The program's operation is controlled by console switches, allowing for adjustments such as program delay (reader speed) and diagnostic checks (e.g., high-speed, brake, clutch, multispeed, and margin checks). Upon detecting an error, the program halts, indicating the location and specific discrepancy, allowing the operator to record the malfunction. An auxiliary program is also provided to generate the required test tape.