PDP-14
| Company type | Division |
|---|---|
| Industry | Computer hardware, Industrial automation |
| Founded | June 1969 |
| Headquarters | United States |
| Products | PDP-14 programmable logic controller |
The PDP-14 was a dedicated computer developed by Digital Equipment Corporation’s Industrial Products Group, intended to supplant industrial relay systems controlling machinery that performed repetitive operations. It was specifically designed to maintain dependable functionality within electrically noisy environments involving electric motors, solenoids, and arc welders—situations that typically posed challenges for conventional electronic devices. As an initial tier in factory automation, the PDP-14 operated as a programmable logic controller (PLC) and was capable of interfacing with a standard DEC PDP-8 minicomputer.
The inaugural unit was delivered in June 1969 to oversee a gear grinding machine. The concept of this "programmable machine controller" was officially patented in 1973.
The PDP-14 executed Boolean equations commonly represented through “ladder diagrams” and incorporated programmable read-only memory (PROM). Programming was performed on a PDP-8 system and verified via a direct connection; during testing, the PDP-8 provided instructions before these were ultimately transferred into the PROM.
Subsequent iterations, including the PDP-14/30—which did not maintain binary compatible status—utilized the physical packaging technology from the PDP-8. Other versions comprised the PDP-14/35 and a budget-friendly PDP-14/L variant featuring reduced input/output capacity.
Hardware
The 12-bit architecture of the PDP-14 supported up to 4K words for its instruction set. Its hardware setup consisted of a control unit paired with multiple external modules:
- I-boxes (BX14) handled discrete input signals, accommodating up to 256 input points.
- O-boxes (BY14) managed control over as many as 255 actuator outputs.
- A-boxes contained timer modules used for timing-related operations or retentive storage modules that preserved data through power interruptions; these shared their output address space with O-boxes.
- S-boxes resembled O-boxes but lacked physical outputs and were utilized to store intermediate computational results within the shared output address space.
The combined output address space across O-, A-, and S-boxes was capped at 255 addresses or fewer.
Registers
The PDP-14 included seven 12-bit registers:
IRPC1andPC2MBSPAREINPUTandOUTPUT
Instructions
Prominent instructions within the PDP-14 instruction set were:
TRR: Facilitated data transfer among selected registers. ThePC1andSPAREregisters could be incremented or decremented during transfer.JMS: Jumped to a subroutine at a specified 12-bit address, storing the return point inPC2.JMR: Returned from subroutine by jumping to the address contained inPC2, effectively moving control fromPC2back toPC1.SKP: IncrementedPC1by one, effectively skipping an instruction.
Other instructions provided functionalities such as testing ON/OFF states and issuing set commands like SYN (to turn "Y" ON) and SYF (to turn "Y" OFF).
Software
Programming for the PDP-14 was initially handled exclusively by DEC. Subsequently, development moved onto the PDP-8, where a software tool called SIM-14 emulated the PDP-14 environment, enabling programming and testing without requiring access to the original hardware.