Artifact: Telemetry Multiplexer, Pulse Code Modulation (PCM), Apollo Command Module, Block II

Apollo Command Module PuIse-Code Modulation (PCM) Telemetry Equipment (manufactured by Radiation, Inc., Melbourne, Fla.) This unit weighs 42.1 pounds and is 13 by 7 by 14 inches. It received and sampled analog, parallel digital and serial digital information, which consists of astronaut biomedical, spacecraft operation, and scientific data; and converted it to a single serial output for transmission to earth. This single-output signal was routed to the Premodulation Processor (See following artifact for an example of an the Premod Processor) for transmission to the ground or to Data Storage Equipment (DSE).

The pulse-code modulated telemetry equipment was located in the lower equipment bay. Incoming signals were of three general types: high-level analog, parallel digital and serial digital. Two modes of operation were possible: the high (normal) bit-rate mode of 51.2 kilobits per second and the low- (reduced) bit-rate mode of 1.6 kilobits per second. The analog multiplexer accommodated up to 365 high-level analog inputs in the high-bit rate mode. These analog signals were gated through the multiplexer, the high-speed gates, and then fed into the coder. In the coder, the 0-to-5 volt analog signal was converted to an 8-bit binary digital representation of the sample value. This S-bit word was parallel-transferred into the digital multiplexer where it was combined with 38 external 8-bit digital parallel inputs to form the output in non-return-to zero (NRZ) format. The digital parallel information was transferred into the output register and combined with the digital serial input, and then transferred serially into the data transfer buffer. From here the information was passed on to the premodulation processor for preparation for transmission.

The pulse-code modulation telemetry equipment received 512-kiloHertz and 1-Hertz timing signals from the central timing equipment. If this source failed, its programmer used an internal timing reference. The timing source being used was telemetered. Two calibration voltages were also telemetered as a confidence check of the operation of the telemetry equipment.