In order to develop a software-only test environment for the James Webb Space Telescope (JWST) mission, a solution was needed to send commands and receive telemetry between the TCTS (Telemetry and Command Test Set) and CMM-S card. The as-is solution requires the utilization of commercial off-the-shelf hardware (RT Logic Telemetrix T501 processor) and custom CMM-S hardware.

The T501 emulator implements the behavior of the RT Logic Telemetrix T501 processor using only software. The T501 emulator provides two simple TCP/IP interfaces: the first interface conforms to the JWST TCTS specification, and the second interface provides a simple mechanism to downlink Virtual Channel Data Units (VCDUs) and receive uplinked Communications Link Transmission Units (CLTUs) from TCTS.

In the JWST test bed, the TCTS software communicates with the T501 over TCP/IP, and the T501 converts the TCP/IP packets into LVDS signals for the CMM-S card. In the software-only testbed, the T501 emulator replaces the physical T501 and, instead of LVDS signals, provides a simple, easy-to-use, TCP/IP socket interface.

The T501 emulator to TCTS interface is broken down into two main classes: sessions and services. Each session in the T50l opens exactly one TCP/IP socket. Depending on the session type, access may be granted to zero or more services. Each service represents a particular facet of the T501 operation.

In the command pathway, the TCTS software sends one or more CLTUs wrapped in an RTL (Run-Time Library) message specifying the send_cltus directive. One of the parameters for the send_cltus directive is the CLTU bytestream represented as ASCII hexadecimal values. The T501 emulator will convert the ASCII hexadecimal values into binary format, locate any and all CLTUs in the bytestream, and perform some basic sanity checks. Each accepted CLTU is sent to the CMM-S model individually over a TCP/IP socket. Any rejected CLTUs are logged.

In the telemetry pathway, the T501 emulator receives VCDUs individually from the CMM-S model over a TCP/IP socket. The T50I emulator wraps each VCDU with an RTL CADU header and sends it to the TCTS software.

This work was done by Daniel Nawrocki of Athena Sciences for Goddard Space Flight Center. GSC-16740-1

Test & Measurement Tech Briefs Magazine

This article first appeared in the February, 2015 issue of Test & Measurement Tech Briefs Magazine.

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