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Friday, July 2, 2010

Serial Relay Control Fixture

During my career as an electronic technician, I built many different test fixtures for both engineering and production testing. One thing nearly all automated test set-ups need is some PC-driven interface for controlling power and signals during testing. Enter the serial relay control fixture.

The serial relay fixture detailed here is used in an automated test tower to control power and signals to and from each of up to 8 assemblies, or units being tested (UUTs). This device consists of a 12 volt power supply, six RS-­232 relay boards, connectors for each type of port, and quite a bit of inter­connect wiring - all contained in an aluminum housing. As is shown in the above photo, there is both an exhaust fan as well as a filtered air intake port for ventilation. The cover is reversed in the photo showing the opened box - when assembled, the fan is normally positioned near the power supply to quickly remove the heat it generates. Each of the serial relay boards is uniquely addressable and contains 8 relays. Each of the relays is a DPDT unit which can be individually controlled with RS­232 commands. Since there are six boards in the fixture, there are a total of 48 relays available for use.

Eight of these relays controlled power to each of the UUTs, most of the remaining ones switched signals between the UUTs and various pieces of gear. The RS-232 relay boards, a "commercial off-the-shelf" item, are PIC microcontroller based. These boards were designed such that up to 255 of them may be connected in parallel off the same RS-232 buss. Prior to being interconnected in this way, each one must be programmed individually by connecting it to a PC and assigning it a unique identifier number (address). Once this is done, each board can remember its address even if power is disconnected.

This fixture, as well as the complete refrigerator-size test tower it is a part of, is controlled via a Windows PC running LabVIEW software. The LabVIEW code performs two essential functions: it handles all the signals which control the sequence of the tests, and performs data acquisition via test gear equipped with GPIB or RS-232 ports. Each of the UUTs also communicates with another of the the PC's serial ports through a 16 port serial console switch - also controlled via the LabVIEW program.

While this serial relay control fixture was strictly used for control of an automated test tower, one could clearly use this same technology to accomplish other tasks - such as controlling industrial processes, or performing building automation functions.

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