Sensing circuit sets up communications

Burlington, MA--At roadside weigh stations, vehicle weight is typically displayed inside the control room. A driver must wait for the operator's print ticket to know the vehicle's status.

Remote displays installed in the driver's view get the driver involved in the process. If the scale responds immediately, the driver knows the vehicle is positioned correctly. Weight is read directly, enabling quick response if there is a discrepancy.

After a technician installs a remote display, the receiver's communications parameters are set up to match the weighing amplifier's transmission characteristics. Conscientious setup ensures an accurate readout. Traditionally, display installers determine parameters such as baud rate, data format, and parity from documentation, then use DIP switches to set up the display's receiver. Because various data formats evolved over the years, installing remote displays at weigh stations can prove tricky and time consuming, especially when you consider that documentation is often unavailable. In such instances, most installers rely on a trial-and-error approach to set up communications.

"Distributors in the weighing industry wanted several sizes of remote displays that were quick, easy, and foolproof to install," says Scanning Devices president David Chanoux. The design goal: develop the Smart Remote Display (SRD) so it can automatically determine transmitted output parameters and display correct scale weight without using documentation, programming, or mechanical switches. An installer simply mounts the display, connects to line power, and then connects the signal input.

The design solution combines hardware and software. Engineers used PADS-PCB from PADS Software Inc. to design the circuitry and specify the parts and connections. For software development, engineers turned to Scanning Devices' Excaliber package. Excaliber produces microcode for the microcontroller from the PC's text file. A two-conductor, non-polarized input connection enables linking either RS-232 or 20-mA lines with a single connector. Pushing the learn button starts the automatic communications configuration and makes connection to various signal transmitters fast and easy. "Connecting to a signal transmitter is a snap with the learn button," says Scanning Devices President David Chanoux.

Automatic setup works without a need for knowledge of the transmitting instrumentation. The only requirement: about 50 characters of asynchronous serial ASCII data in RS-232 or 20 mA format. Logic in the circuitry automatically determines the polarity and format of the signal, digitizes the data, and sends it to a Phillips-Signetics 2691 Universal Asynchronous Receiver Transmitter or UART.

While the UART attempts to convert data from serial to parallel, an Intel 8051 microcontroller monitors the UART's success/failure rate and statistically analyzes to minimize failures. Once the transmission characteristics are determined, the microcontroller sets up the UART and stores the setup in an EEPROM. Each time the display powers up, the microcontroller reloads the UART with the correct parameters.

The transmitted signal first passes through two inputs to the voltage-sensing circuit. After undergoing logic operations, the input yields two outputs. Outputs determine if the signal is RS-232 or 20 mA, then route it appropriately through the voltage or current circuit. For example, if the signal is determined to be a voltage, the next step is polarity determination. If it's 20 mA, then active/passive determination is made. An iterative process continues until the correct standard is specified.

The system sets the baud rate after analyzing the minimum/maximum input frequencies. Then statistics on the UART's success rate provide the correct data format. After resolving parity with similar logic, the incoming data are compared to standard manufacturers' message formats stored in the microcontroller. "As manufacturers send us their standard message formats, we just add it to the library and add another loop to the logic," says Chanoux.

More than 200 SRDs are installed at weigh stations across the country. A patent application on the method and apparatus for automatic communications configuration is applied for, but not yet issued. "Easy installation and set up make the SRD a success," says Chanoux.

Additional details...Contact David Chanoux, Scanning Devices, Box 192, Lexington, MA 02173, (617) 272-5135

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