UART Wildcard User Guide

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RS422/485 Configuration Jumpers

The remaining jumpers on the UART Wildcard enable conversion from RS422 to RS485, and the installation of optional RS422/485 termination networks. There are three jumpers for each serial port. The silkscreen on the UART Wildcard designates "Port1" and "Port 2" as the two jumper regions. Within each region is a pair of jumpers labeled "RS485", and an additional jumper labeled "Term", which stands for "termination".

RS485 Jumpers

Two jumpers on each port configure that port to be either RS422 or RS485 capable. In RS422 mode two wires are used for receive and two for transmit. In RS485 mode only one pair is used for both transmit and receive; the RS422 transmit and receive pairs are shorted together with a pair of onboard jumpers. To use the half duplex RS485 protocol on a given serial port, install the two jumper caps on the two jumpers labeled "RS485" in that port’s jumper area (labeled "Port1" or "Port2"). To use RS422, remove the jumper caps from the "RS485" jumpers. If the port is software configured for RS232 the jumpers have no influence. The remaining protocol configuration is software programmable as described below.

Optional RS422/485 Termination Network Jumpers

Each serial communications link can be thought of as a "transmission line" with a characteristic impedance. When the signals on the transmission line encounter a non-matching impedance, a portion of the signal energy is reflected, thereby degrading the signal-to-noise ratio of the communications link. One advantage of the differential signaling scheme used by the RS422 and RS485 protocols is that a straightforward termination network can be installed at the end of the transmission line to minimize unwanted reflections. The UART Wildcard provides a default termination network for each serial port. To install the termination network, simply place a jumper cap on the jumper labeled "Term" (termination) in that port’s jumper area (labeled "Port1" or "Port2"). This places a 100 Ω resistor in series with a 0.1 μF capacitor across the differential signal conductors. The capacitor blocks DC current that would greatly increase the power drain of the circuit. The 100 Ω resistor provides the effective termination resistance seen by the high frequency bit transitions.

Please note that termination networks should be installed on multi-drop RS485 links only at the two terminal nodes in the network. At high baud rates, the termination network may reduce the amplitude of the received pulses, so make sure that you test any termination scheme that you put in place in your custom application.

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