The RS-485 mode is set using 3 jumpers, and by selecting the RS- 485 Mode for DTR Operation on the com Port.
RS-485 Network Biasing
In a RS-485 network, the transmitter tri-states to a high impedance state. Unlike RS-422 where the transmitter holds the TD(B) line high and the TD(A) line low (Mark state) when not transmitting data.
To establish and maintain the Mark state for all RS-485 receivers, Biasing is required. Biasing maintains a positive voltage difference from RD(B) relative to RD(A) above the receiver threshold of 200mV for all devices on that section of the network. If the biasing is not maintained, the first data bit of a signal may not be detected, and one or more devices may not respond to commands, or may operate intermittently.
The RS-485 receiver on the 3PCIO1 is biased with a 4.7 KΩ pull-up resistor on the Receive Data B line and a 4.7 KΩ pull-down resistor on the Receive Data A line. These values are usually adequate for networks without termination and 31 or fewer nodes. When termination is used, biasing must be increased, calculated according to the termination value and number of nodes. Through hole pads are provided for adding smaller value pull-up and pull-down resistors.
Port Bias: (Low: R2/14, Hi: R3/13) R2 = R3 = 4.7K
Combination of termination and bias must not fall below 54 ohms.
Termination: Not installed. Not required for baud rates 19.2K or less.
Termination Resistor Location: R15 (R15 ≥ 120 ohms)
Documentation Number 3PCIO1-0903 Manual | Chapter 6 | 47 |
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