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package equipped with the EMCP 3 control panel in much the same way an operator does from the panel. The host device connects to the EMCP through a half-duplex RS-485 serial link. The data is transmitted in a binary format over the serial connection. The host device acts as Modbus Master, and the EMCP 3 electronic controller acts as the Modbus Slave, also called a Remote Transmitter Unit (RTU). The host device initiates all communication, sending commands or requests for information to the EMCP 3. The EMCP 3 then takes action based on the query and/or sends a response to the query over Modbus. Various data rates are available, and can be configured on the EMCP 3 control panel. See the Software Configuration section below for instructions. The options are the following: 2400, 4800, 9600, 14400, 19200, 28800, 38400, and 57600 baud. Wiring The SCADA communication wires are brought out of the EMCP 3 as part of the 70-pin AMP connector. The pins, as designated on the AMP connector, are shown in Table 1. Table 1: Modbus Pins on 70-pin Connector Pin # 3 4 5
Name MODBUS – MODBUS Reference MODBUS +
Description Optically isolated half-duplex differential (-) for Modbus Half-duplex RS-485 Reference for Modbus Optically isolated half-duplex differential (+) for Modbus
Modbus data transmission over RS-485 is accomplished over a single twisted pair for differential signaling with transmit and receive alternating over the same wire pair (MODBUS+ and MODBUS-). The Reference should be connected to the shield, to prevent ground loop currents. Figure 26 shows a possible wiring configuration to a RS485 device. Figure 27 shows a possible wiring configuration to a RS-485 to RS-232 converter. See documentation of connected device to verify wiring configuration. EMCP 3 AMP connector
RS-485 DB-9 connector
MODBUS - 3
5 TxD/RxD*
*
MODBUS + 5
9 TxD/RxD+
MODBUS Ref 4
1 COMMON
* - For multipoint network, terminate with 180ohm resistors at ends of network trunk
Figure 26: Possible wiring configuration to an RS-485 device
