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6 Serial Communications
6.1. RS485 Port – Modbus or JC N2
0240 0340 0440 0640 1000 1540 Option
INTEGRA 1000 and 1540 offer the option of an RS485 communication port for direct connection
to SCADA systems. This port can be used for either an RS485 Modbus RTU slave, or as a
Johnson Controls N2 protocol slave. Choice of reply protocol is made by the Integra on the
basis of the format of request, so that a Modbus request receives a Modbus reply, and an N2
protocol request receives an N2 protocol reply.
6.2 Modbus® Implementation
This section provides basic information for the integration of the product to a Modbus network.
If background information or more details of the Integra implementation is required please refer
to our “Guide to RS485 Communications and the Modbus Protocol”, available on our CD
catalogue or from any recognised supplier.
The Modbus‚ protocol establishes the format for the master's query by placing into it the device
address, a function code defining the requested action, any data to be sent, and an error
checking field. The slave's response message is also constructed using Modbus protocol. It
contains fields confirming the action taken, any data to be returned, and an error-checking field.
If an error occurs in receipt of the message, or if the slave is unable to perform the requested
action, the slave will construct an error message and send it as it’s response. Framing errors
receive no response from the Integra.
The electrical interface is 2-wire RS485, via 3 screw terminals. Connection should be made using
twisted pair screened cable (Typically 22 gauge Belden 8761 or equivalent). All "A" and "B"
connections are daisy chained together. The screens should also be connected to the “Gnd”
terminal. To avoid the possibility of loop currents, an Earth connection should be made at only
one point on the network.
Line topology may or may not require terminating loads depending on the type and length of
cable used. Loop (ring) topology does not require any termination load.
The impedance of the termination load should match the impedance of the cable and be at both
ends of the line. The cable should be terminated at each end with a 120 ohm (0.25 Watt min.)
resistor.
A total maximum length of 3900 feet (1200 metres) is allowed for the RS485 network. A
maximum of 32 electrical nodes can be connected, including the controller.
The address of each Integra 1000/1540 can be set to any value between 1 and 247. Broadcast
mode (address 0) is not supported.
The maximum latency time of an Integra 1000/1540 is 150ms i.e. this is the amount of time that
can pass before the first response character is output. The supervisory programme must allow
this period of time to elapse before assuming that the Integra is not going to respond.
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Integra 1540, 1000, 0640, 0440, 0340, 0240 Issue 1 04/03