Installation and Operating Instructions - Crompton Instruments

RS485 DIGITAL OUTPUT - MODBUS IMPLEMENTATION
The SPR offers a RS485 communication INTERFACE for direct connection to SCADA systems
using the MODBUS RTU protocol. 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
occurred 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 its response.
The electrical interface is 2-wire RS485 (half duplex), via a 2 part connector. 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 one point on the network. See the connection diagram for details.
A total maximum length of 1200M (4000 ft) is allowed for the RS485 network. A maximum of 32
electrical nodes can be connected, including the controller. The cable should be terminated with
a resistor at each end.
The address of each SPR can be set to any value between 1 and 247. Broadcast mode (address
0) is not supported.
The maximum latency time of an SPR 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 SPR is not going to respond.
1. Heavy Modbus traffic can affect the loop time of the product.
2. If an invalid or out of range number is written into a holding register, the SPR will
attempt to set that register to the nearest valid number. It is particularly
important when setting up or modifying trip parameters that all data values are
written and then read back and verified.
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