EN-44
5.3 Modbus
5.3.1 Modbus RTU connection diagram
5.3.2 RS485 connection close-up
The components required for connection to the remote and/or local ModBus supervision system are as follows:
- An asynchronous half duplex RS485 serial card in RTU mode, connected to each controller.
- A standard RS485/USB converter for connection to a PC (not supplied by the manufacturer). The converter
can be connected to any network RS485 card.
- An electrical network using an AWG20/22 shielded cable (not supplied by the manufacturer) comprising a
twisted pair and shielding with a max length of 1000 m. This network must never run parallel to power cables
at a distance of less than 50 cm. These cables may cross, but perpendicularly. You are requested not to
form a loop with the network cable or the earth braid, and to properly separate the various cable families
(control, power, earth and communication bus).
- A supervision program installed on a PC (not supplied by the manufacturer).
An 120Ω ¼W electrical resistor must be connected to the RS485 serial card in last position on the bus, as shown in
the connection diagram.
The format of the data frame is as follows: 8 data bits, stop bits and adjustable parity on 2 words (2 bytes), high-order,
low-order. The data format (16 bits, signed) is standard for Modbus except for analogue data which is in the format
"Integer divided by 10".
The codes for the Modbus functions used are:
- 1 or 2: Read n bits
- 3 or 4: Read multiple registers (16 bits)
- 5: Write one bit
- 6: Write one register
- 8: Read diagnostics counters
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