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FO 935
OUTPUT RS485 MODBUS
INTRODUCTION TO THE MODBUS INSIDE MODULE
The MODBUS INSIDE expansion module is built in the monitoring unit and allows data transfer on a RS485 line with
MODBUS RTU protocol, maximum 32 devices.
OPERATING NOTES
For the module to work correctly, it is necessary to set the RS485 network set-up parameters: address, baud rate, parity
bit.
See programming steps 32 to 37 on page 20.
The serial communication of the temperature control monitoring unit is active only when the FO 935 D is in temperature
control mode in one of the intended modes (Scan, Auto, Man and T.Max).
When other functions such as programming, programming display and relay test are activated, the ModBus
communication is temporarily deactivated.
DATA TRANSMISSION ON MODBUS NETWORK
The internal MODBUS INSIDE module is used to connect the FO 935 D control unit to an RS485 network with Modbus
RTU protocol in order to read the data indicated in the modbus table, page. 25, and to be able to write those indicated in
the notes section for remote programming, the module is always in slave mode.
The FO 935 D control unit is in communication with the network only when it is in temperature reading mode, while it is
inactive when in the following modes: display, programming and test relays.
RS485 ELECTRICAL CONNECTIONS
As far as the signal cable to be used in order to ensure the correct network operation is concerned, we recommend you
follow the provisions of the EIA RS485 standard which suggests using a 24AWG twisted pair.
The twisted pair that connects units in RS485 might need a 120 ohm end resistor on the last unit of the series.
Connect the twisted pair paying attention to polarities and lay the network avoiding to make sharp bends or ring windings
in order not to modify line impedance. If necessary, the GND terminal for grounding is also available.
Always position the RS485 twisted pair far from power cables.
DATA FRAME
The frame in asynchronous transmission consists of:1 start bit, 8 data bits, 1 parity bit (even or odd, if the parity has been
set) and 1 stop bit.
With parity NO (none) it is possible to select N-1 (1 stop bit) or N-2 (2 stop bit).
The permitted baud rates are:2400, 4800, 9600, 19200 and 38400.
If not otherwise specified, the word length (DATA) is 16 bits.
DATA PACKET
A complete request/response sequence is composed as follows:
Master's request:
SLAVE ADDRESS
- 1 byte
FUNCTION CODE
- 1 byte
DATA
- variable, depends on the function code
CRC
- 2 byte
Slave request:
SLAVE ADDRESS
- 1 byte
FUNCTION CODE
- 1 byte
DATA
- variable, depends on the function code
CRC
- 2 byte
1MN0095 REV.0
(ONLY FO 935 D VERSION)
