KROHNE ALTOSONIC V Modbus EN - Manual (Page 9)

ALTOSONIC V
ModBus Manual 0300 rev07 E 7.30855.35.00 Page 9 of 64
4 PHYSICAL COMMUNICATION LAYER
The Modbus protocol is a half-duplex protocol. The physical layer can be half or full duplex.
The Modbus driver supports both half (RS485) and full (RS232/RS422) duplex communication layers.
In case of RS485, the parameter 3.8 MODBUS_UART_HALF_DUPLEX must be turned on. The
transmitter is activated when the UFP-V transmits data.
The RS485 receiver may not be disabled e.g. the transmitted data must also be received by the UFP-
V for correct functioning!
4.1 When using RS232 to RS485 converters
•
Always use isolated converters!
•
Use the types that enable the transmitter by means of the Request To Send signal.
• Use the parameter 3.4 MODBUS_UART_RTS_MODE to define whether a high or a low level
enables the transmitter.
•
Check if the terminator resistor corresponds with the characteristic line impedance.
•
Use pull-up and pull down resistors for fail safe operation.
• If possible, use the Serial Communication port that uses Interrupt Request 3.
4.2 When using serial I/O cards with RS485 drivers
• Use the types that enable the transmitter by means of the Request To Send signal.
•
Use the parameter 3.4 MODBUS_UART_RTS_MODE to define whether a high or a low level
enables the transmitter.
•
Check if the terminator resistor corresponds with the characteristic line impedance.
•
Use pull-up and pull down resistors for fail safe operation.
• Set the IO-address and Interrupt number to the correct values.
•
When possible, use Interrupt Request 3.