Felix Instruments F-901B, Reference Manual, Page: 8 / 19

Page 7 of 18
MODBUS
The F901B supports the standard Modbus protocol in both RTU and TCP/IP mode:

TCP/IP: the factory default mode. Upon powering up, the F901B initializes the Modbus TCP/IP protocol via
the default static IP address 192.168.1.50 port 502 (This address is configurable via
Modbus commands ).

RTU: The F901B sensor can be configurated to operating in RTU (RS485) mode by setting the
MODBUS_MODE register and swapping the internal cables inside the sensor. See below for RTU
specifications. Please contact Felix Instruments for details on how to configure sensor hardware in RTU
mode.
RTU RS485 Configuration
Pin Description
Pin Name Type Description
1 3.3V Reference potential 3.3V Reference Voltage
2 GND Reference potential Local device ground
3 UART_TX Digital Output Firmware update interface
4 B (D-) Bus In/Out Driver output and receiver input
5 A (D+) Bus In/Out Driver output and receiver input
6 UART_RX Digital Input Firmware update interface
7 BOOT Digital Input Firmware update interface
8 RESET Digital Input Microcontroller reset input (Active-Low) Firmware
update interface
IMPORTANT : use pin 4,5 (B/A) and GND for Modbus RS485 communication. Pin 3,6,7,8 are reserved for firmware
updating and are 3.3V tolerant.
Sampling cycle Cleaning cycle
Trigger interval
idle time
8 7 6 5 4 3 2 1
RJ-45
Plug
RJ-45
Socket
1 2 3 4 5 6 7 8
1 2 3 4 5 6 7 8
Figure 3 Trigger measurement timing
Front
Bottom
Front