OPERATION ST51A/ST75A/ST75AV Mass Flow Meters
56 Fluid Components International LLC
Modbus Operation
The ST51A/ST75A/ST75AV offers Modbus as one of its digital communication protocol, but unlike the other digital communication
protocols Modbus only offers set up and configuration for the totalizer variables and K Factor corrections. Refer to Modbus Connections on
page 24 for Modbus wiring details.
The Modbus physical layer uses the flow meter’s asynchronous RS-485 serial port. There is no high speed Modbus over Ethernet. The
flow meter is configurable to operate in one of two basic traditional transmission serial interface modes: RTU and ASCII message coding.
The ST51A/ST75A/ST75AV offers the process variable parameters (value) in floating point form, which are organized as single or double
precision floating point registers. These registers are the 4000 and the 5000 group registers, both of which are accessed using Modbus 03
and 04 function codes. Refer to Table 20, page 58, for an overview of the registers.
Setting Up the ST51A/ST75A/ST75AV for Modbus Operation
1. Use an FC88 or PC (see Instrument Configuration and Setup Using the Service Port (RS-232), page 27 for details) to access the
instrument’s main menu:
Mode?
1=RS-232 2=Modbus
3=HART 4=Config
5=Update Firmware >
2. Enter 4 to select the instrument’s configuration menu:
**Configure**
Configure?
1=ST51A/75A
2=Modbus
3=HART >
3. Enter 2 to select Modbus configuration. The first configurable parameter in the Modbus configuration menu sequence, Modbus Node,
displays:
**Modbus Setup**
Modbus Node: 1
Change? (Y/N)
4. Each parameter is displayed with its currently programmed value, followed by a “Change?” prompt asking whether or not to change it.
Accept the existing parameter by entering N or ENTER to go to the next parameter. Or, enter Y to make a change.
**Modbus Setup**
Modbus Node: 1
Change? (Y/N)
5. Mode (ASCII or RTU), Baud Rate (9600 or 19200), Data Bits (7 or 8), Parity (No Parity, Odd, or Even), and Stop Bits (1 or 2), in that
order, follow the Modbus Node parameter. Make changes to any of these parameters as required for your application.
6. After stepping through all parameters in the sequence, the serial interface display indicates Modbus setup completion as shown
below. Enter ESC to exit the Modbus setup menu.
**Modbus**
(ESC to exit)
MB_CommPortCfg
Modbus setup OK.
Modbus Init Done
7. With the main menu on display (as shown above in step 1) select the Modbus option (2) to operate the instrument in the Modbus mode. The
serial interface responds with the lines shown below. The instrument is now ready to be used with the Modbus software of your choice.
**Modbus**
(ESC to exit)
Modbus Init Done
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