18
Control
MODBUS-RTU
MODBUS is an application-layer messaging protocol that formats data for communications
over serial RS-232 or RS-485. The instrument supports MODBUS-RTU protocol, with data
transmitted through Pin 6 of the 6-pin connector.
MODBUS-RTU can be used to control:
•
taring
•
data collection
•
setpoint control
•
totalizer
•
batch dispensing
•
changing the gas
•
control loop adjustment.
Note:
For more information on MODBUS-RTU communication commands, please see
the MODBUS FAQ at
alicat.com/using-your-alicat/faq-modbus/
and the MODBUS-RTU
manual at
Communicating Via MODBUS-RTU
The following MODBUS commands are supported:
3
Read holding register
6
Write single register
16
Write multiple registers
Use the instrument registers to interact with the instrument.
Note that the response to function code 6 (Write Single Register) will echo the value provided
by the master in the write and not the value of the register after the write. For example, if
the setpoint register were written to a value that is not valid for the instrument, it will be
adjusted inside the register; however, the value returned will be the original setpoint value.
If the real value is desired, a read of the register must be issued. This is consistent with the
MODBUS specification and demanded by some MODBUS implementations.
Reading undefined registers may return 0 or a Bad Address error.
MODBUS Address
Each instrument must have a unique address, which will be an integer from 1 to 247.
To find the MODBUS address
: read register 45.
To assign a MODBUS address
: write an integer from 1 to 247 to register 45.
Changing the Baud Rate
BASIS 2 units can communicate at the baud rates shown in the table below.
Value
Corresponding Baud Rate
0
4800
1
9600
2
19200
3
38400 (default)
4
57600
5
115200
