Hach Sigma 950, Instrument Manual

Page: 113 / 146

111
SCADA-Modbus
®
System Guidelines
With the exception of baud rate, the communication parameters must not vary from this
format.
E.8 User Memory Customizing
The most familiar component of existing SCADA networks today is the Programmable
Logic Controller (PLC). Because the network integrator is most familiar with this type of
device, the flow meter emulation of an existing PLC simplifies the process of integrating
the manufacturer’s instrumentation into the SCADA network. Modbus ASCII uses a
referencing system to identify the various types of memory inputs and outputs. Each
reference number has a leading digit that identifies its data type (discrete input, discrete
output, register input, register output) followed by a string of digits that indicates its
location in RAM (
Table 40 ).
The memory data is stored in 16-bit words. Within the predefined function codes of the
Modbus ASCII protocol, the data fields are subject to interpretation by the device
manufacturer. For example, the 950 Flow Meter places temperature information in
registers 40001-40002.
E.9 Modbus ASCII Function Codes Supported
Currently, the 950 Flow Meter is capable of a read-only function to retrieve channel and
total flow information. All data addresses in the Modbus ASCII message are referenced to
zero. Therefore, a reference to holding register 40001 is addressed as register 0000. The
function code field specifies the type of register accessed; therefore, the 4XXXX is
implicit.
Function 03: Read Holding Registers
Reads the register (4X reference) contents of the 950 Flow Meter as defined in the tables
that follow.
Table 40 Modbus ASCII Memory Input/Output Referencing System
Reference
Indicator
Reference Type Meaning
0xxxx discrete output or coil binary
1xxxx discrete input binary
3xxxx input register real
4xxxx output holding register real
6xxxx extended memory register real