YSI 6500 Environmental Process Monitor - Manual (Page 47)

Proper Use and Care of the 6500
Section 5
IMPLEMENTATION OF MODBUS SYSTEM
The 6500 Modbus system can be implemented if your application requires digital data
management of one or many parameters on a single data channel. This system involves a simple
interface between the 6500 Monitor and a Programmable Logic Controller (PLC). Configuration
of the PLC should be done only by programmers and engineers with PLC experience. The
following section provides basic information for the programmer who is involved in
6500/Modbus set up. Additional advice on implementing the 6500 Modbus system can be
obtained by contacting YSI Customer Service.
The interaction between the PLC and the 6500 is simple. The PLC writes to and reads from the
6500 using just two of the dozens of Modbus commands. The 6500 makes use of the Modbus
register system to transfer data. It will respond to two Modbus commands, “Read Holding
Registers” and “Preset Multiple Registers”. All other Modbus commands are unsupported and
ignored if given.
The 6500 supports both RS-232 and RS-485 communications protocols. Labeling of RS-485
channels sometimes varies. When the 6500 transmits through the RS-485 port, it drives channel
A high and channel B low during the active state, and in the idle state drives channel A low and
channel B high.
There are 4 main register areas to deal with the parameters:
• Parameter type
• Parameter status
• IEEE floating point parameter data
• Scaled integer parameter data
First, the PLC writes to the parameter type registers to indicate which parameter measurements it
wants. Programmers can enable and disable these sonde parameters by writing to these registers
using the "Preset Multiple Register" command. Each parameter and unit is identified by a
specific code available from the YSI master sensor list.
The data must then be read from the 6500 by the PLC. The 6500 maintains a current set of data
in the holding registers. Use the "Read Holding Registers" command to obtain the most recent
set of data from sondes connected to the 6500. Each parameter from each sonde is stored in a
different register (or register pair). Programmers may choose from two sets of data registers: one
contains a set of IEEE floating point data, and the other contains a set of scaled integer data.
The 6500 also maintains parameter status information in another set of registers. These registers
contain information regarding the status of the parameters in each register. These can also be
read using the “Read Holding Registers” command.
Each of these areas is 15 registers long, except for the floating point data area which is 15 register
pairs long. The first register (or register pair for the floating point data) in each area corresponds
to the first parameter, the second corresponds to the second parameter, etc.
In general if you attempt to read from a reserved or unused area, the 6500 will return a value of
“0”.
YSI Incorporated 6500 Environmental Process Monitor 44