Instruction manual

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MODBUS SYSTEM SET UP
This section describes the functioning of the Model 1700 controller and Model 1708/1716
RTUs in a MODBUS system.
MODBUS OPTION
The Model 1700 controller has an optional implementation of the industry standard
MODBUS protocol. This option is provided by means of a replacement EPROM containing
code to support the MODBUS protocol when communicating with the PC running the
MODBUS application. The Model 1708 and Model 1716 RTUs do not require any
modification to support the MODBUS protocol if they are communicating only with the
Model 1700. There are optional EPROMs for both of the RTU models that will allow them to
be interfaced directly to the PC, by-passing the Model 1700 controller. The MODBUS
implementation makes the devices compatible with nearly all SCADA programs as well as
with many PLCs.
The Model 1700 is compatible with most MODBUS programs that use five-digit MODBUS
addresses and limit themselves to the functions listed in Table 10.
Table 10. Compatible MODBUS Functions
MODBUS Function Function Code Used for...
Read Coil Status 01 Digital Outputs
Read Input Status 02 Digital Inputs
Read Holding Registers 03 Analog Outputs, Counters, Accumulators
Read Input Registers 04 Analog Inputs
Force Single Coil 05 Digital Outputs
Preset Single Register 06 Analog Outputs, Counters, Accumulators
Force Multiple Coils 0F Digital Outputs
Preset Multiple Registers 10 Analog Outputs, Counters, Accumulators
The correspondence between Zetron and MODBUS I/O terminology is listed in Table 11.
Table 11. Zetron and MODBUS I/O Terminology
Zetron I/O Type MODBUS I/O Type
Digital/Relay Output Coil
Digital Input Input Status
Analog Input Input Register
Analog Output Holding Register
Accumulator Holding Register
Counter Holding Register
Run-time Holding Register