Teledyne 2101 Field Wizard, Section, Modbus Protocol, example, a process control computer, module

2101 Field Wizard
Section 4	Modbus Protocol
		By accessing these registers you can obtain the current value of
		whatever parameter you desire. The reading(s) can then be dis-
		played or stored wherever you designate as a destination; for
		example, a process control computer.
		Note
		Level, flow, velocity, and temperature data is stored in metric
		units only.
		Not all registers are limited to read-only data storage. You can
		also use some registers for control purposes. For example, by
		writing a “1” value to register 24 (“Identify Module” register), you
		will tell a 2100 module to light the LED on the front of the
		module.
4.2.1	Establishing	There are several different communications protocols supported
	Communication	in the 2100 series that require auto-baud rate detection. Because
		of this, each time a modbus connection is made, the module uses
		a polling mechanism to repeatedly send a command until a

response is received. It may take up to 20 command retries before the module has identified the baud rate and a response is received.

4.2.2Module Addressing When connecting to a site via a Modbus OPC server, you use a dedicated line of communication to that module or stack from the OPC server, which can be a dedicated communications cable (direct connection) or a dedicated phone number (modem).

When you are using a direct connection, you are dedicating a specified COM port on the computer, and that COM port deter- mines the site to which you are connecting.

When you are using a modem, the dedicated line is defined by the site's phone number.

If you connect more than one 2100 Series module at a site, the Modbus OPC server, while using the shared communication line for all of the modules within the network, must have some way to differentiate between the modules. When sending a command to a specific module, the command has an address field. This allows the server software to talk to, as well as control, the specified module, while ignoring other modules in the same stack or site.

Each module capable of Modbus Protocol communication will automatically create its own specific ASCII address within the site, using:

•The model numbers of the modules

•The user-defined module names

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