User`s manual

Watlow EZ-ZONE™ Communications • 4 • Chapter 2 Mobbus RTU & TCP

Chapter 2 Modbus RTU & TCP

Modbus Remote Terminal Unit (RTU) and Modbus TCP

Gould Modicon, now called AEG Schneider, created the protocol refered to as "Modbus" used in process

control systems. Modbus provides the advantage of being extremely reliable in exchanging information, a

highly desirable feature for industrial data communications. This protocol works on the principle of packet

exchanges. The packet contains the address of the controller to receive the information, a command field

that says what is to be done with the information, and several fields of data. Reading from these registers

retrieves all information in the controller. Each of these registers are listed in this user’s manual (Opera-

tions, Setup, Profiling, & Factory Pages). You will need this list to determine where the data is located. The

last item sent in the packet is a field to ensure the data is received intact. This is called a cyclical redundancy

check-sum (CRC). All information exchanged is in hexadecimal numbers.

Many parameter values within the PM controller are four bytes in length and require two Modbus regis-

ters. By default, the low register number contains the two lower bytes and the high register number contains

the two higher bytes. If it makes your programming easier you may reverse this Modbus default where the

low register number contains the two higher bytes and the high register number contains the two lower

bytes. This setting can be modified in the PM controller Setup pages under the "Com" menu.

Modbus RTU is typically deployed over serial connections where Modbus TCP is deployed over the Eth-

ernet physical layer. If it is desired to acquire more information on Modbus RTU or Modbus TCP direct your

browser to: http://www.modbus.org.

When purchasing a third-party software package, be sure to look for a package that is Modbus RTU/TCP

compatible or has Modbus drivers included. Most third-party packages require you to specify the Modbus

registers of the controller to setup and use the package. Again, all Modbus addresses can be found in this

manual.

Note: Excessive writes to the PM may cause premature EEPROM failure. For more detail see the section entitled "Saving

Settings to Nonvolatile Memory".

Default PM Communication Parameters (Modbus RTU)

If your model number has a one in the identified placeholder (PM_ _ _ _ _- [1] _ _ _ _ _ _) then these de-

faults apply.

Protocol ([PCoL]) = Modbus

Address ([Ad;M]) = 1

Baud Rate ( [bAUd]) = 9600

Parity ([PAr]) = none

To change or view the PM communication defaults follow the steps below:

1. Push and hold the up and down arrow keys on the front panel for six seconds to go the the Setup Menu.

2. Push the up or down arrow key until [`CoM] (Communications Menu) appears in upper display and

[`SEt] in the lower display.

3. Push the green Advance Key ‰ to enter the Communications Menu. The upper display shows [Mod],

and the lower display shows [PCoL].

4. Push the green Advance Key ‰ to change the Modbus address. The upper display shows [1], and the

lower display shows [Ad;M]. Use the up arrow key to change the Modbus address.

5. Push the green Advance Key ‰ to change the baud rate. The upper display shows [9600], and the lower

display shows [bAUd]. Use the up and or down arrow key to change the baud rate. Push the Advance Key

‰ to change parity. The upper display shows [nonE] and lower display shows [PAr]. Use the up and or

down arrow key to change the parity.