Modbus Messaging and Access Control

Modbus Messaging Service

The 171 CBU 98090 and 171 CBU 98091 processors provides a Modbus Messaging service that permits remote devices to connect to–and communicate with–the processor. The processor supports up to 16 simultaneous Modbus TCP server connections.

Controlling Access

Use the Access Control list in the Messaging tab to restrict access to the processor in its role as Modbus TCP server. When Access Control is enabled, only devices with IP addresses added to the list can communicate with the processor over TCP port 502. The Access Control list can contain up to 128 devices.

If you enable Access Control, consider granting access to the following devices:

your maintenance laptop
every target device to which the processor adapter may send a Modbus TCP message

When Access Control is disabled, any Ethernet device can establish a TCP connection with the processor.

Navigating to the Messaging Configuration Tab

To open the Messaging tab:

Step	Action
1	In the Project Browser, navigate to the Project > Communication > Networks node.
2	Double-click the appropriate Ethernet network to open its configuration dialog.
3	Select the Messaging tab to open that page:

4	Enter the parameter settings for Access Control. Refer to the Access Control configuration parameters (below).
5	Click the validate check box in the upper tool bar to confirm the Modbus Messaging parameter settings.

Configuring Access Control

To enable the access control function:

Step	Action
1	In the Connection Configuration area, select Access Control. The Access Control list is enabled.
2	To grant a remote device permission to access the processor, complete a row in the list as follows: Place a check mark in the Access column. Type in the IP address for the remote device. The added device is permitted to open a TCP connection to the processor when Access Control is enabled.
3	Repeat step 2 for every remote device–up to 128 devices–that you want to be able to open a TCP connection to the processor.

Step

Action

1

In the Connection Configuration area, select Access Control. The Access Control list is enabled.

2

To grant a remote device permission to access the processor, complete a row in the list as follows:

Place a check mark in the Access column.
Type in the IP address for the remote device.

The added device is permitted to open a TCP connection to the processor when Access Control is enabled.

3

Repeat step 2 for every remote device–up to 128 devices–that you want to be able to open a TCP connection to the processor.

NOTE: If you select the Access Control check box but do not enter device addresses in the Access Control list, no Ethernet communications to or from the processor can occur.

Modbus TCP Messaging Commands

The processor supports the following Modbus TCP commands

Function Code / Sub-Code	Description
FC01	Read coils.
FC02	Read discrete inputs.
FC03	Read multiple registers.
FC04	Read input registers.
FC05	Write single coil.
FC06	Write single register.
FC07	Read exception status.
FC08 / 00	Return query data.
FC08 / 11	Return bus message count.
FC08 / 12	Return bus communication detected error count.
FC08 / 13	Return bus exception detected error count.
FC08 / 14	Return slave message count.
FC08 / 15	Return slave no response count
FC08 / 16	Return slave NAK count
FC08 / 17	Return slave busy count.
FC08 / 18	Return bus character overrun count.
FC08 / 21	Diagnostics Ethernet
FC08 / 22	Retrieving Ethernet channel-specific data from an Ethernet port
FC11	Get communications event counter: for serial line only.
FC12	Get communications event log: for serial line only.
FC15	Write multiple coils.
FC16	Write multiple registers.
FC22	Mask write register.
FC23	Read/write multiple registers.
FC43 / 14	Read device identification.