Communications fails if Access Control is checked off in Connection Configuration for Modbus protocol.

Explain why communications fails if the 'Access Control' option is checked off in the 'Connection Configutation'
section for a configuration that is using Modbus protocol to communicate to a Modbus device that is connected
to a '174CEV300xx Modbus to Ethernet' bridge from a Premium Ethernet port..

Product Line
174CEV300xx Modbus to Ethernet'

Environment
Windows 7

If a Modbus slave device is attached to a Premium Ethernet port through a '174CEV300xx Modbus to Ethernet'
bridge and the XWAY communications table is configured for Modbus protocol, the communications between
the Premium Ethernet port and the Modbus slave device will work as expected. However, if the option for Access
Control is checked off in the 'Connection Configutation' table found in the PL7 'XWAY Messaging tab for the Ethernet
port configutation, the communications will fail.

The reason for the failure is because the configuration is incorrectly defined as Modbus. The protocol needs
to be configured as UNITE.. This may seem strange because looking at the application from a user's perspective,
the connected devices are Modbus devices.

To help clarify this, the following will explain how the communications between the devices are performed.

The remote Modbus device connected to the bridge uses Modbus protocol to transmit Modbus messages
between the Modbus device and the '174CEV300xx Ethernet to Modbus' bridge. The bridge then
transmits the Modbus message by encapsulating the Modbus message within a TCP/IP message.
The XWAY table defines the protocol that is used to transmit the message over the XWAY bus to the
Premium's CPU via the XWAY bus on the Premium backplane. UNITE protocol is used to transmit the
messages from the modules on the backplane to the Premium CPU. When the TCP/IP message is received
at the Ethernet port, it is broken down and the Modbus message is extracted from it and re-encapsulted in a
UNITE message that is then sent to the CPU.

When an Ethernet to Modbus bridge is used to communicate from a remote Modbus device to the PLC, the
address that will be examined by Access control is the bridges IP address. The Xway Address must be defined
as the network and station address of the PLC. The station address for a Modbus device must have a value of 100
added to the PLC's address (i.e., If the PLC's address is 5, the station address is defined as 105.). Because the
communication request will take place through the bridge, the bridges IP address needs to be entered in the
Connection Configuration table as shown below



The Access Control allows the user to specify devices (i.e., remote device, bridges, SCADA, PCs, etc.) that have
access to the Premium Ethernet port.. When this feature is checked, only the devices that appear in the 'Connection
Configuration' table and have the Acess checkbox selected, will be able to exchange data with the PLC. No other
device will be able to communicate and exchange data with it.

When the connection is received from the remote device, verification of the IP address of the remote machine
is performed. This will occur only if access is activated by the user checking off the option in Connection
Configuration . This test consists of checking that this address is on a list of remote machines authorized to
connect. The test also checks for the communications protocol. If the test is positive the connection is opened.
If the test fails, the connection will remain closed.

If the configured protocol is defined as Modbus, the test will look for a message that contains the Modbus
protocol. However, it will not find a message with Modbus protocol because the Modbus message will be
encapsulated within a TCP/IP protocol.

We are currently working on correcting the applicable documentation to include a better explanation and possibly
modifying the PL7 configurations screens to make it friendlier to configure this type of application.