Rockwell Automation 2711P Creating Modbus Applications - Manual (Page 13)

Publication 2711P-UM002B-EN-P - March 2007
Modbus KEPServer Drivers 13
Modbus ASCII Modbus ASCII protocol is typically used to connect to other ASCII
devices that support the Modbus ASCII protocol. KEPServer support
includes:
• Modbus ASCII compatible devices
• Flow Computers using Daniels/Omni/Elliott register addressing
Modbus/TCP Modbus/TCP is a Modbus messaging protocol over Ethernet TCP/IP
and is intended for supervision and control of automation equipment.
The most common use of this protocol is for Ethernet attachment of
PLCs, I/O modules, and gateways to other simple field buses or I/O
networks.
The Modbus/TCP KEPServer driver supports Modbus and Mailbox
device models.
Modbus Device Model
The most common Modbus device model is where the driver connects
to physical devices (e.g. Modicon TSX Quantum, other Modbus Open
Ethernet compatible devices) and acts as a device on the network
with a device ID equivalent to the machine's IP address. The driver
accepts any unsolicited commands it receives and attempts to process
them as if it were another PLC.
MailBox
The Mailbox model determines the manner unsolicited requests are
handled. By defining a mailbox device, the driver does not act like a
PLC on the network (as described above). Instead, it acts as a storage
area for each and every mailbox device defined. When the driver
receives an unsolicited command, the driver detects the IP address the
message came from and places the data in the storage area allocated
for the device. If the message comes from a device with an IP address
that has not been defined as a mailbox device, the message is not
processed. Any client application that reads/writes to this type of
device, reads/writes to the storage area contained in the driver, not
the physical device.