Impulse 463E - Manual (Page 58)

© Sealevel Systems, Inc. - 55 - SeaI/O User Manual
Modbus RTU
The Modbus RTU specification is a serial line style of communication where the
packets are determined by a series of timeouts. The last byte of an arriving packet is
defined as no more bits have been clocked into the shift registers for 3.5 character
times (i.e., no more bytes have been received for at least 3.5 character times). The
layout of a Modbus RTU packet is shown below.
Address Payload CRC-16
The payload consists of function code, followed by the function specific data. That
data is well defined within the Modbus Application Protocol specification. The
extended commands, defined in this manual, are good examples of how the payload
of a Modbus command is designed.
Modbus TCP
The SeaI/O E-Series (Ethernet) modules utilize the Modbus TCP communications
package. The libraries provided with the SeaI/O modules will operate virtually the
same as the Modbus RTU libraries.
The Modbus TCP protocol is slightly easier to use because there is no CRC attached.
Instead, Modbus TCP relies on the TCP implementation to ensure the message gets
to the sender without error.
The defined port for Modbus TCP communications is 502. This port has been
reserved on TCP modules that implement Modbus TCP. All Ethernet enabled SeaI/O
modules support a bridging mode, where all connected modules can respond to
Modbus commands as if they were a single unit. The modules utilize their unique
Slave ID (Unit ID) and will respond to a command that the Ethernet bridge will then
transmit back to the user. The header of the TCP message is named the MBAP
Header, and is designed as follows:
Tx ID (h) Tx ID (l) Protocol ID (h) Protocol ID (l) Length (h) Length (l) Unit ID
Following the MBAP header is the same payload of the RTU messages described in
the Modbus Application Protocol Specification.