CHAPTER 6: OPERATIONAL GUIDE
OPERATIONAL GUIDE
MULTINET4 MULTI-PORT SERIAL SERVER & MANAGED SWITCH – INSTRUCTION MANUAL
6–25
FIGURE 6–4: Format of a Modbus ASCII Packet
Modbus RTU (depicted in the figure below) uses binary message encoding with a cyclic
redundancy check (CRC). Each message begins with a silent interval of at least 3.5
characters times and ends with a similar silent interval.
FIGURE 6–5: Format of a Modbus RTU Packet
6.7.3
Network Protocol
The Modbus/TCP format (depicted in the figure below) strips the message framing and
LRC/CRC from the normal Modbus packet and prepends a Modbus/TCP header consisting
of a 2-byte Transaction ID (set by the client and echoed by the server), a 2-byte Protocol ID
(always 0-0), and a 2-byte length. The device address byte (now referred to as the unit
identifier) and the function byte are preserved and are followed by a variable amount of
data. This information is then delivered as the payload of a TCP/IP packet. The Modbus
LRC/CRC is not included because it is redundant with the CRC provided by the link layer
(that is, Ethernet).
FIGURE 6–6: Format of a Modbus/TCP Packet
6.7.4
Exception Handling
The Modbus/TCP client and server on Multinet4 can optionally generate and forward
Modbus exception codes when certain communication or configuration failures occur.
Specifically, the client will generate a GATEWAY PATH UNAVAILABLE exception message
(exception code 0x0A) and pass it back to the master device if a remote address has not
been configured for the destination device. The server will generate a similar message if a
local device entry has not been configured for the destination device address. The
message is sent to the client, which then forwards the exception to the Modbus master
device.
