Network Protocol; Exception Handling - GE Multinet 4 Instruction Manual

CHAPTER 6: OPERATIONAL GUIDE
6.7.3

Network Protocol

6.7.4

Exception Handling

MULTINET4 MULTI-PORT SERIAL SERVER & MANAGED SWITCH – INSTRUCTION MANUAL
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
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
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.
OPERATIONAL GUIDE
6–25