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7 MODBUS TCP SLAVE COMMS
7.1
INSTALLATION
The installation of the Modbus link consists of connecting a standard Ethernet cable between the RJ45
connector at the rear of the unit to a host computer either directly or via a network. A 'straight-through' cable
can be used in either case (i.e. a cross-over cable is not required).
7.2
INTRODUCTION
MODBUS TCP allows the instrument to act as a 'slave' device to one or more host computers connected via
the RJ45 connector at the rear of the recorder. Each recorder must have a unique Internet Protocol (IP)
address, set up as described in Section 4.2.1 (Network.Interface).
MODBUS TCP (Transmission Control Protocol) is a variant of the MODBUS family of communications protocols
intended for supervision and control of automated equipment specifically covering the use of MODBUS
messaging in an intranet or internet environment, using TCP/IP protocols. Much of the MODBUS detail in this
manual is derived from the document openmbus.doc, available at http://www.modbus.org/default.htm. The
above mentioned document also includes implementation guidelines for users.
Note: The Modbus protocol allows a maximum of 255 data bytes to be read from or written to in one
transaction. For this reason, the maximum number of standard (16 bit) registers that can be
accessed in one transaction is 255/2 = 127 and the maximum number of IEEE (32-bit)
registers is 127/2 = 63.
7.2.1 Function Codes
Modbus function codes 3, 4, 6, 8 and 16, defined in Figure 107, are supported and are fully described in Section
7.3, below.
Code
03
04
06
08
16
DIAGNOSTIC CODES
Function code 08, subfunction 00 (Return query data) echoes the query (Loop back).
September 18
FEC920: USER GUIDE
Modbus definition
Read holding registers
Read input registers
Preset single register
Diagnostics
Preset multiple registers
Figure 107 MODBUS Function code definition
A5E45696052A Rev-AA
Description
Reads the binary contents if holding registers. In this
implementation codes 3 and 4 are identical in operation.
Reads the binary contents if holding registers. In this
implementation codes 3 and 4 are identical in operation.
Writes a single value to a single register.
Performs a simple loop back test.
Writes values to multiple holding registers.
Page 115
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