100/180 mm PAPERLESS GRAPHIC RECORDER: USER GUIDE
HA028910
Issue 13 Sep 16
User Guide
Page 281
8
MODBUS TCP SLAVE COMMS
8.1 INSTALLATION
The installation of the Modbus link consists of connecting a standard Ethernet cable between the RJ45
connector at the rear of the recorder either:
1. Directly to a host computer, using a crossover cable
2. To a host computer via a network, using a ‘straight through’ cable.
8.2 INTRODUCTION
MODBUS TCP allows one or more recorders to act as ‘slave’ devices 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 4.5
.
MODBUS TCP (Transmission Control Protocol) is a variant of the MODBUS family of communications
protocols intended for supervision and control of automated equipment specifi cally covering the use of
MODBUS messaging in an intranet or internet environment, using TCP/IP protocols. Much of the MOD-
BUS 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.
8.2.1 Function Codes
MODBUS function codes 3, 4, 6, 8 and 16, defi ned in table 8.2.1a below, are supported and are fully de-
scribed in
section 8.5
, below.
Table 8.2.1a MODBUS Function code defi nition
DIAGNOSTIC CODES
Function code 08, subfunction 00 (Return query data) echoes the query (Loop back).
Reads the binary contents of holding registers.
In this implementation, code 03 is identical with code 04.
Read holding registers
Read input registers
Pre-set single register
Diagnostics
Pre-set multiple Registers
Reads the binary contents of input registers. In this
implementation, code 04 is identical with code 03.
Writes a single value to a single register.
Obtains communications diagnostics information
Writes values to multiple holding registers
03
04
06
08
16
Code
MODBUS definition
Description
