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Chapter 8. Gigabit Ethernet (GbE) Interface Overview and Operation
Modbus TCP Server Operation
8.5
The Modbus TCP Server supports up to 16 simultaneous connections. These connections
are not shared with any other applications. Other TCP-based application protocols such as
SRTP Server use a different set of TCP connections.
8.5.1
Modbus Conformance Classes
PACSystems Modbus TCP Server supports Modbus Conformance classes 0, 1, and 2.
The RXi GbE Ethernet interface has been certified by the Modbus TCP Conformance Test
Laboratory to be in conformance with Conformance Test Policy Version 2.1.
8.5.2
Server Protocol Services
The Modbus TCP Server responds to incoming Request Connection, Terminate Connection
and Request Service messages. The client with which the server is interacting should wait for
the server's response before issuing the next Request Service, otherwise requests could
be lost.
There is no inactivity timeout in the server. If a client opens a connection, that connection
stays open until the connection is terminated.
8.5.3
Station Manager Support
The Modbus TCP Server supports the standard Station Manager monitor mode commands:
STAT, TALLY, and TRACE. The Modbus TCP Server task letter is "o".
8.5.4
Reference Mapping
The Modbus protocol's reference table definition is different from the internal structure of the
PACSystems reference tables. Modbus refers to Holding Register, Input Register, Input
Discrete and Coil tables; PACSystems uses Discrete Input (%I), Discrete Output (%Q),
Analog Input (%AI), Register (%R), and Word (%W) reference tables for Modbus data. The
following table shows how each Modbus table is mapped to the PACSystems reference
tables.
Modbus Reference Tables
Modbus File
F525,R2880
(16-bit words)
Modbus File Access Table
The Modbus File Access table is mapped exclusively to PACSystems %W memory.
146
Modbus Holding
Access
Register Table
(6xxxx)
(4xxxx)
---
---
---
---
---
---
1 – 32640
---
(16-bit words)
F1,R1 –
---
PACSystems* RXi Distributed IO Controller User Manual
Modbus Input
Modbus Input
Register Table
Discrete Table
(3xxxx)
(1xxxx)
1 – 32768
---
(bits)
1 – 32640
(16-bit words)
---
---
---
Modbus
Coil Table
(0xxxx)
%I1 – 32768
---
(bits)
%AI1 – 32640
---
---
(16-bit words)
1 – 32768
%Q1 – 32768
---
(bits)
(bits)
%R1 – 32640
---
---
(16-bit words)
---
---
PACSystems
Reference
Tables
%W1 –
5,242,880
(16-bit words)
GFK-2816F
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