Modbus® communications protocol – Alpha Technologies Cordex Controller Ver.3.1x User Manual

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12. Modbus® Communications Protocol

This feature enables CXC communication of alarms and live signals using Modbus protocol (messaging

structure developed by Modicon®.
A message is encoded in Remote Terminal Unit (RTU) data format and is communicated upon query via

Transmission Control Protocol (TCP) to any Modbus supervisor or master. Reference: Modbus application

protocol specification v1.1a (www.Modbus-IDA.org).
The CXC acts as a server on Ethernet networks known as Modbus TCP RTU: TCP/IP over Ethernet,

default TCP port = 502. The server is always up and running when CXC starts up; ready to respond to a

client query.
CXC communications cannot be configured as Modbus gateway, to behave as both server and client at

the same time, and does not support Modbus Plus high speed token passing network.
Modbus protocol offers service specified by function codes (see tables provided). A function code is an

element of a Modbus request/reply Protocol Data Unit (PDU). The CXC supports the following:
• 0x01 (Read Coils): Read CXC relay status bits,
• 0x02 (Read Discrete inputs): Read alarm tables status bits,
• 0x03 (Read Holding registers): Read CXC live signals from signals tables,
• 0x04 (Read Input registers): Read CXC relay or alarm table status registers.

Table S — Table N–CXC Modbus PDU address definition for function code 0x01 (read coils)

PDU Address

Variable Name

Variable Description

Response Data Format

0x0001

Relay1_Status_Bit

Relay 1 (K1)

BINARY

0x0002

Relay2_Status_Bit

Relay 2 (K2)

BINARY

0x0003

Relay3_Status_Bit

Relay 3 (K3)

BINARY

0x0004

Relay4_Status_Bit

Relay 4 (K4)

BINARY

0x0005

Relay5_Status_Bit

Relay 5 (K5)

BINARY

0x0006

Relay6_Status_Bit

Relay 6 (K6)

BINARY

0x0007

Relay7_Status_Bit

Relay 7 (K7)

BINARY

0x0008

Relay8_Status_Bit

Relay 8 (K8)

BINARY

0x0009

Relay9_Status_Bit

Relay 9 (K9)

BINARY

0x000A

Relay10_Status_Bit

Relay 10 (K10)

BINARY

0x000B

Relay11_Status_Bit

Relay 11 (K11)

BINARY

0x000C

Relay12_Status_Bit

Relay 12 (K12)

BINARY

0x000D

Relay13_Status_Bit

Relay 13 (K13)

BINARY

0x000E

Relay14_Status_Bit

Relay 14 (K14)

BINARY

0x000F

Relay15_Status_Bit

Relay 15 (K15)

BINARY

0x0010

Relay16_Status_Bit

Relay 16 (K16)

BINARY

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