ProSoft MVI56E-MNETCR, Руководство пользователя

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MVI56E-MNETCR ♦ ControlLogix Platform Reference
Modbus TCP/IP Multi-Client Enhanced Communications Module for Remote Chassis User Manual
ProSoft Technology, Inc. Page 125 of 161
5.5 Modbus Protocol Specification
The following pages give additional reference information regarding the Modbus
protocol commands supported by the MVI56E-MNETCR.
5.5.1 About the MODBUS/TCP Protocol
MODBUS is a widely-used protocol originally developed by Modicon in 1978.
Since that time, the protocol has been adopted as a standard throughout the
automation industry.
The original MODBUS specification uses a serial connection to communicate
commands and data between Client and server devices on a network. Later
enhancements to the protocol allow communication over Ethernet networks using
TCP/IP as a "wrapper" for the MODBUS protocol. This protocol is known as
MODBUS/TCP.
MODBUS/TCP is a Client/server protocol. The Client establishes a connection to
the remote server. When the connection is established, the Client sends the
MODBUS/TCP commands to the server. The MVI56E-MNETCR module
simulates up to 30 Clients.
Aside from the benefits of Ethernet versus serial communications (including
performance, distance, and flexibility) for industrial networks, the MODBUS/TCP
protocol allows for remote administration and control of devices over an Internet
connection. It is important to note that not all Internet protocols are implemented
in the module; for example, HTTP and SMTP protocols are not available.
Nevertheless, the efficiency, scalability, and low cost of a MODBUS/TCP network
make this an ideal solution for industrial applications.
The MVI56E-MNETCR module acts as an input/output module between devices
on a MODBUS/TCP network and the Rockwell Automation backplane. The
module uses an internal database to pass data and commands between the
processor and the server devices on the MODBUS/TCP network.
5.5.2 Read Coil Status (Function Code 01)
Query
This function allows the user to obtain the ON/OFF status of logic coils used to
control discrete outputs from the addressed server only. Broadcast mode is not
supported with this function code. In addition to the server address and function
fields, the message requires that the information field contain the initial coil
address to be read (Starting Address) and the number of locations that will be
interrogated to obtain status data.
The addressing allows up to 2000 coils to be obtained at each request; however,
the specific server device may have restrictions that lower the maximum quantity.
The coils are numbered from zero; (coil number 1 = zero, coil number 2 = one,
coil number 3 = two, and so on).
The following table is a sample read output status request to read coils 0020 to
0056 from server device number 11.