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Modbus/TCP Scale Data Communication Option - Benefits and Sample Application

Modbus/TCP Scale Data Communication Option - Benefits and Sample Application

OVERVIEW
Modbus is one of the most popular Programmable Logic Controller (PLC) communication protocols used in the industrial world. Modbus/TCP is a popular variant of the Modbus communication protocol. The Modbus/TCP standard allows devices to communicate with PLCs and operator panels with installed PLC-based software by embedding Modbus messages inside TCP/IP frames.

 

MODBUS/TCP BACKGROUND
Modbus/TCP combines Ethernet, TCP/IP, and the Modbus protocol. A synergy is created by combining these standards, leveraging the most common physical network with a universally common networking standard and a vendor neutral communication standard. This process results in a truly open, accessible network for the exchange of process data.

The use of Modbus/TCP provides a totally scaleable solution from ten nodes to ten thousand nodes without the risk of compromise that other multicast techniques would run. Significant benefits of Modbus/TCP over Modbus include the ability to remotely access devices over the internet, the ease of implementation, and the ability to take advantage of existing networking hardware.

Modbus/TCP embeds a Modbus frame into a TCP frame in a simple manner. The basics of Modbus/TCP communication consist of a request message and a response message from the scale. Typically the request is for a weight value, sometimes combined with other necessary data (such as the current scale status).

 

APPLICATION DESCRIPTION
Data Collection and Remote Configuration

A manufacturing plant wants to station three 10 lb capacity Doran Model 4300 checkweighers on the production floor and transmit all weighments to a PC, which collects and stores all the weighment data. Several different products will be weighed on this line for confirmation that all parts are present in the box before shipment to the customer. Each product will have different over, under, and accept checkweigh tolerances. The checkweigh tolerances will be programmed by the PC.

Although RS-232 is available in each scale, only one serial port is available on the PC for communication. An RS485 to RS232 converter is used to connect the PC RS232 port to the RS485 bus. As the finished product is weighed, the weight data is stored in the scale’s data buffer. The PC is programmed to poll each scale, one at a time, for the weighments stored in the data buffer.

When the product line is changed to manufacture a different product, the PC sends out a command to each scale to reprogram the under, over, and accept checkweigh tolerances.

Once programmed, each scale is polled for its current checkweigh settings to confirm that the correct settings were received before the next product is checkweighed.