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4-20mA Scale Data Communications Option - Benefits and Sample Application

4-20mA Scale Data Communications Option - Benefits and Sample Application

OVERVIEW

4-20mA has been available as a communications protocol for data collection and systems monitoring and control since the 1960s. 4-20mA is still widely used today with remote indicators, PLC’s, and legacy equipment. Although dated, 4-20mA offers an ease of use and affordability that many other communications protocols cannot.

4-20mA BACKGROUND

The simplicity of 4-20mA communications starts with a current loop circuit that provides the supply voltage and measuring current with only two wires. A base current signal of 4mA provides an indication of a loss of signal due to a poor connection or to a broken wire resulting in a 0mA current signal. In addition, a current signal is immune to any electrical interference and can be transmitted over long distances.

Continuing the simplicity of 4-20mA is the measuring current output in relation to weight on the scale. A 4mA signal is equal to 0% of full capacity and a 20mA signal relates to 100% of capacity. For instance, a scale calibrated to a capacity of 100 lbs will have the following signal output, based upon the displayed weight:

 

Displayed Weight 4-20mA Output Signal

0.00 lb 4mA

50.00 lb 12mA

100.00 lb 20mA

Gross Overload Error 24mA

Negative Weight or

Gross Underload Error

3.5mA

 

Many third parties supply 4-20mA instrumentation that can typically be programmed to read in weight units or as a percentage of total capacity. Be aware that most instrumentation available is based upon 8- bit technology, which will reduce Doran’s 12-bit accuracy. If maximum accuracy is required, be sure to purchase 4-20mA instrumentation that operates on 12-bit or higher technology.

To be compatible with all third party instrumentation, the Doran 4-20mA option has two settings, active or passive output. When set to active, the Doran 4-20mA option provides the supply voltage and controls the measuring current. Setting the 4-20mA option to passive requires an external supply voltage or instrumentation that provides the supply voltage.

Although no calibration is necessary to configure the Doran 4-20mA option, minor adjustments to the measurement current output can be made to compensate for any irregularities in third party instrumentation. In addition, Doran’s 4-20mA option provides a test mode where a 4mA and 20mA signal can be generated in order to calibrate the third party indicator.

 

APPLICATION DESCRIPTION

PLC System Control
A manufacturing plant wants to station three 500 lb capacity scales on the production floor and transmit all weighments to a PLC, which will control a filling process. Although RS-232 is available in each scale, using three 4-20mA inputs is a much more affordable option for the PLC. Furthermore, the PLC programmer finds it much easier to set up and configure inputs from a 4-20mA signal.

An empty 55-gallon drum is set on each scale platform, and a remote PLC-based start switch is pressed to begin the filling process for each scale. The PLC activates a relay to open a valve for each scale. The PLC monitors the three 4-20mA input levels, deactivating the valve control relays at the proper weight limit to stop the filling process.