Current signal 4 20 mA circuit. Current loop interfaces

The RZU-420 current loop calibrator is designed to set unified current signals of 4…20 mA in the process of testing automation systems, as well as to control the magnitude of current and voltage. The current loop can be powered both from the system under test and from the instrument.

Execution of the device - portable, with autonomous food from batteries. It is also possible to power the device from a 220 V network using an external network adapter.

The device has an intuitive interface and is easy to use. The wide functionality of the RZU-420, ergonomics and low cost make it indispensable for the APCS adjuster during commissioning. The use of RZU-420 can significantly reduce the commissioning time.

The RZU-420 current loop calibrator has been extensively tested in real work conditions and received positive marks in all technical checks and tests.

RZU-420 capabilities

• Simultaneous display of the current reference on the display with an accuracy of a thousandth of a mA and display of the output reference as a percentage of the 4 ... 20 mA scale with an accuracy of a tenth of a percent.
• Current setting range: 0 ... 25 mA (on a scale with a linear dependence).
• RZU-420 has the ability to measure current loop parameters such as current I and voltage U.
• The device can operate both from an external power source and from the built-in one. Switching modes is done by pressing a key on the instrument panel with a constant display of the selected power mode on the display.
• The device allows both smooth setting of current with a resolution of 0.1% of the scale, and step-by-step setting of current every 1 mA. Also, RZU-420 allows you to generate a signal of 4 ... 20 mA in function setting mode: meander, saw, triangle, sinusoid. Switching of the task mode is performed by a key on the front panel of the device with permanent display of the selected mode on the display.
• The device has an indication of breaking the current loop. When the current loop breaks, the message “break” lights up on the LCD indicator.
• The device has an indication of the state of the battery power, constantly displayed on the display, which allows you to calculate the expected time of operation from a given set of batteries.
• The display of the device is backlit for the ability to work in low light conditions.
• The maximum intrinsic reference/measurement error is only ±0.1%.
• The case of the device is made of impact-resistant plastic with IP20 dust and moisture protection level.
• There is a certificate of the measuring instrument.

Nizhny Novgorod

This article is a continuation of a series of publications in the journal ISUP, devoted to normalizing *, **, *** ****. The article "Transformation of similar to similar in measurement and control systems" (ISUP. 2012. No. 1) was devoted to normalizing, which convert unified input signals into unified output signals.

Why the 4…20 mA signal?

The wide distribution of the current unified signal 4 ... 20 mA is explained by the following reasons:
- the transmission of current signals is not affected by the resistance of the connecting wires, so the requirements for the diameter and length of the connecting wires, and hence the cost, are reduced;
- the current signal works on a low-resistance (compared to the resistance of the signal source) load, so the induced electromagnetic interference in current circuits is small compared to similar circuits that use voltage signals;
- a break in the transmission line of the current signal 4 ... 20 mA is unambiguously and easily determined by measuring systems by the zero current level in the circuit (under normal conditions, it should be at least 4 mA);
- a current signal of 4…20 mA allows not only to transmit a useful information signal, but also to provide power to the normalizing converter itself: the minimum allowable level of 4 mA is sufficient to power modern electronic devices.

Characteristics of current loop transducers 4…20 mA

Consider the main characteristics and features that must be considered when choosing. As an example, let us give the normalizing transducers NPSI-GRTP, produced by the research and production company "KontrAvt" (Fig. 2).

Rice. 2. Appearance of NPSI-GRTP - converters manufactured by NPF "KontrAvt" with galvanic separation of 1, 2, 4 channels of the current loop

Designed to perform only two main functions:
- measurement of an active current signal 4…20 mA and its conversion into the same active current signal 4…20 mA with a conversion factor of 1 and with high speed;
- galvanic separation of input and output signals of the current loop.

The main conversion error of the NSI-GRTP is 0.1%, the temperature stability is 0.005% / °C. Working temperature range - from -40 to +70 °C. Insulation voltage - 1500 V. Speed ​​- 5 ms.

Options for connecting to sources of active and passive signals are shown in fig. 3 and 4. In the latter case, an additional power supply is required.

Rice. 3. Connecting NPSI-GRTP converters to an active source

Rice. 4. Connecting NPSI-GRTP converters to a passive source using an additional power supply unit BP

In measurement systems where it is necessary to separate the input signals, the source of the input signal, as a rule, is the measuring sensors (MT), and the receivers are the secondary measuring instruments (MT) (regulators, controllers, recorders, etc.).

In control systems where separation of output signals is required, the sources are control devices (CU) (regulators, controllers, recorders, etc.), and the receivers are actuating devices (ID) with current control (membrane actuators (MIM), thyristor controllers, frequency converters, etc.).

It is noteworthy that the NPSI-GRTP converter, manufactured by , does not require a separate power supply. It is powered by an input active current source 4…20 mA. At the same time, an active signal of 4…20 mA is also formed at the output, and an additional source in the output circuits is not required. Therefore, the solution based on current loop separators, which is used in NSI-GRTP, is very economical.

There are three modifications of the converter: . They differ in the number of channels (1, 2, 4, respectively) and design (Fig. 2). A single-channel transducer is housed in a small-sized narrow case only 8.5 mm wide (dimensions 91.5 × 62.5 × 8.5 mm), two-channel and four-channel - in a case 22.5 mm wide (dimensions 115 × 105 × 22.5 mm ). Converters with galvanic isolation are used in systems with tens and hundreds of signals, for these systems the placement of such a number of converters in structural shells (cabinets) becomes a major problem. The key factor here is the width of one conversion channel along the DIN rail. in 1-, 2- and 4-channel versions they have an extremely small "channel width": 8.5, 11.25 and 5.63 mm, respectively.

It should be noted that in the multi-channel modifications of NSI-GRTP2 and NPSI-GRTP4, all channels are completely unrelated to each other. From this point of view, the performance of one of the channels does not affect the operation of other channels in any way. That is why one of the arguments against multi-channel converters - "one channel burns out, and the entire multi-channel device stops working, and this dramatically reduces the safety and stability of the system" - does not work. But such an important positive property of multi-channel systems as a lower "channel price" is fully manifested. Two- and four-channel modifications of the transducers are equipped with screw connectors, which facilitate their installation, maintenance and repair (replacement).

In a number of tasks, it is required to apply a 4 ... 20 mA signal to several galvanically isolated receivers. To do this, you can use both single-channel converters NPSI-GRTP1, and multi-channel NPSI-GRTP2 and NPSI-GRTP4. Connection diagrams are shown in fig. 5.

Rice. 5. The use of single-channel and two-channel converters for signal multiplication "1 to 2"

For ease of installation and maintenance, the connection of external connections in a single-channel modification is made by spring terminal connectors, and in two- and four-channel modifications - by detachable screw connectors.

Rice. 6. Connection of external lines using detachable terminal connectors

Thus, the new line of converters for separating the 4…20 mA current loop, presented by KontrAvt Research and Production Company, can reasonably be called a compact and economical solution that can compete in terms of characteristics with the corresponding imported counterparts. The converters are provided for trial operation, so the user has the opportunity to test the devices in operation, evaluate their characteristics and make an informed decision on the appropriateness of their use.
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And from the previous sections it is clear that unprocessed signals from very diverse and the range of their change extends from a few millivolts (for a thermocouple) to more than a hundred volts for a tachogenerator. In addition, they can be caused by changes in DC voltage, AC voltage, or even resistance. Therefore, it is quite obvious that if the analog input boards are only in the range of signals, then it is necessary to use some .

The origin of the input signal can be represented as shown in Fig. 4.13. The primary signal from the sensor on site is converted into standard signal, and the combination of the sensor and this device is called the transmitter or . After that, a standardized signal carrying information about the measured variable can be applied to a conventional analog input board.

A natural question arises: what should this standardized signal be? Analog signals are low level signals and are therefore subject to interference (or noise as they are most commonly called). A signal represented by electric current is less affected by noise than a signal represented by voltage, so a current loop is usually chosen. The converter and the receiving device are connected according to the scheme shown in fig. 4.14, and the current signal on the receiving side is converted into voltage using a ballast resistor. The current loop can be used with several receivers (such as a meter, chart recorder or PLC input) connected in series.

The most common standard represents an analog signal as a current with a range of 4-20mA, where 4mA is the minimum signal level and 20mA is the maximum. If, for example, a pressure transmitter gives a 4-20 mA signal representing a pressure in the range 0-10 bar, then 8 bar would correspond to a current of 8 x (20 - 4)/10 + 4 = 16.8 mA. The 4-20 mA signal is often converted to a 1-5 V signal using a 250 ohm ballast resistor.

The 4 mA "zero" signal (called bias) serves two purposes. First, it is used as a damage to the transducer or cable cord. If a converter or cord break occurs, or a short circuit occurs in the communication line, then the current through the ballast resistor will be zero, which corresponds to a "negative" signal of 0 V on the receiving side. This can be very easily detected and used as an "inverter failure" alarm.

The 4mA bias current also simplifies layout. On fig. 4.14 it was assumed that the converter had a local

Rice. 4.15. 2-wire transmitter 4-20 mA

power source and provided a current signal. A similar arrangement is possible, but the more common (and simpler) arrangement shown in Fig. 4.15. Here, the power supply (typically 24-30 VDC) is placed on the side of the receiving device, and the signal lines serve both to power the converter and to carry current. The converter draws current from the power supply in the range of 4-20 mA in accordance with the measured signal. This current, as before, is converted into voltage using a ballast resistor.

The 4 mA offset provides the current required by the transmitter for normal operation. Obviously, this cannot be achieved if the signal range is 0-20 mA. The converters included under the scheme of fig. 4.15 are usually called two-wire.

Fundamentals of 4..20mA Current Loop Operation

Since the 1950s, the current loop has been used to transmit data from transducers in monitoring and control processes. With low implementation costs, high noise immunity and the ability to transmit signals over long distances, the current loop has proven to be particularly suitable for industrial environments. This material is devoted to the description of the basic principles of the current loop, the basics of design, configuration.

Using current to transmit data from the converter

Industrial grade sensors often use a current signal to transmit data, unlike most other transducers such as thermocouples or strain gauges that use a voltage signal. Although converters that use voltage as a communication parameter are indeed effective in many industrial applications, there are a number of applications where the use of current characteristics is preferable. A significant disadvantage when using voltage for signal transmission in industrial conditions is the weakening of the signal when it is transmitted over long distances due to the presence of resistance in wired communication lines. You can, of course, use high input impedance devices to get around signal loss. However, such devices will be very sensitive to noise generated by nearby motors, drive belts, or broadcast transmitters.

According to Kirchhoff's first law, the sum of the currents flowing into a node is equal to the sum of the currents flowing out of the node.
In theory, the current flowing at the beginning of the circuit should reach its end in full,
as shown in Fig.1. one.

Fig.1. According to Kirchhoff's first law, the current at the beginning of the circuit is equal to the current at its end.

This is the basic principle on which the measurement loop operates. Measuring current anywhere in the current loop (measuring loop) gives the same result. By using current signals and data acquisition receivers with low input impedance, industrial applications can benefit greatly from improved noise immunity and increased link length.

Current loop components
The main components of the current loop include a DC source, a sensor, a data acquisition device, and wires connecting them in a row, as shown in Figure 2.

Fig.2. Functional diagram of the current loop.

A DC source provides power to the system. The transmitter regulates the current in the wires from 4 to 20 mA, where 4 mA is a live zero and 20 mA is the maximum signal.
0 mA (no current) means open circuit. The data acquisition device measures the regulated current. An efficient and accurate method of measuring current is to install a precision shunt resistor at the input of the measurement amplifier of the data acquisition device (in Fig. 2) to convert the current into a measurement voltage, in order to ultimately obtain a result that unambiguously reflects the signal at the output of the converter.

To help you better understand how the current loop works, consider as an example a system design with a converter that has the following specifications:

The transducer is used to measure pressure
The transmitter is located 2000 feet from the measuring device
The current measured by the data acquisition device provides the operator with information about the amount of pressure applied to the transducer

Considering the example, we begin with the selection of a suitable converter.

Current System Design

Converter selection

The first step in designing a current system is choosing a transducer. Regardless of the type of measured quantity (flow, pressure, temperature, etc.), an important factor in choosing a transmitter is its operating voltage. Only connecting the power supply to the converter allows you to adjust the amount of current in the communication line. The voltage value of the power supply must be within acceptable limits: more than the minimum required, less than the maximum value, which can damage the inverter.

For the example current system, the selected transducer measures pressure and has an operating voltage of 12 to 30 V. When the transducer is selected, the current signal must be correctly measured to provide an accurate representation of the pressure applied to the transmitter.

Selecting a Data Acquisition Device for Current Measurement

An important aspect to pay attention to when building a current system is to prevent the appearance of a current loop in the ground circuit. A common technique in such cases is isolation. By using insulation, you can avoid the influence of the ground loop, the occurrence of which is explained in Fig. 3.

Fig.3. Ground loop

Ground loops are formed when two terminals are connected in a circuit at different potential locations. This difference leads to the appearance of additional current in the communication line, which can lead to measurement errors.
Data Acquisition Isolation refers to the electrical separation of the signal source ground from the instrument input amplifier ground, as shown in Figure 4.

Since no current can flow through the isolation barrier, the ground points of the amplifier and signal source are at the same potential. This eliminates the possibility of inadvertently creating a ground loop.

Fig.4. Common-mode voltage and signal voltage in an isolated circuit

The isolation also prevents damage to the DAQ device in the presence of high common-mode voltages. Common mode is a voltage of the same polarity that is present at both inputs of an instrumentation amplifier. For example, in Fig.4. both the positive (+) and negative (-) inputs of the amplifier have +14 V common mode voltage. Many data acquisition devices have a maximum input range of ±10 V. If the data acquisition device is not isolated and the common mode voltage is outside the maximum input range, you could damage the device. Although the normal (signal) voltage at the input of the amplifier in Fig. 4 is only +2 V, adding +14 V can result in a voltage of +16 V
(The signal voltage is the voltage between the “+” and “-” of the amplifier, the operating voltage is the sum of normal and common mode voltage), which is a dangerous voltage level for devices with lower operating voltage.

With isolation, the common point of the amplifier is electrically separated from ground zero. In the circuit in Figure 4, the potential at the common point of the amplifier is "raised" to +14 V. This technique causes the input voltage value to drop from 16 to 2 V. Now that data is being collected, the device is no longer at risk of overvoltage damage. (Note that insulators have a maximum common mode voltage they can reject.)

Once the data collector is isolated and secured, the last step in configuring the current loop is to select an appropriate power source.

Power Supply Selection

Determining which power supply best suits your needs is easy. When operating in a current loop, the power supply must provide a voltage equal to or greater than the sum of the voltage drops across all elements of the system.

The data acquisition device in our example uses a precision shunt to measure current.
It is necessary to calculate the voltage drop across this resistor. A typical shunt resistor has a resistance of 249 Ω. Basic calculations for current loop current range 4 .. 20 mA
show the following:

I*R=U
0.004A*249Ω=0.996V
0.02A*249Ω=4.98V

With a 249 Ω shunt, we can remove the voltage in the range from 1 to 5 V by linking the voltage value at the input of the data collector with the value of the output signal of the pressure transducer.
As already mentioned, the pressure transmitter requires a minimum operating voltage of 12 V with a maximum of 30 V. Adding the voltage drop across the precision shunt resistor to the operating voltage of the transmitter gives the following:

12V+ 5V=17V

At first glance, a voltage of 17V is enough. However, it is necessary to take into account the additional load on the power supply, which is created by wires that have electrical resistance.
In cases where the sensor is located far from the measuring instruments, you must take into account the wire resistance factor when calculating the current loop. Copper wires have DC resistance that is directly proportional to their length. With the pressure transmitter in this example, you need to account for 2000 feet of line length when determining the operating voltage of the power supply. Linear resistance of a single-core copper cable is 2.62 Ω/100 ft. Accounting for this resistance gives the following:

The resistance of one strand 2000 feet long will be 2000 * 2.62 / 100 = 52.4 m.
The voltage drop on one core will be 0.02 * 52.4 = 1.048 V.
To complete the circuit, two wires are needed, then the length of the communication line is doubled, and
the total voltage drop would be 2.096 volts. The total would be about 2.1 volts due to the converter being 2000 feet away from the secondary. Summing up the voltage drops on all elements of the circuit, we get:
2.096V + 12V+ 5V=19.096V

If you used 17 V to power the circuit in question, then the voltage applied to the pressure transmitter will be below the minimum operating voltage due to the drop in wire resistance and shunt resistor. Selecting a typical 24V power supply will satisfy the power requirements of the inverter. Additionally, there is a voltage margin in order to place the pressure sensor at a greater distance.

With the right choice of transducer, data acquisition device, cable lengths and power supply, the design of a simple current loop is complete. For more complex applications, you can include additional measurement channels in the system.

Yuri Kurtsevoi (Maxim Integrated)

Highly integrated analog current loop signal conditioner 4-20 mA MAX12900 productionMaxim integrated can convert PWM— a microcontroller signal that does not have a built-in DAC into a loop 4 signal… 20mA fortwo-, three-orfourwired configurations .

The 4…20 mA current loop is one of the most popular data transmission methods in many industries today. Due to its resistance to interference when transmitting a signal from transmitter to receiver, it is ideally suited for such applications. Another advantage is the relative simplicity and budget of the method. Although, of course, the need to control the voltage drop in some sections of the circuit and for a number of other parameters often leads to more complex circuits and an increase in the cost of the solution. Table 1 summarizes the advantages and disadvantages of the 4…20 mA current loop communication method.

Table 1. Advantages and disadvantages of the 4…20 mA current loop

Advantages	Flaws
Basic standard in many industries	One current loop corresponds to only one data channel Ability to pass value to only one variable
Easy to connect and set up	For simultaneous operation of several data channels (for transmitting values ​​of several variables), it is required to create the same number of current loops. But using too many wires can lead to ground loop problems if the independent loops are not properly insulated.
The signal does not degrade with increasing distance	Channel isolation issues increase with the number of channels
Less susceptibility to interference	–
No current indicates an error in the data link	–

All sensors with 4…20 mA interface, depending on the configuration, can be divided into three groups:

1. two-wire (loop-powered) sensor 4…20 mA;
2. three-wire sensor 4…20 mA;
3. four-wire sensor 4…20 mA.

The most convenient configuration is a loop-powered solution. However, if the sensor itself consumes more than 3…4 mA from the budget of the 4…20 mA loop, then an additional power supply will have to be used for its operation. When connecting such sensors, you will need to use a 4-wire configuration. The 3-wire configuration is a simplified version of the previous one that combines the sensor's positive power lead with the current loop (Figure 1b). Figure 1 shows all the configurations described above. Table 2 lists the advantages and disadvantages of each.

Table 2. Advantages and disadvantages of sensors with different connection schemes

Configuration	2 wire	3-wire	4 wire
Advantages	No local power supply needed; low cost; suitable for aggressive environments	More economical than the four-wire option; ease of implementation; the ability to use display devices and other devices that require additional power; the ability to use powerful outputs, relays	External power; the ability to transmit a variable signal; power circuit isolation; the ability to use display devices and other devices that require additional power; the ability to use powerful outputs, relays
Flaws	Voltage drops across sections of the loop can cause problems; there are restrictions on the consumption of the circuit	Lack of power loop isolation; power lines and loops must be implemented with care	higher cost; more wires; not applicable in aggressive environments

Using the MAX12900 in Sensor Circuits with 2-, 3-, or 4-Wire Current Loop Configurations

The MAX12900 is a highly integrated, ultra-low power analog signal conditioner for sensors with a 2…20 mA transmitter. 10 modules are built into its compact body:

• LDO converter with a wide input voltage range;
• circuits for processing PWM-modulated signals for two inputs;
• two low-power, low-drift op-amps;
• one op-amp with low offset voltage drift and wide bandwidth;
• two diagnostic comparators;
• power-on controller with a good power quality indication output (power-good output);
• low drift voltage reference.

A key advantage of the MAX12900 is that it can convert a PWM signal from a microcontroller that does not have an onboard DAC into a 4…20mA loop signal for two-, three-, or four-wire configurations. Thus, it is the equivalent of a combination of a low power, high resolution DAC, a PWM signal processor, two processing circuits, and an active filter with an integrated low power op amp. Two signal processing circuits ensure a stable PWM amplitude despite fluctuations in signal amplitude, temperature and supply voltage changes. A wide bandwidth amplifier coupled with a discrete transistor converts input voltage to output current and allows HART® and FOUNDATION Fieldbus H1 signal modulation. The low-bias op amp and low-drift voltage reference ensure minimal error over a wide temperature range. The low power op amp and comparators are the building blocks of advanced diagnostic systems. Power rail monitoring, output current measurement, and open circuit detection are some examples of the diagnostic capabilities of such systems. All of this, along with high accuracy and low overall power consumption, makes the MAX12900 an ideal device for smart current loop sensors.

Using the MAX12900 as a 2-wire transmitter (loop powered)

Figure 2 shows a simplified block diagram and model of how the MAX12900 can be configured as part of a loop powered sensor. This configuration is required for systems operating in harsh environments and must comply with ATEX Directive 94/9/EC and be IECEx certified. Such an implementation of the sensor circuit is possible only in cases where the transmitter consumes less than 4 mA. The PWM signals generated by the microcontroller are fed to special PWM signal conditioning and processing circuits built into the MAX12900. Using one of the built-in operational amplifiers and an external RC circuit, you can create a low-pass filter. External transistors are used to convert voltage to current.

Figure 3 shows the electrical circuit-level implementation of the two-wire current loop that powers the sensor (note that the entire block highlighted in turquoise is integrated into the MAX12900).

One of the most common sensors of this type are temperature sensors. Let's try to design a MAX12900 temperature sensor transmitter using a precision thermocouple and a dedicated thermocouple signal converter (MAX31856). The MAX31856 processes the thermocouple signal and transmits the data over the SPI interface. Thus, in order to read the sensor and generate PWM signals for the MAX12900, a microcontroller must be used. The MAX12900EVKIT uses an STM32L071 microcontroller for this task. The key to this design is to estimate the worst-case power budget (maximum current draw for all operating temperatures and voltages). Based on this, a decision can be made on the use of one or another configuration of the current loop: two-, three- or four-wire.

According to the MAX12900EV datasheet, the total consumption of the low-power MCU and the MAX12900 is 3.5 mA for the worst case. The MAX31856 draws a maximum of 2mA at 3.3V supply voltage (Table 3). Thus, the total consumption exceeds 4 mA, which means that it is not possible to implement a two-wire transmitter.

Table 3. Consumption of temperature sensor components

Using the MAX12900 in a 3-Wire Transmitter Circuit

Having excluded the possibility of using a two-wire solution, let's see what is the possibility of designing a three-wire circuit. The first thing to keep in mind is that only one positive power pin can be used for both data transfer and circuit power. 24V (from the PLC) is too high for the microcontroller and MAX31856, which require 3.3V to operate. There are several approaches to solve this problem. The first is to use a DC/DC converter such as the MAX17550 to convert 24V to 3.3V, as shown in Figure 4. The MAX17550 is an ultra-compact, high efficiency synchronous DC/DC buck converter with up to 25mA output current. The MAX12930 digital dual channel isolator is used to isolate the sensor/MCU PWM interface with the MAX12900. In Figure 4, the components in the dotted box are in an isolated power domain with a floating ground that is different from the PLC ground.

Another approach to solve the power problem is to use the MAX15006AATT+ ultra-low-current linear voltage converter, which can provide 3.3V with a load current of up to 50mA, as shown in Figure 5.

The second problem to keep in mind when designing such sensors is the floating ground of the transmitter. The sensor itself, the microcontroller, and the MAX12900 - the transmitter for communication - must share a common ground bus. At the same time, the potential of this earth is a floating potential with respect to the PLC earth. The state of the floating ground depends on the data transmitted and the load level of the loop. Several approaches have been taken to solve this problem, such as using a dual-channel, low-power MAX12930 (as shown in Figure 4) to isolate the PWMA and PWMB inputs from the transmitter.

An alternative approach is to use an active circuit that constantly monitors and controls the common ground level of the microcontroller and the sensor. This implementation option becomes possible and convenient due to the presence of a general-purpose op amp, namely OP2, integrated into the MAX12900. This circuit also requires the use of an external low voltage n-channel MOSFET Q3 and a general purpose PNP transistor Q4 to match the voltage drops across RLOAD and RSENSE.

Using the MAX12900 in 4-Wire Transmitter Applications

We've looked at how the MAX12900 can be applied to two- and three-wire transmitters. Implementing a 4-wire solution is very easy compared to these, as there are separate power and ground circuits for the sensor and PLC.

Conclusion

Maxim Integrated's ultra-low power MAX12900 analog signal conditioner for 4...20mA transmitters offers unsurpassed flexibility in a variety of applications and is ideal for use in industrial sensors for control and automation systems that need to be converted to a 4...20mA current loop signal.