Universal, 4-20 mA, two wire industrial transmitter

This post unifies together the following posts:

Industrial, 4-20 mA current loop, measuring basics I.

Industrial, 4-20 mA current loop, measuring basics II.

Industrial, 4-20 mA current loop, measuring basics III.

Industrial, 4-20 mA current loop, measuring basics IV.

Internal workings of process controllers I

. Internal workings of process controllers II.

There are a lot of types of transmitters nowadays. Usually a separate one (a specific type) is needed for thermocouple measurement, an other one (an other type) is needed for level measurement…

The basic concept of this post is to present a way for a universally usable transmitter, which can accept almost any type of input (Pt100, pH probe, rotation, level, light…) and produce a configurable, standard 4-20 mA output.

In practice all the physical properties of the world – that can be measured – are first converted into one of the following signals: voltage or frequency. This is because these two signals can be measured directly and most accurately with microcontrollers.

An accurate band-gap voltage reference is needed to measure voltage with high precision. Measuring frequency is equal to measuring time since f = 1 / T. Frequency measurement only needs a precise crystal oscillator which is always incorporated in processor based systems.

Let’s see how some real world measurements are working in practice.



Light input measuring PCB

Temperature (PT100):


Pt100 RTD input measuring PCB



Rotation input measuring PCB



pH input measuring PCB

Low level current:


Low level current input measuring PCB

Putting these altogether:

Universally usable transmitter needs to have – a high resolution A/D converter (practically >= 24 bits resolution), – a lower resolution (thus lower power consumption) A/D (this is usually the microcontroller’s onboard 10, 12 or 16 bit A/D), – frequency input(s) (which can receive signals from a quadrature encoder) – some digital inputs (for limit switches and/or digital signals).

MCU section:

The inputs above provide raw measurements without any correction, since all the linearisation and mathematical functions are calculated by the microcontroller. That’s where fuction block programmable process controllers’ theory has to be taken into account.

Power supply:

The whole transmitter’s power supply has to provide the appropriate voltage levels for every type of measurements. Notice that not every type of measurement can be realised within the 2-wire 4-20 mA transmitter’s avaliable power budget.

Tipically a position measurement based on the differential transformer design exceeds this power need, so this type of measurement will be actually a 3- or 4-wire transmitter.

Output signal:

Traditionally a 4-20 mA output signal is the only output for a 2-wire transmitter. Going further in complexity, there can be multiple analogue outputs along with the digital outputs as well (either relay contacts or push-pull voltage outputs).

Using analog signals tends to be obsolete nowadays, a digital bus system is usually used for measurements forwarding. There are a lot of industrial bus systems used in the world, form the basic, serial line with some ASCII protocol to the complex ethernet based ones.

All the communication buses use one of the three types of interface:

– UART for serial line based buses (RS-232, RS-485, Modbus RTU, Profibus, Interbus…)

– CAN bus (CanOpen, DeviceNet, SafetyBus …)

– Ethernet (EtherNet/IP, EtherCat, ModbusTCP…)

Fortunately UART and CAN interface is common in microcontrollers, so these not require extra hardware. If one needs ethernet based communication, then a MAC+PHY IC is needed. The big advantage is that only one type of transmitter is needed which reduces the cost of storing spare parts, and simplifies designs.


Base PCB shema

Since there are some types of power supplies depending on the supply type (110V or 230V AC input, 24V DC or 12 V DC input, 4-20 mA current loop…) the power supply unit (PSU) has to be modular (a separate PCB) and also this is the case with the communication line (Com PCB), so the base PCB has to be divided into three PCBs as it is shown below:

vazlat_2Base PCB connections

The so called interface (actually the connections) between the PCBs are shown below:


Base PCB interface connections

The input section is just named as Measurement PCB (Meas PCB) and has all the connections which are necessary for precise measurement, the power supply has minimal connections. The communication section has 4 data lines which are just named as Com1 – Com4 since they can be RX – TX – (and optionally DIR) in case of UART, or CanTX – CanRX in case of CAN bus, or TX+ – TX- – RX+ – RX- for Ethernet connection.

To sum it up:

A universally applicable transmitter must be modular (to suit for every application), and has to be “freely” configurable (from the software side) in order to maximally adjust every piece into the specific application.

Its input connector varies with the measured property, but this connector incorporated in the measurement PCB, so every meas. PCB has its own connector.

The CPU PCB is common, it includes the calculation algorithm

The PSU PCB varies with the power and analog output needs.

The communication PCB is optional. It is to be omitted in case of a 4-20 mA transmitter, but it is a must for a universal 3- or 4-wire intelligent transmitter.

The very little detailed schematic is shown:

principleUniversal transmitter data processing and flow

With these basic informations it is easy to set up either a 2 wire, universally programmable transmitter, or a more complex process controller, and even a universally programmable audio processor – amplifier.

The principle is the same everywhere: being most widely usable, being modular and being configurable (programmable).


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