Industrial PLC Signal Conversion

In industrial controls there are many different reasons to convert an electrical or electronic signal. Often this is necessary to allow one device to communicate with another, particularly when they are produced by different manufacturers. In this instance it is best to revert to 'industry standard' signal types.

How To Convert A Temperature Signal To Analogue 4-20mA or 0-10V

Temperature sensors provide outputs in many different formats and ranges. Many PLCs require specific input modules to read thermocouples and PT100 sensors. These modules can be costly and may have long lead times. A good solution to this problem is to convert the temperature input to a widely used analogue signal such as 4-20mA or 0-10V.

We recommend the Novus TxRail-USB and TxIsoRail for these applications. One important difference between the models is that the TxIsoRail offers galvanic isolation.

How To Convert 0-10V To 4-20mA Analogue Signal

A common requirement in industrial controls is to convert one analogue signal type to another. Perhaps the equipment reading the signal only supports one type of analogue input. This is not only true for PLCs but other types of electronic control equipment such as Variable Speed Drives (VSDs) and proportional valves.

We recommend the TxIsoRail with 4-20mA Output for converting 0-10VDC to 4-20mA. 

How To Convert 4-20mA To 0-10V Analogue Signal

Two models of TxIsoRail are available providing the option to also convert 4-20mA to 0-10V. You can use the 0-10V output version in this scenario.

Additionally there is a simple method using a 500ohm resistor which can be used depending on the application. In the example below we use a 500ohm resistor at the input terminals of a proportional valve to convert the current signal (0-20mA) to the required voltage signal (0-10V).

It is important to note that when using a resistor to convert a 4-20mA signal the minimum voltage at the receiving device will be 2V. This is due to the higher low end signal of 4mA. In this scenario it may be possible to scale the input receiver to compensate for the signal range of 2-10 VDC.

How To Split An Analogue 4-20mA Signal Into Two Channels

There are often instances where one common signal needs to be directed to two independent devices. Consider a level instrument that needs to provide local indication on a digital display but also feedback to a PLC analogue input. In this case you might assume one 4-20mA signal is not sufficient. The solution here is to split the signal.

We recommend TxIsoloop-2 for use as an analogue signal splitter.

How To Read An Analogue Signal Via Modbus-RTU

Remote devices provide challenges when transmitting signals over long distances. They can also be difficult to integrate with legacy systems. Modbus RTU is a useful tool as a default communications protocol. The analogue signals can be converted to Registers and read via a Modbus Master. A data cable can provide a faster route back compared with installing multiple analogue cables. 

We recommend the DigiRail-2A for up to 2 analogue inputs and DigiRail Connect for mixed signal types including digital inputs.

How To Convert Modbus RTU to Modbus TCP

Modbus RTU is a widely used protocol associated with RS485 serial networks. This functionality is often built in to field devices and meters. Many modern PLCs and SCADA systems have switched to Modbus TCP which uses ethernet. So there is a requirement for devices that can facilitate communication between the field device and Modbus TCP enabled controller.

Our solution is a simple transparent gateway that converts the requests from the controller into Modbus RTU messages that the field devices can interpret and respond back. This type of Gateway is extremely simple to setup with no requirement to map registers.

We recommend the DigiRail OEE for this gateway application.