Two-Wire vs. Four-Wire Transmitter For Analog Process Signals - What to Consider?

I/O modules are an integral part of process signal connectivity.
Image courtesy of Acromag

Transmitters are everywhere in process control. They take a sensor output signal,amplify and condition it, then send it to monitoring and decision making devices. The most common analog electrical signal used for transmitting process control signals is a 4-20 mA (milliampere) current flow. It has succeeded in its adoption for a number of reasons, not the least of which are its resistance to interference and ability to transmit a signal across a substantial length of cable.

Aside from the sensor connection, there are two basic wiring schemes for these devices. The simplest employs just two conductors to transmit the signal and coincidentally provide operating power for the transmitter electronics. This type of transmitter is commonly referred to as a "loop powered" or "two-wire" device. A DC power supply, typically 24 volts, is wired in series with the 4-20 mA output signal and the transmitter derives its operating power from this source. Loop powered devices generally consume very little power, but process designers must consider the total resistance imposed on the loop by all connected devices. The cable, unless the length is monstrous, poses a measurable but comparatively small resistance. Careful consideration should be given to the resistance imposed by receiving devices, especially if there are several in series, receiving the loop signal. The output voltage of the power supply and the maximum tolerable voltage of the connected devices will serve as limiting factors on loop instrument quantity. Where they can be applied, two-wire transmitters offer a straight forward solution for delivery of analog process measurement signals.

A "four-wire" transmitter gets its name from, you guessed it, the two pairs of wires used to provide operating power and a signal transmission path. Provided with a separate power source, possibly even 120 volts AC, this transmitter type will often be found in applications where the sensor may have power requirements that cannot be met with the limitations inherent in the loop powered device. While it may seem that the separate power supply negates the need to consider total resistance load on the signal loop, this is not the case. The signal loop still will be limited by the DC power supply that serves as the driving force of the loop.

In many cases, the question of "two-wire or four-wire" will be answered by the transmitter manufacturer. Since the two-wire scheme is a less burdensome installation, it may be the only product offering when a suitable device can be designed for an application. That said, a diligent search will probably find two and four-wire versions of transmitters for almost every application.

What are some decision making guidelines?

• Some types of transmitters have sufficiently high power requirements that they cannot be loop powered. In this case, four-wire may be the only option.
• For low resistance loads, use 2 wire transmitters for a simpler installation.
• Allow some headroom in the loop resistance to accommodate at least one added receiving device in the future. For example, a temperature signal may serve as an input to a controller now, but need to service a recording device potentially added in the future.
• Distance should not be mindlessly overlooked, but is generally not a limiting factor, as most installations would be compatible with the distance limitations for two- or four-wire device output signals.
• When signal transmission distances become unwieldy, due to cabling costs or other factors, consider a wireless transmitter instead of a wired device.

An important aspect of applying 4-20 mA signal loops is to maintain the capability to add another receiving device to the circuit. The use of information in the form of process signals has been growing for a long time and is likely to continue. It is certainly easier to wire an additional device into an existing loop, than to install an additional sensor, transmitter, power supply, and cabling to accommodate the additional device.

Share your process measurement requirements and challenges with process instrumentation experts, leveraging your own process knowledge and experience with their product application expertise to develop complete and effective solutions.


.

Signal Splitter Solves Multiple Device Connection Challenge

Selecting the right I/O modules can solve signal
transmission challenges.
Image courtesy Acromag

Industrial process measurement and control requires the transmission of signals from point to point with no significant distortion. Even with the growing prevalence of wireless signal transmission, over-wire transmission of signals is still a primary means of connecting one device to another.

In the cabled process measurement and control world, the 4 to 20 milliampere signal is generally considered the standard for transmitting analog control and measurement signals over any distance.There is an immense array of instrumentation and controllers available for use with 4-20 mA signals, so expertise in routing and delivering those signals should be part of your process measurement and control skill set.

Like just about everything else, routing 4-20 mA signals presents its own set of challenges that require some thought and planning to overcome. Electrical interference is always a concern and must be prevented from impacting the operation of measurement and control devices. Additionally, there must be sufficient power in the signal loop to accommodate the resistance load of connected devices. There are other considerations, but I'm going to focus on these two.

One scenario that can present significant issues is multiple devices requiring connection to the same signal, but with great distance between them. A simple solution can be implemented using an isolated signal splitter.

Features of these units making them an attractive, single box, solution:

• One 4-20 ma input channel for the measuring or controlling device.
• The input signal is retransmitted as identical isolated 4-20 ma signals
• Galvanic isolation from input to output
• Isolation between channels for safety and increased noise immunity. Fault in one output channel does not impact the operation of the other channels.
• Reliable operation in industrial environments, with protection from RFI, EMI, ESD, and surges.
• Low radiated emissions in accordance with CE requirements. 
• DIN-rail mounting of the unit
• Plug-in terminal blocks

If you have a very long signal loop, connecting multiple devices, consider breaking the devices into two groups that may allow for a substantially shorter cable length for each group. Connect each group to one of the isolated outputs of the splitter, giving each group of instruments the identical signal without the risks or impractically of an excessively long cable run.

There are other devices available that may combine special characteristics that solve your signal transmission and processing challenges. Contact a product specialist and discuss your existing or anticipated project requirements. Combine your process knowledge and experience with the extensive product knowledge of a professional sales engineer and produce the best possible outcome.

Loop Powered Isolated Signal Transmitters and Splitters from Instrument Specialties, Inc.