The single most common way to wire an analog loop wrong is to get power backwards: connecting two devices that both want to supply the loop's power, or two that both wait for the other to supply it. The vocabulary that prevents this is passive versus active, or equivalently sink versus source. Every device in a 4-20 mA loop plays one of these two roles, and a working loop needs exactly one power source. This guide explains the two roles, the four ways to combine them, how to read each device's role off its datasheet, and why a mismatch leaves you with a dead loop and a zero reading.
Passive vs Active 4-20 mA in one line: In a 4-20 mA loop, an active (source) device supplies the loop power and drives current through the circuit, while a passive (sink) device draws current from a loop that something else is powering. A complete loop needs exactly one active device supplying power and one passive device consuming it. If both ends are active, they fight; if both are passive, nobody powers the loop - either way the loop is dead and reads zero.
Every 4-20 mA loop needs one thing above all: a source of loop power. An active device contains or is connected to a power supply and pushes current out of its terminals; it is the source of the energy that flows around the loop. A passive device has no loop supply of its own - it modulates or reads current in a loop that is already energized by something else, drawing power rather than providing it. Passive is often called sink because it accepts current flowing into it; active is often called source because it sends current out.
The confusion arises because both transmitters and input cards come in both flavors. A transmitter can be active (it has a power feed and sources its own 4-20 mA output) or passive (a 2-wire loop-powered device that only regulates current the input card supplies). Likewise, an analog input channel can be active (it provides loop power and just reads the current) or passive (it expects the current to arrive already powered from the field). The words describe the role, not the type of device, so you have to determine the role of each end independently.
The rule that ties it together is simple and unforgiving: a working loop has exactly one active device and one passive device. The active device supplies the power; the passive device uses it. Getting this right is the whole game of analog wiring, and it is entirely separate from polarity, cable, and range. Before you connect anything, you decide which device is going to power the loop, confirm the other one is content to be powered, and then wire them so the current flows in one continuous path.
There are four ways to pair the two roles, and only two of them work. An active transmitter into a passive input works: the transmitter sources the current, the input sinks and reads it. A passive transmitter into an active input works too: the input supplies loop power, and the 2-wire loop-powered transmitter regulates the current the input is pushing. These are the two correct configurations, and between them they cover the vast majority of real installations.
The other two combinations fail. An active transmitter into an active input means two devices are both trying to supply power - they fight over the loop, and depending on the circuit you get a wrong reading, a saturated reading, or a tripped supply. A passive transmitter into a passive input is worse in its way: neither device supplies power, so no current flows at all, and the loop simply sits dead at zero. In both failure cases the reading is meaningless, and the mistake is invisible until you check which device was supposed to power the loop.
The reason this trips people up is that the terminals look identical in all four cases - two screws, a plus and a minus - so nothing physical stops you from making the wrong pair. The fix when you have a mismatch is to change the role of one end. If both are passive, add an external loop power supply, or reconfigure the input card to its active (loop-powered) mode so it supplies the current. If both are active, remove one power source, usually by setting the input card to passive so it just reads the current the transmitter is already sourcing.
The reliable way to avoid a mismatch is to read each device's role off its datasheet before wiring. On a transmitter, look for the topology: a 2-wire loop-powered device is passive, and a separately powered device that lists a sourced or active 4-20 mA output is active. On an input card, the terminology varies - some manufacturers say the channel provides loop power or transmitter power (active), others say it expects an externally powered signal (passive), and many cards can be configured either way in software or by a jumper. When the wording is ambiguous, the wiring diagram in the manual settles it, because it shows whether a supply symbol sits inside the card or outside it.
When a loop reads dead zero and you suspect a role mismatch, the fastest check is to ask which device was supposed to power the loop and confirm that it actually is. A milliamp reading of zero with the supply present usually means no source - both ends are passive, or the one active end is not configured to supply. A reading pinned at full scale or an over-range fault can mean two sources fighting. Breaking the loop and measuring current with a meter in series tells you immediately whether any power is flowing, and inserting a known loop supply temporarily can prove whether a passive-passive pair springs to life.
In a SCADA context this matters because the symptom - a channel stuck at zero - looks the same whether the loop is mis-wired, the transmitter has failed, or the cable is cut. A cloud SCADA platform such as Merobix does not resolve field wiring for you, but by historizing every channel it separates a wiring fault from a process event: a point that read valid data and then went to zero points to a device or cable failure, whereas a point that never read anything from the day it was commissioned points squarely at a passive-versus-active mismatch that was never correct. Knowing which kind of zero you are looking at saves a truck roll.
An active device supplies the loop power and sources current out of its terminals; a passive device has no loop supply of its own and only draws current from a loop that something else has energized. Active is also called source, passive is also called sink. A complete loop needs exactly one active device to power it and one passive device to consume that power.
A dead-zero reading with the supply present often means nothing is powering the loop - both the transmitter and the input are passive, so no current flows. It can also mean a mismatch was never correct from commissioning, a cut cable, or a failed transmitter. Measuring current with a meter in series tells you whether any power is flowing, and confirming which device was meant to power the loop pinpoints a passive-passive mismatch.
Check the datasheet or wiring diagram. A card that says it provides loop power or transmitter power is active, and it pairs with a passive 2-wire transmitter. A card that expects an externally powered signal is passive, and it pairs with an active, self-powered transmitter. Many cards can be configured either way with a jumper or in software, so verify the actual setting rather than assuming the default.
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