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4–20 mA, RS485 or SDI-12: How to Choose a Soil Sensor Interface

A soil sensor can report a plausible value and still be the wrong purchase if its interface does not match the controller, logger or power budget already installed. This guide compares 4–20 mA, RS485 and SDI-12, and lists the integration fields to request before ordering.

Updated October 3, 2026How Equipvia researches
Data logger application for a field weather and soil monitoring station
The interface decision starts with the receiving device: 4–20 mA for a documented PLC loop, RS485 for a register-based bus, SDI-12 for a logger-managed sensor port. Data logger application photograph via Wikimedia Commons (CC BY-SA 3.0). View current RIKA listing.

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The interface is a system decision. A soil sensor can report a plausible moisture value and still be the wrong purchase if its electrical interface does not match the controller, logger or power budget already installed. The decision is not simply analog versus digital. The buyer must define what is being measured, which device requests or receives it, how many sensors share the connection, how the value is scaled, and what the system should do when the signal is missing.

This guide owns the interface decision: 4–20 mA current loop, RS485 with a documented protocol, or SDI-12 for a compatible environmental logger. The same sensing principle can be offered with several outputs, but the wiring, data model and commissioning work are not interchangeable.

Start with the receiving device, not the marketplace title

The first line of an RFQ should name the receiving device and its input type. A 4–20 mA sensor needs a compatible current input and a defined loop power arrangement. An RS485 sensor needs a bus master, electrical settings and a protocol or register map. An SDI-12 sensor needs a recorder or controller that can issue SDI-12 commands, provide the required power and interpret the returned fields. A listing that says "RS485/SDI-12" is a discovery lead, not proof that both interfaces are present on the quoted variant.

Interface What the buyer receives Where it tends to fit Main procurement risk
4–20 mA A current value scaled to one measured variable or one configured channel. Existing PLC, RTU or irrigation controller with analog current input; simple point-to-point loops. The listing may not state scaling, loop resistance, fault current, isolation, measured variable or whether power and signal share the same conductors.
RS485 with Modbus or another protocol Digital registers or frames carrying one or more values, status fields and sometimes configuration. Multi-parameter sensors, PLC/SCADA systems and installations where a digital bus reduces analog input count. RS485 is the physical layer, not the data meaning. Without register map, baud/parity, address and termination guidance, integration is not specified.
SDI-12 A low-power serial exchange in which the logger addresses a sensor, starts a measurement and requests the returned data. Environmental and agricultural data loggers that already support SDI-12 and need several smart sensors on a shared port. The recorder must support the protocol and the buyer must map returned fields and units. Address conflicts and undocumented commands can stop a multi-sensor bus.
0–5 V or 0–10 V A voltage proportional to a configured variable. Short, simple analog runs where the controller already has the correct voltage input. Voltage drop, grounding and input scaling become installation concerns; do not treat a voltage option as equivalent to a current loop.

What the interface changes in practice

4–20 mA for a fixed PLC channel

A current loop is attractive when the site already has analog input cards and the buyer wants a direct value with little software interpretation. The loop still needs a complete specification: the measured variable, engineering-unit mapping, lower and upper limits, supply voltage, maximum loop resistance, cable and shield practice, and the behavior used to indicate a broken wire or an out-of-range condition. If the sensor offers several outputs, the quotation must identify the exact output variant; a product family page is not enough.

RS485 when the data model matters

RS485 can carry more than one variable and can reduce the number of analog channels, but the bus does not define the register contents. The buyer needs the protocol name, register map, byte order, units, signed or unsigned interpretation, address range, baud rate, parity, stop bits, response timeout and termination recommendation. "Modbus" without those fields is not an integration document. The supplier should also state whether the selected variant transmits temperature, EC or status in addition to moisture.

SDI-12 for logger-led environmental sensing

SDI-12 is a command-and-response protocol designed for sensors and data recorders. Acclima's manual describes a three-wire connection, 1200-baud ASCII communication, a unique address for each device and measurement commands that return the time until data are ready and the number of values prepared [E01]. LI-COR's logger guidance shows the practical steps: connect power, ground and data, discover the sensor, assign a unique address and map the returned outputs to logged fields [E02].

The protocol does not automatically tell the logger what every returned number means. NexSens notes that the parameter and unit mapping is stored in the logger or application rather than encoded by SDI-12 itself [E03]. Ask for the manufacturer's command table and data dictionary, not only the phrase "SDI-12 compatible."

Do not use one interface rule for every deployment

Buyer situation A defensible starting choice Questions that decide the final choice
Existing PLC with spare 4–20 mA inputs 4–20 mA when one stable variable per channel is enough and the loop power budget is acceptable. What is the exact scale? What is the fault-state current? Is the output moisture only, or does the selected variant include temperature?
PLC or SCADA with several soil variables per location RS485/Modbus when the controller supports the protocol and the supplier provides a complete register map. How many values, what units, what byte order, what address range, what response duration? Are addresses configurable? What happens when two sensors share a port?
Environmental logger with SDI-12 support SDI-12 when the logger can power and address the sensor and the returned fields are documented. Which revision, which commands, what measurement time and current draw, and how are fields mapped to units?
Mixed legacy and new equipment Choose the interface that the receiving device natively supports, or budget for a documented gateway. Who owns scaling, timestamps, buffering, diagnostics and firmware updates after the conversion?

A manufacturer can sell the same sensing element in several output variants. ICT International's SMT-100 documentation is a useful example: it lists SDI-12, RS485 variants, voltage outputs and a 4–20 mA version, while noting that the 4–20 mA version measures soil moisture only [E04]. That is why a buyer must specify the output code and measured variables together.

The hidden integration fields to request

  • Exact interface code and part number for the quoted variant, including whether the output is simultaneous or an either/or option.
  • Measured variables, engineering units, range, resolution and status or diagnostic fields for each output.
  • For 4–20 mA: two-wire or four-wire wiring, supply range, maximum loop resistance, scaling, fault current and isolation.
  • For RS485: protocol, register map, address procedure, baud/parity/stop bits, byte order, response timing, termination and maximum bus layout.
  • For SDI-12: supported revision, command list, measurement time, current draw, default address, address-change command and field-to-unit mapping.
  • Cable length, connector pinout, shield or drain-wire practice, surge protection and grounding instructions.
  • Firmware revision, calibration method, raw-data access and behavior after power loss, bus timeout or communication failure.
  • A bench-test procedure showing the value received by the actual PLC, logger or gateway before shipment.

Two sourcing candidates with different interface roles

The following are sourcing candidates, not certified recommendations. Marketplace fields are stated information and must be confirmed against the quotation and the exact output variant. Supplier identity, protocol documentation, calibration, field performance and affiliate tracking remain separate checks.

Candidate — RIKA RK520-01

Candidate — INFWIN MT20-A

RIKA fits a buyer who already owns analog or RS485 control hardware and wants a probe with multiple output options. INFWIN MT20-A fits a buyer with an SDI-12-capable logger and a need for a low-power serial node. Neither listing proves interoperability with the buyer's controller; both require a bench test using the actual receiver.

Commission the interface before commissioning the crop

First, connect one sensor to the actual receiving device on a bench. For 4–20 mA, inject or observe known loop values and verify the controller's engineering-unit scaling. For RS485, read the identification or register values, change the address only after recording the default, and verify byte order and units. For SDI-12, discover the device, assign a unique address, issue the measurement command and compare every returned field with the manufacturer's data dictionary.

Second, create a controlled change at the sensor and confirm that the same change appears in the logger or PLC without a unit or sign error. Record supply voltage, cable length, firmware, configuration and timestamps. A value that changes on a dashboard is not enough; the acceptance record should show the raw signal or register, the converted value and the expected physical condition.

Third, unplug the signal, interrupt power and create a bus or loop fault. Verify the receiver's alarm, stale-data behavior and recovery sequence. For a control system, a missing measurement must not be silently treated as a fresh, safe reading. Only after the failure path is understood should the buyer connect multiple sensors or automate irrigation actions.

Bottom line

Choose the interface that your receiving device can document and test. Use 4–20 mA for a clearly defined PLC loop, RS485 when the digital register model and bus settings are available, and SDI-12 when a compatible environmental logger should manage low-power smart sensors. Do not approve a listing because it names several protocols. Require the exact variant, data dictionary, wiring, power, fault behavior and bench-test result. RIKA is a multi-output PLC-facing candidate; INFWIN MT20-A is an SDI-12 logger candidate. Both remain sourcing candidates until the quoted configuration passes the buyer's own receiver test.

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