Yes: temperature and humidity reach a screen by one of three paths, and the path decides the screen. A 4-20 mA transmitter needs an input built for current loops, a Modbus RTU sensor needs a panel set to the sensor's baud and parity, and a value already in a controller needs any screen with a driver for that controller. Size for the room at 20 ft of viewing distance per inch of capital height.
Yes, any screen can show temperature and humidity, but only after the signal path is decided. The measurement reaches a display one of three ways: as a 4-20 mA current loop that needs an input built to measure it, as Modbus RTU data on an RS-485 bus that needs a panel set to the sensor's baud and parity, or as a value already scaled inside a controller, which any screen with the right driver can read. Pick the screen by the path first and by size second. Get the path right and the numbers appear on the first power-up; get it wrong and you have a lit screen with nothing on it. This page gives the three paths, the settings each one needs, the character height the room demands, and a worked dock-door example.
Match the signal path before the screen
The decision order is signal path, then size, then mount. Read the label on the sensor or transmitter before you shop for a screen. A two-wire transmitter marked 4-20 mA is an analog source: it needs a receiver that measures loop current and scales it, and no screen without such an input will read it, whatever its resolution. A sensor with an RS-485 terminal and a register list in its manual is a Modbus RTU slave: it needs a panel that polls it as a master. When a controller already brings both values in, the screen only has to talk to that controller.
Each path ends in a working display. None of them starts with the diagonal size.
The three ways measurements reach a screen
| What the sensor gives you | What the screen must have | What you buy | Pick this when |
|---|---|---|---|
| 4-20 mA on two wires, loop-powered | one or two analog inputs that measure 4-20 mA and scale each range | a loop-powered panel meter, or a panel meter or HMI with analog input channels | the transmitter is loop-wired and no controller reads it |
| Modbus RTU on RS-485 | a serial master port set to the sensor's baud, parity and stop bits | an HMI panel with an RS-485 port, or a Modbus RTU display head | the sensor is digital and no controller polls it |
| Values already in a controller | a driver for that controller's protocol and port | any HMI that polls the controller | a PLC or RTU already reads and scales the sensors |
| Sensor with a built-in display | nothing extra | a wall sensor whose LCD shows the reading | one reading point is enough and nobody needs the value elsewhere |
Some room sensors combine their own LCD with an RS-485 port speaking Modbus RTU, so the sensor is its own display. Its datasheet then also lists the supply, commonly 10 to 28 V DC, and a range such as 0 to 99.9% RH.
A 4-20 mA transmitter needs a loop input
A 4-20 mA loop sends its whole range as current: 4 mA at scale zero, 20 mA at full scale, 16 mA of span. The 4 mA live zero is the useful part. A panel reading 4 mA knows the value is zero; a panel reading 0 mA knows the wire is broken. That is why current loops beat voltage outputs on any cable run past the bench, as the comparison of voltage and current analog signals for sensor runs explains.
Scaling happens in the receiver:
value = (I - 4 mA) / 16 mA x span
A transmitter scaled 0 to 100 °C that the panel measures at 12 mA shows (12 - 4) / 16 x 100 °C = 50.0 °C. Set that span and offset once, in the meter or panel channel, from the transmitter's own label.
Decision: one value at one point needs only a loop-powered panel meter. Two transmitters, temperature and humidity, landing at the same spot need a panel with two analog input channels, each with its own span and unit.
Safety note: do not break a live loop to insert a meter. The reading faults the moment the loop opens, and on loops that feed alarms or interlocks that trip is not contained to the display. Land the meter on the terminal block during a shutdown, or clamp on.
A Modbus RTU sensor needs the same baud and parity
A digital temperature and humidity sensor on RS-485 is a Modbus RTU slave. It holds its values in registers, and the panel polls those registers as the master. Four settings must match on both ends, and all four come from the sensor manual: slave address, in the 1 to 247 range, baud rate, with 9600 the common default, parity, and stop bits. A panel on the wrong parity hears nothing, so set the four from the manual, not from the last job.
The manual also lists the register numbers, the data type and the decimal place. Room sensors typically report temperature in 0.1 °C steps and relative humidity as 0.0 to 99.9% RH, so the panel is told one decimal place per register. If the number arrives but looks absurd, the data type is the usual culprit; the article on an HMI panel showing nonsense numbers from a BCD analog register walks that down. Wiring is a bus, not a star: sensors daisy-chain on the RS-485 pair, and the steps in putting an HMI panel on a custom Modbus RTU slave apply to a sensor unchanged.
Decision: sensors are digital and no controller is in the circuit, buy a panel that speaks Modbus RTU out of its serial port. Where a controller already polls the sensors, let the screen talk to the controller instead.
A PLC that already reads the sensor needs only a driver
When a controller already brings both values in, the screen's job is to talk to the controller, and the sensors drop out of the decision. Any HMI with a driver for that controller's protocol can poll it. Confirm two things before ordering: the port on the controller, RS-232 or RS-485, and the driver for its protocol, set at the baud the controller port already runs. If the panel replaces an older one, checking the PLC serial protocol before the swap avoids ordering a panel that cannot speak to the controller at all.
One more path covers a second board in a lobby or on the floor. A signage screen, like the panel-mount units in our display range, plays images and video from microSD or USB. It has no sensor inputs and is not an HMI, so it shows measurements only as content that is published to it: an export job writes an image with the current values and the screen plays it offline. Use it where people want a big notice board that refreshes on publish, and keep the live number on an HMI where someone acts on it.
Decision: the controller exists, buy for the controller. The screen is chosen by its driver, not by its panel size.
Size the screen for the room
Once the path is fixed, size comes from one rule. The highway lettering figure (FHWA) gives 30 ft of viewing distance per inch of capital letter height; for numbers people read at a glance, use 20 ft per inch. Line spacing sits at 1.7 times the capital height.
capital height (in) = viewing distance (ft) / 20 for a board read while walking
A board read from 40 ft needs 2 in capitals; from 15 ft, 0.75 in. Diagonal, aspect ratio and mounting are covered in the guide to choosing a screen size; the character height here is the number that decides whether the panel is big enough.
Example: a dock door board read from 40 ft
Scenario: two 4-20 mA transmitters, temperature and humidity, are already wired to a PLC analog card in the electrical room. The dock office wall faces the loading floor 40 ft away, and the board must show two lines: temperature and relative humidity.
Character height: 40 ft / 20 ft per inch = 2.0 in capitals. Line spacing: 1.7 x 2.0 = 3.4 in. Block height for two lines: 2.0 + 3.4 + 2.0 = 7.4 in. A 16:9 panel stands 0.49 times its diagonal tall, from 9 / sqrt(16 x 16 + 9 x 9) = 9 / 18.36 = 0.49, so the text block alone needs 7.4 / 0.49 = 15.1 in of diagonal with no margin left for a header. Specify the 19 to 22 inch diagonal class and the block fits with room for labels and a timestamp.
Signal decision: the PLC exists and already scales the transmitters, so the dock board is an HMI that polls the PLC over the port the controller already runs, set to that port's baud and parity. The transmitters and the analog cards need no changes.
End result: 2 in capitals on a 19 to 22 inch panel, polled from the PLC, readable across 40 ft of dock floor.
What to ask for before you order
Send this list to whoever quotes the screen:
- Each sensor's output and range from its label, for example 4-20 mA over 0 to 100 °C, or 0 to 99.9% RH.
- For Modbus RTU: slave address, baud, parity, stop bits, register numbers and data types, from the sensor manual.
- For a controller path: the protocol, the port type, and the port's baud and parity.
- The viewing distance in feet and the number of lines the board must show.
- The supply available where the screen mounts, and the ambient temperature there in both the summer and the winter case.
That last item matters because the screen's own operating range is in its datasheet, and a dock in winter is a colder place than the office it was quoted from.
Recommendation: Read the sensor label first, then buy for the path. A 4-20 mA transmitter goes to a loop-powered panel meter, or to a panel with analog input channels when temperature and humidity land together. A Modbus RTU sensor goes to a panel set to its slave address, baud and parity, with 9600 as the usual starting baud. A value already in a PLC goes to any HMI with a driver for that controller at the port's own baud. Where one reading point is enough, buy the sensor with the built-in display and skip the extra hardware. Size every option with 20 ft of viewing distance per inch of capital height, and line spacing at 1.7 times that height.
Questions
The screen powers up but shows no reading. Where do I look first?
Check the signal at the receiver before the screen. On a 4-20 mA input, measure the loop current: about 4 mA at scale zero and 20 mA at full scale means the transmitter is alive, and 0 mA means a broken wire or no loop supply. On Modbus RTU, silence or discarded frames point to baud, parity or wiring, starting with the RS-485 A and B lines swapped at one device. On a controller path, confirm the driver, port and station address.
Why does the humidity show as a number between 0 and 1?
Some sensors report relative humidity as a fraction from 0.000 to 1.000, while the panel expects percent. Multiply that register by 100 in the panel's scaling or decimal-point setting and it reads 0 to 99.9% RH. The sensor manual's register list states the unit and decimal place for each value, so check there before changing anything else. The sensor is reading correctly; the scaling lives on the panel.
How often should the panel poll the sensor?
Room air moves slowly, so a poll every 1 to 2 s is enough for a wall board. Polling faster only loads the RS-485 bus, which matters once a dozen slaves share it; polling slower lets an open door drift unnoticed between updates. Take the sensor's own measurement interval from its manual. A panel cannot show an update the sensor has not produced yet, whatever the poll rate.
Can temperature and humidity come from two different sensors onto one screen?
Yes. On Modbus RTU each value is a register, so two sensors with different slave addresses share one RS-485 bus and both lines appear on one panel. On analog inputs, one transmitter feeds one channel, so a two-value panel needs two input channels. Give each value its own span, unit and decimal place, and label the lines so temperature and humidity cannot be read swapped.
Planning information, not legal or engineering advice. Rules differ by place: check codes and permits with your local authority and your licensed trades.