Modbus device shows no values

On the gateway page, the Modbus device shows Timeout with No data since and a time, or Unknown with No data received yet. If the gateway itself shows Offline, follow Gateway is offline first.

An owner, admin or technician changes the address, registers and bus settings. If values do arrive but they are wrong, follow Values are wrong.

A new Modbus device shows no values yet

The gateway has never forwarded a value from this Modbus device. Automatic monitoring then raises no alert. If everything below is correct, the Modbus device is not answering, and you continue at Modbus RTU or Modbus TCP.

The gateway has not read yet

Wait one polling interval, 300 seconds (5 minutes) by default, and look again. Over the SIM, the gateway reads no more often than the SIM interval. How both intervals work is covered in Choose the polling interval.

The configuration did not reach the gateway

If sending fails, the portal reports Modbus device saved. The configuration could not be sent to the gateway yet. It is sent along with the next change you save to a Modbus device on this gateway. when you add the Modbus device. Saving a register counts as such a change, even if you change nothing.

1. Open the Modbus device

   On the gateway page, under Modbus devices, click the Modbus device.

2. Open a register

   In the register list, click the pencil next to a register (Register bewerken, Edit register).

3. Save the register

   Click Opslaan (Save), even without a change. The portal sends the configuration to the gateway and reports Register opgeslagen (Register saved).

Other messages are covered in Message when adding or saving a Modbus device.

The Modbus device has no registers

Without registers, the gateway reads nothing. The page of the Modbus device then shows Nog geen registers. Voeg er een toe om dit Modbus apparaat uit te lezen. (No registers yet. Add one to read this Modbus device.) Add them as described in Set up registers.

The register address or function is wrong

A Modbus device answers with an error code to an address or register type it does not know. The value then stays empty. A classic number such as 40001 in the Adres (address) column is not converted, because the portal counts from 0. A register that you add on the page of the Modbus device is set to Read input registers (4) by default.

If the manual calls the value a holding register, choose Read holding registers (3) in Functie (function). Convert the address and function as described in Enter a register address from the manual.

A Modbus RTU device does not answer

The gateway gets no usable answer over the RS485 bus. If other Modbus devices on the same bus do give values, the bus and gateway work and the fault is at this Modbus device.

The slave address is wrong

Compare Slave-adres (slave address) under Verbinding (connection) on the page of the Modbus device with the address on the Modbus device itself. Change one of the two. In the portal, you do that as in Change the connection afterwards.

Baud rate, parity or stop bits differ

Under Verbinding, Seriële bus (serial bus) shows the setting of the gateway, 9600 8N1 by default. Set the Modbus device to exactly this. You change the bus itself in Gateway settings, and that then applies to all Modbus RTU devices on the gateway.

Two Modbus devices have the same address

Two answers at the same time collide, and the gateway reads neither. The address where one of the two should have been stays silent. If exactly two Modbus devices are missing, look at this first. Give each Modbus device on the bus its own address.

A and B are swapped or the bus is broken

Swapped wires give no answer and damage nothing. Swap A and B at the Modbus device. Also check the signal ground, the terminals and the power supply of the Modbus device. If no Modbus device on the bus gives values, first check the wires on D+ and D- of the gateway.

The wiring is covered in Connect the RS485 bus. If answers drop out now and then, follow Answers on RS485 drop out or are garbled.

A second master is on the bus

If a PLC or another system reads the same RS485 bus, its requests collide with those of the gateway. Remove the second master from the bus. If both must read the Modbus device, use Modbus TCP when the Modbus device has a network connection and allows more than one connection.

A Modbus TCP device does not answer

The gateway gets no answer on the IP address, port and unit ID from the portal.

The IP address, port or unit ID is wrong

Compare IP-adres (IP address), Poort (port) and Unit-ID (unit ID) under Verbinding with the settings of the Modbus device. If IP-adres is empty, the gateway skips the Modbus device. If the Modbus device gets its IP address through DHCP, it can change after a restart. Give it a fixed IP address.

Where a Modbus TCP device must be connected so that the gateway can reach it is covered in Connect to the network via WAN and LAN.

Another system keeps the connection busy

Some Modbus TCP devices, such as many inverters, allow only one Modbus connection at a time. If other software keeps that connection open, the gateway gets no answer. Close that software.

A Modbus device that worked no longer shows values

After 5 times the polling interval without a value, and never within 15 minutes, the status is Timeout. If the check Modbus device not reporting is On, an alert follows as well. How that check works is covered in Automatic monitoring. If the bus has changed or the Modbus device has been replaced, also go through the causes under Modbus RTU and Modbus TCP.

The gateway is searching

During automatic search, the gateway pauses reading all its Modbus devices. Reading then continues by itself.

Reading on the gateway has stalled

If all Modbus devices of an online gateway fall silent at the same time, the portal sends the configuration again by itself. This happens after three polling rounds, and never within 10 minutes. Reading then starts again. If you do not want to wait, save a register again yourself as described above. If the values stay missing, check the power supply of the Modbus devices and the bus.

Measure on site with Diagnostics

You find out why a Modbus device does not answer on site with ModbusCloud Diagnostics on a laptop, through a USB-RS485 adapter or the network. For RS485, first remove the A and B wires of the gateway from the bus, because two masters give garbled answers.

How to connect is covered in Connect and find a Modbus device with Diagnostics. The messages are covered in Find faults with the Diagnostics tab and the console. Then connect the gateway again.

Contact support

If this does not solve it, contact us through the support page. Send the serial number of the gateway and the brand and type of the Modbus device. Add the address and the bus settings on the Modbus device, and a photo of the connections. If you measured with Diagnostics, paste the text of the report as well.