6.0 Ethernet [T-Series Datasheet] | LabJack
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6.0 Ethernet [T-Series Datasheet]


Communication Protocol:  Modbus TCP

Connector Type: RJ-45 Socket, Cat 5

Compatible: 10/100Base-T, Auto-MDIX

PoE (Power over Ethernet): Not built-in.  See PoE App Note.

Max Cable Length: 100 meters typical

Max Packet Size: 1040 bytes/packet (TCP), 64 bytes/packet (UDP)

The T-series devices each have a 10/100Base-T Ethernet connection. This connection only provides communication, so power must be provided through the USB connector. Refer to this WiFi and Ethernet tutorial to get started.


T-series devices will respond to connections or packets on following ports:

  • 502 (TCP) - Responds to Modbus TCP packets received over this port.
  • 702 (TCP) - Sends spontaneous data such as stream to devices connected to this port.
  • 52362 (UDP) - Responds to Modbus UDP packets received over this port. Usually used to search for T-series devices on a network.

If you need a wireless connection instead of Ethernet, you can buy a wireless bridge. Connect the T-series device to the bridge and the bridge will connect to the wireless network. Find more information in the Convert Ethernet to WiFi App Note.

"But I Don't Know How To Speak Ethernet?"

Don't worry, the LJM Library takes care of the Ethernet details.  You just make our Open call to get a handle to a particular device (e.g. HandleA), and then make simple calls such as eReadName(HandleA, "AIN0") which returns the voltage from analog input 0.  We have examples for many development environments and even some simple ready-to-run software.

Config (_DEFAULT) Registers

Use the following _DEFAULT registers to configure Ethernet:

Ethernet Config Registers
Name Start Address Type Access

ETHERNET_IP_DEFAULT             The IP address of wired Ethernet after a power-cycle to the device.

49150 UINT32 R/W

ETHERNET_SUBNET_DEFAULT             The subnet of wired Ethernet after a power-cycle to the device.

49152 UINT32 R/W

ETHERNET_GATEWAY_DEFAULT             The gateway of wired Ethernet after a power-cycle to the device.

49154 UINT32 R/W

ETHERNET_DNS_DEFAULT             The DNS of wired Ethernet after a power-cycle to the device.

49156 UINT32 R/W

ETHERNET_ALTDNS_DEFAULT             The Alt DNS of wired Ethernet after a power-cycle to the device.

49158 UINT32 R/W

ETHERNET_DHCP_ENABLE_DEFAULT             The Enabled/Disabled state of Ethernet DHCP after a power-cycle to the device.

49160 UINT16 R/W

ETHERNET_APPLY_SETTINGS             Writing 1 to this register power-cycles Ethernet. It tells the device to waits 1s before turning off Ethernet and then 500ms before turning it back on.

49190 UINT32 W

These registers can also be read.  Configure the Ethernet network configurations in Kipling software.

These _DEFAULT registers are non-volatile.  Whatever value you write to a _DEFAULT register will be retained through a reboot or power-cycle.  New values written to these _DEFAULT registers are not applied until power-up or until a 1 is written to ETHERNET_APPLY_SETTINGS.

Status Registers

Use the following read-only registers to read the status of Ethernet:

Ethernet Status Registers
Name Start Address Type Access

ETHERNET_IP             Read the current IP address of wired Ethernet.

49100 UINT32 R

ETHERNET_SUBNET             Read the current subnet of wired Ethernet.

49102 UINT32 R

ETHERNET_GATEWAY             Read the current gateway of wired Ethernet.

49104 UINT32 R

ETHERNET_DNS             Read the current DNS of wired Ethernet.

49106 UINT32 R

ETHERNET_ALTDNS             Read the current Alt DNS of wired Ethernet.

49108 UINT32 R

ETHERNET_DHCP_ENABLE             Read the current Enabled/Disabled state of Ethernet DHCP.

49110 UINT16 R

Config (_DEFAULT) vs. Status Registers

Example: If you write ETHERNET_IP_DEFAULT="" (you would actually write/read the 32-bit numeric equivalent—not an IP string), then that value will be retained through reboots and is the default IP address.  If DHCP is disabled, this will be the static IP of the device and what you get if you read ETHERNET_IP.  If DHCP is enabled, then a read of ETHERNET_IP will return the IP set by the DHCP server.


The following registers configure Ethernet power:

Name Start Address Type Access

POWER_ETHERNET             The current ON/OFF state of the Ethernet module.

48003 UINT16 R/W

POWER_ETHERNET_DEFAULT             The ON/OFF state of the Ethernet module after a power-cycle to the device. Provided to optionally reduce power consumption.

48053 UINT16 R/W

Some Examples

Read IP Example: To read the wired IP Address of a device, perform a Modbus read of address 49100.  The value will be returned as an unsigned 32-bit number, such as 3232235691.  Change this number to an IP address by converting each binary group to an octet, and adding decimal points as necessary.  The result in this case would be "".

Change IP Example: To change the Ethernet IP Address of a device, perform a Modbus write to address 49150.  The value must be passed as an unsigned 32-bit number, such as 3232235691.  Change this IP address "" by converting each octet to a binary group, and sticking them together.

Default IP Address

The default is DHCP enabled.  If your network does not have a DHCP server, there are a few options to talk to the device so you can change Ethernet settings: 

  • Use USB.
  • Connect to a network with a router (DHCP server).
  • Use the SPC-to-AIN3 jumper to temporarily force a static IP.


The Ethernet connection on any T-series device has 1500 volts of galvanic isolation.  All power supplies shipped by LabJack Corporation with T-series devices have at least 500 volts of isolation.

Note that if you power the T-series device from a USB host/hub, ground from the host/hub is typically connected to upstream USB ground, which often finds its way back to AC mains ground and thus there would be no isolation.

LEDs on the Ethernet jack

Both the green and orange LEDs on the Ethernet jack will illuminate on connection to an active Ethernet cable. The orange LED turns on when an active link is detected and blinks when packets are received/processed. The green LED illuminates when the connection is 100Mbps.

The orange LED is closest to the USB connector.

UDP Discovery-Only Mode - T7 only


Firmware minimum: 1.0284

It is theoretically possible that an unintended UDP packet could be received by a T-series device and processed. However, this is statistically unlikely. By one calculation, the statistical likelihood of this happening is less than 2.7E-10 percent.

However, if your application needs ultimate reliability or your network has Modbus UDP packets broadcast on port 52362, you can enable a discovery-only mode. Discovery-only mode disables all Modbus functionality except a small list of registers that LJM uses to identify devices. The error UDP_DISCOVERY_ONLY_MODE_IS_ENABLED (2317) indicates that an operation attempted to read/write a register outside of this list while discovery-only mode was enabled.

Use ETHERNET_UDP_DISCOVERY_ONLY to enable/disable discovery-only mode until the device is power-cycled:

Name Start Address Type Access

ETHERNET_UDP_DISCOVERY_ONLY             Restricts which Modbus operations are allowed over UDP. When set to 1, only the registers needed for discovering units can be read and any other operation will throw an error.

49115 UINT16 R

Use the _DEFAULT register version to set the startup behavior for discovery-only mode:

Name Start Address Type Access

ETHERNET_UDP_DISCOVERY_ONLY_DEFAULT             The Enabled/Disabled state of ETHERNET_UDP_DISCOVERY_ONLY after a power-cycle to the device.

49165 UINT16 R

There are separate WiFi-only configurations for discovery-only mode via WiFi.