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App Notes

Software & Driver


5.0 USB

Communication Protocol: Modbus TCP

Connector Type: USB-B Receptacle

Compatible: USB 1.1+

Max Cable Length: 5 meters

Max Packet Size: 64 bytes/packet


Power is supplied to the T7 through the 5V USB connection.  If the Ethernet or Wi-Fi connection is preferred for communication, use the provided AC USB 5V adapter for power.  When used for communication, it is a full-speed USB connection compatible with USB version 1.1 or higher.

Interface - Talk to the T7

Modbus TCP is the protocol used by all connections on the T7 (USB, Ethernet, WiFi).  If you want to handle USB communication yourself (find/open/write/read/close), you can use the Modbus protocol.  Most customers, however, will use our LJM library which provides convenient device discovery, high-level functions, and programming flexibility. 

Power Considerations

USB ground is connected to the T7 ground (GND), and USB ground is generally the same as the ground of the PC chassis and AC mains, since standard USB is non-isolated.

It electrical isolation between the T7 and host is desired, use Ethernet (or WiFi) instead, or add a USB isolator.

Any host port or self-powered hub port should provide 4.75 - 5.25 volts at any current up to 500 mA, per USB specifications. We have found that some are not up to spec, however, and at higher currents the supplied voltage drops below 4.75 volts. See Appendix A-5 for details on T7 power supply requirements.

If designing your own driver...

The LabJack vendor ID is 0x0CD5. The product ID for the T7 is 0x0007.

The USB interface consists of the normal bidirectional control endpoint (0 OUT & IN), 3 used bulk endpoints (1 OUT, 2 IN, 3 IN), and 1 dummy endpoint (3 OUT). Endpoint 1 consists of a 64 byte OUT endpoint (address = 0x01). Endpoint 2 consists of a 64 byte IN endpoint (address = 0x82). Endpoint 3 consists of a dummy OUT endpoint (address = 0x03) and a 64 byte IN endpoint (address = 0x83). Endpoint 3 OUT is not supported by the firmware, and should never be used.

All commands should always be sent on Endpoint 1, and the responses to commands will always be on Endpoint 2. Endpoint 3 is only used to send stream data from the T7 to the host.