We also support a custom function called Modbus Feedback (MBFB) that can handle multiple reads & writes in a single packet. We support standard functions 3 (Read Multiple), 4 (Read One), 6 (Write One),Īnd 16 (Write Multiple). Modbus packets on the U3 and U6 are limited to 64 bytes, including the 2 zeros appended to the command.
Packet size limits for the T4 are USB=64 and Ethernet=1040 bytes. Packet size limits for the T7 are USB=64, Ethernet=1040, and WiFi=500 bytes. With a read of a 32-bit (2 register) value, the value is returned MSW then LSW, where each word is returned MSB then LSB, so the 4 bytes come in order from most significant to least significant. With a read of a 16-bit (single register) value, the 1st byte returned is the MSB (most significant byte) and the 2nd byte returned is the LSB (least significant byte). Modbus is big-endian, which means the most significant value is at the lowest address. AIN0 is actually stored in 2 registers: the MSW (most significant word) is at address 0 and the LSW (least significant word) is at address 1. For example, AIN0 is defined as a 32-bit value that is read starting at address 0. In our Modbus Map we define some 32-bit values, and these are often referred to as a register, but when looking at the details of Modbus protocol, these are actually 2 Modbus registers. In the Modbus spec and in the function documentation below, a register is specifically a 16-bit value. Modbus Registers Are 16-bit LabJack Values Are One or More Modbus Registers The meaning of the registers are defined in the Modbus Map. Any function we support can be used to read or write values from any address. Each address points to a 16-bit value that might be readable, writable, or both. It consists of a register map with addresses from 0 to 65535. Our Modbus TCP interface is quite simple. Sometimes a Server is called the Slave and a Client is called the Master. The U3 and U6 also have deprecated Modbus support.Ī Modbus TCP Client can send a command to the LabJack and get back a response. LabJack T-series devices (the T4 and T7) are Modbus TCP Servers. Lengthy (and confusing) detail about Modbus TCP can be found at, but all the information actually needed is covered in the following.
0 kommentar(er)
