IS MODBUS POLL LIMITED TO ADDRESS 49999 SOFTWARE
When an address greater than 255 is entered, the software automatically switches to 2 byte addressing and stays in this mode for all addresses until the 2 byte addressing is manually turned off.įigure 7.4 shows a simplified block diagram with a master and a slave device communicating over the SPI bus. Two bytes addressing extends the limit on the number of slaves in a network to 65535.īy default, the simply Modbus software uses 1 byte addressing. The master and the slaves would all be required to support this modification. To get beyond this limit, the protocol can be modified to use 2 bytes for the address. The limit defined in the Modbus specification is even lower at 247. Since a single byte is normally used to define the slave address and each slave on a network requires a unique address, the number of slaves on a network is limited to 256. But, on the other hand, some slave devices do support these registers and some master software can access it, especially if custom software is written. And many slave devices do not support maps using the extended registers.
IS MODBUS POLL LIMITED TO ADDRESS 49999 DRIVERS
Many Modbus software drivers (for master PCs) were written with the 40001 to 49999 limits and cannot access extended registers in slave devices. This would correspond to register numbers from 40001 to 105536. If we utilize the remaining data addresses 270F to FFFF, over six times as many registers can be available, 65536 in total. Registers 40001 to 49999 correspond to data addresses 0000 to 270E. Although this is usually enough for most applications, there are cases where more registers would be beneficial. The range of the analog output holding registers is 40001 to 49999, implying that there can be a maximum of 9999 registers. Some devices are built with a fixed map that is defined by the manufacturer, while other devices allow the operator to configure or program a custom map to fit their needs. How the data is stored (data types, byte, and word ordering) Where the data is stored (which tables and data addresses) What the data is (e.g., pressure or temperature readings) A Modbus map is simply a list for an individual slave device that defines the following:
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