I am encountering difficulties with the address configuration of AB VFDs that output RTU 485. Address 8448 has bits ranging from 0 to 15. To access bit 0, I would set the memory to (03) 38448. Similarly, for accessing bit 15, the address would be 38415. Additionally, I have added address "8192" for reading '001 = Freq. Source= P108 [Speed Reference]' with bits 14, 13, and 12 in use. How should I configure this address for our Linux gateway to retrieve data from? Any guidance would be greatly appreciated.
A common issue in Master devices is reading the address as 8448.0 or 8448.1, but it may need to be 48448.0 or 408448.0.
I understand now. To read that particular bit, I would need to use a value like 38448.10. I just realized that this is a 16-bit integer.
The Modbus protocol is limited to using 16-bit integers for requests and responses. The interpretation of the bits within these integers is left to the slave device. The documentation specifies that the bits can be accessed using Modbus Function Code (FC) 03, with a designated memory address of 8448 in the "16-bit Holding Registers" area of the slave memory model. It is important to note that there is a common convention in some Modbus Master configurations where the direction of data transfer and memory model area are treated as separate parameters by adding a prefix to the address. For example, a prefix of "4" is used for 16-bit holding registers, "3" for read-only 16-bit input registers, "1" for read-only discrete inputs, and "0" for discrete outputs. When using this convention, to read the Holding Register at address 8448, you would specify the parameter as READCombined [4, 8448]. The inclusion of the prefix helps differentiate between various memory model areas and addresses within the Modbus system. It is common for Modbus addresses to be presented as either ordinal addresses (1, 2, 3, ...) or address offsets (0, 1, 2, ...) depending on the specific configuration of the Master and Slave devices. This variation is determined by the manufacturers and models of the devices being used. Adding the "3" or other prefixes to addresses like 8448 and 8192 is done to specify the memory model area for the Modbus Master to accurately retrieve the desired data.
When reading a register, it is important to understand the significance of leading numerals. In the context of Modbus registers, the leading numeral (4) indicates a one-based identification system, where the first register is represented as (4)0001. This is different from zero-based notation, where the first register is 0x0000. For example, when attempting to read register 'index value' 8448, it can be represented as either (4)8448 (already one-based) or (4)8449 (if starting from zero-based). This distinction is crucial for accurate data retrieval and interpretation. In another scenario, determining the frequency source from a Modbus register involves analyzing specific bits. In the holding register (4)8192, bits 12, 13, and 14 indicate the frequency source. Depending on the combination of these bits, the source could be identified as No Command, P108 [Speed Reference], or through Comms. It is important to note that while certain information, such as the speed reference value of P108, may be available in other Modbus registers, it may not necessarily be found in register (4)8448. Understanding these nuances is key to effectively utilizing Modbus registers for data acquisition and control purposes.
I am truly grateful for all of your insightful replies; they have really helped me understand this better. I will be sure to provide regular updates on my progress.
Your Linux gateway essentially needs to know how to read the memory addresses. This typically involves writing a script or driver that matches the specifications given by AB VFD. Considering you have to read different bit positions, your script needs to account for these. It should read the address, then apply a bitwise operation to extract the relevant bit (similar to shifting bits). For your address "8192", if you're using bits 14, 13, and 12, you would apply bitwise AND operation with '11100000000000', to mask the other irrelevant bits. A base code in Python could work well for a quick start, but you might also need to account for proper timing and error checking based on the VFDs Modbus protocol. You may need to explore the exact format and structure expected by your VFDs, as this can greatly affect how data is interpreted.
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Answer: - To access specific bits in AB VFDs with RTU 485 output, you can set the memory address accordingly. For example, to access bit 0, the address would be (03) 38448, and for bit 15, it would be 38415.
Answer: - The address "8192" is used for reading '001 = Freq. Source= P108 [Speed Reference]' with bits 14, 13, and 12 in use. To configure this address for data retrieval through a Linux gateway, you may need to adjust the gateway settings to interpret the data correctly.
Answer: - If you are encountering difficulties with address configuration in AB VFDs with RTU 485 output, you may need to double-check the memory addresses specified for accessing specific bits and ensure they align with the device's communication protocol requirements. Additionally, verifying the settings on your Linux gateway for data retrieval is crucial.
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