Configuring Address Interpretation for AB VFD RTU 485 Communication

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.

• 16-07-2024
• JHarbin

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.

• 17-07-2024
• Cotafam

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.

• 17-07-2024
• drbitboy

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.

• 18-07-2024
• danw

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.

• 18-07-2024
• Cotafam

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.

• 07-01-2025
• Noah Collins

It looks like you're on the right track with your address configuration. In a Linux environment, you might want to use a Modbus library (like libmodbus) that understands the addressing scheme you're dealing with. For accessing the AB VFDs data, you would need to set up TCP/IP connections and use Modbus functions to read from or write to these address locations. However, remember the address in Modbus RTU starts from 0. So for address 8448, subtract 1 to get 8447. This may vary for other devices. Also, ensure your gateway's Baud Rate, Data Bits, Parity, and Stop Bits match the settings on your VFDs.

• 24-02-2025
• David Garcia

You'll likely need to use a Modbus function code to read from or write to the address you mentioned. For instance, if you're trying to read from the coil, you can use function code 0x01. When using other systems or protocols, the Modbus address might shift by one, so coil 8192 on your VFD might be address 8193 on your software. Remember, this address refers to the entire 16-bit word, so you will have to manually isolate the bits you're interested in (14, 13, and 12). On your Linux gateway, I recommend using the 'modbus_read_bits' function provided by the libmodbus library. This should help you interface your Linux gateway with the AB VFDs more easily.

• 24-02-2025
• Rebecca Hall