10.2.1. Modbus Assembly Instance Structure

Each ConveyLinx-Ai Family Modules utilizes Modbus register architecture for remote data access over Ethernet. Modbus TCP is a simple protocol for data exchange based upon a query/response mechanism. Each ConveyLinx-Ai Family Modules’s memory structure contains a fixed array of internal data locations that are constructed as Modbus Holding Registers. Each ConveyLinx-Ai Family Modules has a fixed reserve of Holding Registers with each capable of holding a 16-bit numerical integer value. Modbus TCP protocol provides for read/write access to any available Holding Register. The structure of these registers allows for individual ConveyLinx-Ai Family Modules’s to read from and write to specific register address locations to achieve inter-module communications. Certain registers are read from and written to by the EasyRoll software in order to monitor and/or change default configuration values such as MDR speed, direction, type, etc.

*Modbus TCP addressing convention utilizes a “4:xxxx” notation. The “4:” in Modbus protocol designates that the address is a Holding Register and the xxxx is a numerical value representing the offset or index for a specific location. The “xxxx” values used in this document are to be interpreted as if they are for a Modbus PLC which means that the first register address is “4:0001” and that there is no “4:0000” register. Some PLC data structures and PC development environments utilize the “4:0000” designation and their indexes will be offset by 1. Please refer to your PLC or PC application documentation for the Modbus convention used on their platforms.

Supported Modbus TCP Service Codes

Any Modbus TCP capable PC or PLC can connect to any ERSC visible on its network and access Input and Output Holding Register Assemblies. The ERSC supports the following Modbus TCP Service Codes:
• Service Code 3 – Read Holding Register (up to 45 registers per instruction)
• Service Code 6 – Write Single Register
• Service Code 16 – Write Multiple Registers (up to 45 registers per instruction)
• Service Code 23 – Read/Write Multiple Registers (up to 45 registers per instruction)

Properly Addressing Assembly Registers

In the ConveyLinx-Ai Family Modules, the Assembly Registers are “virtual” in that they are not “physical” Module Register Addresses and as such are not directly addressable. The ConveyLinx-Ai Family Modules firmware intercepts the request and builds the assembly data in order to transmit it to the requesting device. Because of this, you must always address an Assembly by using the first address in the assembly in order for the ConveyLinx-Ai Family Modules to return any data.

!When using Input and Output Assemblies with Modbus TCP PLC, it is important to always use the first address shown in the assembly group as the beginning register to read or write regardless of which register in the assembly is needed by the PLC. Trying to access Input or Output Assemblies starting with any register in the assembly other than the first register will cause the ConveyLinx-Ai Family Modules to return an error

For example, for ZPA Mode Assembly Inputs, if you only need to read register 4:1504; your I/O connection set-up in your PLC must use the starting address of 4:1500 and a sufficient length of registers to read (in this example at least 5 registers) in order to get to the desired register. If you set up your PLC to start reading at 4:1504, the ConveyLinx-Ai Family Modules will return an error