Are there any open-source communication protocols for PLCs that would enable input writing to an Rx3i PLC? I'm seeking this solution for hardware simulation and testing of my PLC program. It's important to note that Modbus/TCP is not a viable option for this purpose, as it only supports reading from inputs, not writing to them.
Ozpeter stated: "No, Modbus/TCP is not functional for writing to inputs; it only allows reading from them." Click to expand. Is this restriction related to the PLC itself, or is it an inherent limitation of the Modbus protocol? From my understanding, Functions 5 and 6 in Modbus are designed to write to coils and registers, respectively.
User Cardosocea inquired: "Is this an inherent restriction of the Programmable Logic Controller (PLC), or is it a perceived limitation of Modbus communication protocol? From my understanding, Function Codes 5 and 6 are designed to write to coils and registers in Modbus. Can anyone clarify?" Upon further investigation, it appears to be a limitation associated with Modbus itself.
Utilize Modbus to write data to %Q or %R memory locations in the RX3i PLC. After this, you can transfer the %Q or %R data to the %I memory using ladder logic. To ensure proper execution, place the rung responsible for this data transfer at the start of your program, as the standard input scan occurs just before the execution of the logic. Alternatively, consider using a 90-30 PLC, which supports the widely-documented SNPX protocol. This protocol allows writing directly to %I memory. However, please note that the RX3i does not support SNPX, which operates over a serial connection rather than Ethernet.
Steve Bailey advises utilizing Modbus to write to %Q or %R memory within the Rx3i system. Once this is established, you can transfer the %Q or %R data to the %I in your ladder logic. It's essential to recognize that the standard input scan occurs just before executing the logic, so ensure that the rung responsible for this data transfer is positioned at the start of your program for optimal performance. On a personal note, I’ve experienced the challenges of this process firsthand and am keen to sidestep any similar issues in the future. Specifically, I'm trying to avoid alterations to the target PLC program. The inputs I wish to simulate are dispersed throughout the input table rather than being consolidated, making the maintenance of such a setup considerably labor-intensive.
Steve Bailey stated: The Rx3i does not offer support for SNPX, as it operates on a serial connection rather than Ethernet. To expand further, does the RX3i provide compatibility with SRTP, which is the Ethernet version of SNP-X?
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