PPI (Point-to-Point Interface) protocol support

[gijzelaerr]

Summary

Implement PPI protocol support for communicating with Siemens S7-200 series PLCs.

Background

PPI (Point-to-Point Interface) is the native communication protocol for Siemens S7-200 series PLCs. While S7-200 is an older product line, it remains widely deployed in legacy industrial installations worldwide. PPI uses serial communication (RS-485) and has a different frame format than the TCP/IP-based S7 protocol.

What needs to be done

• Implement PPI frame encoding/decoding (SD1, SD2, SD3, SC, ED frame types)
• Implement PPI address handling (station addresses 0-126)
• Implement serial port communication layer (via pyserial)
• Implement PPI read/write operations for S7-200 memory areas (V, I, Q, M, S, T, C, SM, AI, AQ)
• Implement token passing mechanism (PPI is a token-ring protocol)
• Consider PPI over TCP support (for serial-to-Ethernet converters)
• Add optional dependency on pyserial
• Add tests (mock serial port or PPI-over-TCP)
• Add documentation and examples

Notes

• PPI is half-duplex on RS-485
• Default baud rate: 9600 (configurable up to 187.5k)
• S7-200 supports up to 4 master connections
• Many S7-200 installations use serial-to-Ethernet converters, so PPI-over-TCP would be valuable