S7-200 Serial PPI

python-snap7 3.2 adds experimental serial PPI support for a PC master talking to one Siemens S7-200 slave. Install the optional serial dependency first:

pip install "python-snap7[ppi]"

Connect with the serial device and PLC station address:

from s7 import PPIClient

with PPIClient("/dev/ttyUSB0", station=2, baudrate=9600) as client:
    data = client.v_read(0, 4)
    client.v_write(10, b"\x01\x02")

On Windows, pass a port such as "COM3". The serial transport uses the PPI 8E1 format. The adapter must provide the physical interface required by the PLC; a serial device name alone does not imply electrical compatibility.

Memory areas

Use PPIArea with read_area() and write_area():

Area	Meaning	Count and data units
`S` / `SM`	Special memory / special markers	Bytes
`I` / `Q` / `M`	Inputs / outputs / markers	Bytes
`V`	Variable memory	Bytes; encoded as DB1 on the wire
`AI` / `AQ`	Analog inputs / outputs	16-bit items
`C` / `T`	Counters / timers	16-bit items

For example:

from s7 import PPIArea, PPIClient

with PPIClient("/dev/ttyUSB0", station=2) as client:
    marker_bytes = client.read_area(PPIArea.M, start=0, count=8)
    counters = client.read_area(PPIArea.C, start=0, count=2)

Limits

The initial implementation supports SD1/SD2 request framing, PDU negotiation, bounded retries, and a single PC-master/single-PLC exchange. It does not implement multimaster token passing or PPI over TCP. Request sizes must fit the negotiated PDU and the PPI frame-size limit. Validate the transport against the target hardware before relying on it in production.

See PPI for all constructor options, frame helpers, and low-level transport classes.