Installing pip and pymodbus on Siemens IoT2020: Complete Guide

Overview

The Siemens SIMATIC IoT2020 (part of the SIMATIC IOT2000 family) ships with a Yocto Linux image that bundles Python 2.7 for gateway scripting, but pip and most pure-Python third-party packages are not pre-installed in the root filesystem. When developers try to bootstrap pip with the upstream get-pip.py script, the installer crashes with ImportError: No module named getpass because the image's Python build was stripped of the getpass module — a dependency of pip itself. The same missing module then blocks any further pip install of pymodbus, flask, or requests.

This article documents the exact root cause of the failure, the verified recovery procedure (restoring getpass.py from the CPython 2.7.3 source tree), the correct sequence to bootstrap pip, and the field-tested steps to install pymodbus for Modbus TCP and Modbus RTU communication with Siemens S7-1200 / S7-1500 controllers and third-party Modbus devices.

Affected Hardware and Image Versions

The getpass defect is specific to the legacy x86 iot2000-example-image built with Poky/Yocto and the python-2.7.3 recipe. The newer ARM-based IoT2050 ships a complete Debian rootfs and is not affected.

Prerequisites

• SIMATIC IoT2020 / IoT2040 with the example image booted from microSD.
• Serial console (115200 8N1 on /dev/ttyS0) or SSH access (root, default password not set on the example image — first-boot autologin).
• The gateway connected to a network with outbound HTTPS to pypi.org and files.pythonhosted.org.
• ~80 MB free space on the rootfs or a USB stick mounted at /mnt/usb.
• Internet access on your engineering workstation to fetch the upstream getpass.py source from the CPython 2.7.3 reference tree.

Root Cause: Why get-pip.py Fails with No module named getpass

The get-pip.py bootstrap script first runs pip.zip from a temporary directory. To confirm Python is functional, the bundled pip package imports pip.vcs.mercurial, which transitively imports pip.download, which in turn imports getpass:

Traceback (most recent call last):
  File "get-pip.py", line 20061, in <module>
    main()
  File "get-pip.py", line 194, in main
    bootstrap(tmpdir=tmpdir)
  File "get-pip.py", line 82, in bootstrap
    import pip
  File "/tmp/tmpSfzewL/pip.zip/pip/__init__.py", line 28, in <module>
  File "/tmp/tmpSfzewL/pip.zip/pip/vcs/mercurial.py", line 9, in <module>
  File "/tmp/tmpSfzewL/pip.zip/pip/download.py", line 5, in <module>
ImportError: No module named getpass

The Yocto recipe for the example image used the python-2.7.3 package and stripped non-essential modules to fit the 8 MB internal flash. getpass, which depends on the optional termios extension, was excluded. pip 1.5+ requires getpass.getpass() for interactive credential prompts and silently fails if the module is missing.

Step 1 — Restore getpass.py to the Python Standard Library

1. From your engineering workstation, open getpass.py on the Enthought CPython 2.7.3 mirror and save the file as getpass.py (UTF-8, LF line endings).
2. Transfer the file to the gateway. With SSH enabled:

   scp getpass.py [email protected]:/tmp/getpass.py

3. On the gateway console, move the file into the standard library directory used by the example image:

   mv /tmp/getpass.py /usr/lib/python2.7/getpass.py
   chmod 644 /usr/lib/python2.7/getpass.py
   chown root:root /usr/lib/python2.7/getpass.py

4. Verify the module loads:

   python -c "import getpass; print getpass.getuser()"
   root

   If root is returned, the module is correctly registered and the bootstrap path is unblocked.

Step 2 — Bootstrap pip on the IoT2020

With getpass restored, either the upstream bootstrap script or the pre-shipped easy_install will work. The easy_install route is preferred on the legacy image because it avoids the SSL/TLS version mismatch that the example image's openssl 0.9.8 has with PyPI's TLS 1.2 endpoints.

Option A — pip via easy_install (recommended on legacy image)

1. Confirm setuptools is present:

   easy_install --version
   setuptools 0.6c11

2. Install pip:

   easy_install pip

3. Verify the pip shim resolves against the standard library (not a vendored copy):

   which pip
   pip --version
   pip 1.5.6 from /usr/lib/python2.7/site-packages/pip (python 2.7)

Option B — pip via get-pip.py (Debian-style images)

On the IoT2050 or any image with full Python 3 and a current OpenSSL, the Python Packaging User Guide procedure works directly:

1. Download the bootstrap script:

   curl -sS https://bootstrap.pypa.io/pip/3.5/get-pip.py -o /tmp/get-pip.py

2. Run it as the unprivileged iot user:

   python3 /tmp/get-pip.py --user

3. Confirm:

   ~/.local/bin/pip --version
   pip 23.x from /home/iot/.local/lib/python3.5/site-packages/pip (python 3.5)

Step 3 — Install pymodbus

pymodbus is a pure-Python implementation of the Modbus protocol maintained by the pymodbus-dev project. It supports Modbus TCP (port 502), Modbus RTU over serial, and Modbus ASCII. Install the release that matches the image's Python version:

1. Install pymodbus (and pyserial for RTU):

   pip install pymodbus==1.5.2 pyserial

2. Confirm installation:

   python -c "import pymodbus; print pymodbus.__version__"
   1.5.2

3. Confirm pyserial is wired to the on-board UART:

   python -c "import serial; ser = serial.Serial('/dev/ttyS0', 9600, 8, 'E', 1); print ser.name"
   /dev/ttyS0

Step 4 — Verify the Installation

Run the four checks below on the gateway console. All four must pass before you move to application code.

1. Python and pip resolution

   python --version && pip --version
   Python 2.7.3
   pip 1.5.6 from /usr/lib/python2.7/site-packages/pip (python 2.7)

2. getpass restored

   python -c "import getpass; print getpass.getuser()"
   root

3. pymodbus import

   python -c "from pymodbus.client.sync import ModbusTcpClient; print 'tcp ok'"
   tcp ok

4. Network reachability to PLC

   ping -c 2 192.168.200.10

   Replace the address with the S7-1200 PROFINET interface IP. The default TIA Portal project assigns 192.168.0.1; on the Siemens reference LoRaWAN project that ships with the example image, the PLC is at 192.168.200.10.

Using pymodbus — Modbus TCP Client Example

The script below polls four holding registers (address 0–3) from a Siemens S7-1200 configured as a Modbus TCP server. The PLC must have the MB_SERVER instruction (S7-1200 firmware ≥ V4.0) or the Modbus TCP library compiled into the user program. Save the file as /root/modbus_tcp_poll.py and run it with python modbus_tcp_poll.py.

#!/usr/bin/env python
# Modbus TCP poll against a Siemens S7-1200 / S7-1500
from pymodbus.client.sync import ModbusTcpClient
from pymodbus.exceptions import ConnectionException
import time

PLC_IP   = "192.168.200.10"
PLC_PORT = 502
UNIT_ID  = 1   # MB_UNIT for S7-1200, default 1

client = ModbusTcpClient(PLC_IP, port=PLC_PORT, timeout=3)
if not client.connect():
    raise SystemExit("Cannot reach PLC at %s:%d" % (PLC_IP, PLC_PORT))

try:
    while True:
        rr = client.read_holding_registers(address=0, count=4, unit=UNIT_ID)
        if rr.isError():
            print "Modbus error:", rr
        else:
            print "Holding [0..3] =", rr.registers
        time.sleep(1.0)
finally:
    client.close()

Expected output on a healthy link:

Holding [0..3] = [0, 0, 0, 0]
Holding [0..3] = [10, 20, 30, 40]

Using pymodbus — Modbus RTU over /dev/ttyS0

For Modbus RTU on the Arduino shield's RS485 transceiver, use the synchronous serial client. The shield's half-duplex direction control is on D3; pymodbus drives it automatically when serial.Serial opens /dev/ttyS0.

#!/usr/bin/env python
# Modbus RTU master over IoT2020 Arduino shield RS485
from pymodbus.client.sync import ModbusSerialClient

client = ModbusSerialClient(
    method='rtu',
    port='/dev/ttyS0',
    baudrate=19200,
    bytesize=8,
    parity='E',
    stopbits=1,
    timeout=1
)

if not client.connect():
    raise SystemExit("Cannot open /dev/ttyS0")

rr = client.read_input_registers(address=0, count=8, unit=1)
if rr.isError():
    print "RTU error:", rr
else:
    print "Input [0..7] =", rr.registers

client.close()

Connecting to a Siemens S7-1200 / S7-1500

The S7-1200 became a Modbus TCP server natively starting with firmware V4.0 (2014). The configuration required in TIA Portal:

1. Add the MODBUS TCP instruction library to the user program (Instructions → Communication → MODBUS TCP). The relevant blocks are MB_SERVER for TCP and the MB_CLIENT for outbound polling — but the IoT2020 acts as the client, so only the server side must be configured.
2. Set the CONNECT parameter to the PLC's IP and port 502.
3. Bind the data block MB_HOLD_REG to a DB of at least 4 words; pymodbus reads Holding[0..3] in the example above.
4. Set the access point MB_UNIT parameter to 1 (default).
5. Compile and download. The PLC's PROFINET interface accepts inbound TCP/502 connections with no firewall rule on the CPU; if a CP is used (e.g., CP 1243-7 LTE), verify the security settings in the CP's WebUI allow inbound TCP/502 from the IoT2020 IP.

Troubleshooting Matrix

Operational Tips and Best Practices

• Pin the package version. The legacy image's pip 1.5 will resolve the latest pymodbus, which dropped Python 2 support after 1.5.2. Pin with pip install pymodbus==1.5.2 on IoT2020/2040.
• Use a virtualenv if space allows. easy_install virtualenv && virtualenv /opt/venv isolates third-party packages from the read-only /usr.
• Snapshot the SD card. After a clean install of pip, pymodbus, pyserial, and your application, dd if=/dev/mmcblk0 of=/mnt/usb/iot2020-good.img to capture a known-good image.
• Migrate to the IoT2050 for new projects. Debian-based images, Python 3, and a maintained pip 23+ eliminate the entire getpass workaround and give you pymodbus 3.5+ with async support.