Network Device Reachability and Ping Limitations in SIMATIC S7-400 and CP 443-1

In SIMATIC S7-400 controllers and CP 443-1 communication processors, you cannot execute an ICMP 'Ping' directly from the PLC user program (LAD/FBD/CFC) to test an unconfigured network device. The CP 443-1 and CPU PROFINET firmware respond to incoming pings from external hosts, but their onboard TCP/IP protocol stacks do not expose an outgoing ICMP Echo Request system function to the user program. To monitor device reachability from the controller, configure a TCP connection in NetPro and evaluate connection status via SFC51 (RDSYSST) or AG_CNTRL (FC10), or execute pings from the WinCC OS Server via VBScript.

Architecture of ICMP and Protocol Stacks in S7-400 and CP 443-1

In industrial automation networks, the Internet Control Message Protocol (ICMP) is widely used by network administrators via the command-line utility ping to test IP connectivity and round-trip latency.

In SIMATIC S7-400 systems utilizing Industrial Ethernet communication processors (such as CP 443-1 6GK7443-1EX30-0XE0 or CPU 410-5H onboard interfaces 6ES7410-5HX08-0AB0):
* The communication processor firmware incorporates an embedded real-time TCP/IP stack.
* The module responds passively to ICMP Echo Requests (ping <CP_IP>) originating from engineering stations, SCADA servers, or network switches.
* However, the controller execution interface exposes only transport-layer application blocks: ISO-on-TCP (RFC 1006), TCP native, UDP, and S7 Communication (BSEND/BRECV, PUT/GET). No system block exists to generate raw ICMP echo requests from user code.

Why Outgoing ICMP Ping is Unsupported from S7 PLC Programs

Industrial controller operating systems deliberately omit outgoing ICMP socket APIs for deterministic execution and cybersecurity:
* Non-Deterministic Bus Latency: ICMP pings rely on variable network routing and response timeouts (typically 1 to 4 seconds). Blocking PLC cyclic interrupt OBs (e.g., OB1 or OB35) while waiting for an ICMP response would cause cycle time overruns and trigger CPU watchdog trips.
* Deterministic Channel Architecture: S7 communication is architected around pre-configured static connection tables compiled in NetPro. Arbitrary ad-hoc socket creation to unconfigured IP addresses violates the determinism required for process automation.
* Security Hardening: Preventing PLCs from generating arbitrary ICMP packets protects industrial subnets from being utilized in network scanning or ping-flood Denial of Service (DoS) attacks.

Alternative PLC Methods: TCP Sockets, AG_CNTRL, and SFC51

If an automation engineer must determine whether a third-party device (such as an analyzer, barcode scanner, or weigh scale) is reachable on the network without configuring a full industrial protocol:

Method / InstructionConfiguration RequirementOperating MechanismReturn Status in S7 ProgramSFC51 (RDSYSST)Configured connection in NetProQueries CPU System Status List (SSL ID 16#0232 or 16#0B91).Returns binary status of configured network connections.AG_CNTRL (FC10)Configured connection in NetProQueries connection status or resets CP connection handles.STATUS word reports connection state (Connected/Disconnected).Open User Communication (TCON)S7-400 PN interface / Open IEDynamically initiates a TCP socket handshake to target IP/port.If target device is unreachable, TCON returns timeout 16#80C4.Modbus TCP Client BlockIP and port configured in DBPolls holding registers cyclically; detects communication timeout.Error output flags communication failure within configured timeout.

Table 1: Alternative methods for testing device reachability in S7-400 programs.

Supervisory Network Monitoring via WinCC OS Scripting

When unconfigured third-party devices must be pinged periodically without consuming PLC communication resources, execute the ping at the Operator Station level:

Implement VBScript in WinCC:
* Create a cyclic VBScript action in WinCC (running every 10 to 30 seconds).
* Use Windows Management Instrumentation (WMI) to issue an asynchronous ping:
vbscript Dim objPing, objStatus Set objPing = GetObject("winmgmts:{impersonationLevel=impersonate}")._ ExecQuery("select * from Win32_PingStatus where address = '192.168.1.50'") For Each objStatus in objPing If objStatus.StatusCode = 0 Then HMIRuntime.Tags("Device_Online").Write 1 Else HMIRuntime.Tags("Device_Online").Write 0 End If Next
Transfer Status to Controller:
* Map the WinCC internal tag Device_Online to an S7-400 controller DB tag via standard Named Connections.
* The PLC receives a clean binary flag indicating network reachability without issuing complex low-level network instructions.

Verification

To confirm device reachability monitoring:

Deploy the configured communication method (e.g., TCON or WinCC VBScript).
With the target device connected, verify that the reachability tag reports 1 (Online).
Disconnect the physical Ethernet cable from the target device:
* Observe that the reachability tag transitions to 0 (Offline) within the configured timeout period.
Reconnect the Ethernet cable: verify that the status restores cleanly to 1 without requiring controller restarts or memory resets.

Limitations and Operational Precautions

Do not execute rapid cyclic pings (e.g., < 1 second) via WinCC VBScript, as WMI queries consume noticeable CPU resources on the OS Server.
Ensure firewall rules between the terminal bus, plant bus, and third-party devices permit ICMP Echo packets if utilizing OS-level pinging.
Industrial Ethernet CPs (6GK7443-1EX30-0XE0) have strict limits on simultaneous active TCP connections (typically 32 to 64 connections); do not exhaust connection resources solely for presence monitoring.

Engineer FAQ

Can an S7-1500 controller ping a network device?

No. Even modern S7-1500 controllers do not provide an outgoing ICMP ping instruction in user logic; they rely on TCON TCP connection handshakes or SNMP network management blocks (SNMP_PAR).

Can SNMP be used to monitor device reachability in PCS 7?

Yes. Siemens provides the PCS 7 Maintenance Station and SIMATIC NET SNMP OPC Server, which monitor device reachability, link states, and port health across managed network switches and third-party devices.

Why does CP 443-1 respond to ping from my PC if the PLC cannot ping?

Because responding to an incoming ICMP ping is handled directly by the CP's onboard network ASIC and firmware stack without involving the S7 CPU operating system.