Resolving S7-1500 TCON Socket Not Opening: Status 7002 Fix

Resolving Siemens S7-1500 TCON Socket Not Opening: Status 7002 BUSY Diagnostic and Fix

Symptom: TCON on a Siemens SIMATIC S7-1500 (CPU 1515-2 PN or comparable) never opens the TCP socket. The block output shows BUSY = TRUE with no ERROR bit, and the STATUS word returns hex 7002. External tools such as Sockettest, netcat, or a custom PC server cannot reach the PLC, and the partner application reports that the CPU is unreachable on the configured port.

Root cause: The connection establishment direction in the configured connection is set opposite to the role the PLC must play. Status 7002 is the normal "waiting for the active partner to call" state, not a fault. The block is behaving correctly; the configuration is wrong.

1. TCON Block Fundamentals on the S7-1500

TCON ("Open communication connection") belongs to the family of Open User Communication (OUC) instructions available in the S7-1200 and S7-1500 instruction set. TIA Portal groups it with TDISCON, TSEND, TRCV, TUSEND, and TURCV under Communications > Open User Communication. TCON allocates the local socket resources, resolves the connection description (ID, type, partner address, port), and either dials out (active) or listens (passive) depending on the connection establishment direction stored in the connection data block.

On the S7-1500, TCON supports four protocol variants selected by connection_type in the connection DB:

Each TCON instance is tied to a single connection description stored in a global data block of type TCON_Param (or the system-generated TCON_Configured_Connection_DB when configured via the wizard). The PLC must have an active PROFINET interface with a valid IP address on the same subnet or routable to the partner.

2. Active vs Passive Connection Establishment

The most misunderstood field on a TCON connection is "Connection establishment" in the connection properties. It controls who initiates the TCP three-way handshake:

• Active (local initiates): The PLC performs connect(). The CPU is the TCP client. The block transitions out of 7002 toward 7000 (idle/established) as soon as the partner accepts the SYN.
• Passive (partner initiates): The PLC performs bind() + listen(). The CPU is the TCP server. The block stays in 7002 until the remote client sends a SYN to the configured port. The block never calls connect(); it only accepts incoming SYNs.

In the client/server model, the client is always the active endpoint. The server is always the passive endpoint. This is the rule the Siemens TIA Portal wizard enforces implicitly through the connection establishment field. Setting the CPU as passive while expecting it to be the "client" guarantees that 7002 will persist indefinitely.

The directional rule is universal: the side that calls connect() is active; the side that calls accept() (after listen()) is passive. Both sides cannot be active, and both sides cannot be passive. A common operator error is to declare both ends "passive" because the term sounds like "the safer choice"; in TCP semantics, passive means it accepts, not that it polls.

3. Decoding Status 7002 on TCON

On S7-1500 firmware V2.0 and later (TIA Portal V13 SP1 and newer), TCON returns a 16-bit status code in the STATUS output and updates the STATE field of the connection DB. Status 7002 (hex) is the standard "connection establishment in progress" state. The mapping is documented in the TIA Portal help for the TCON instruction and in the system diagnostics buffer reference.

If the STATUS field reads 7002 indefinitely with ERROR = FALSE, the TCON state machine is parked in STATE = 2. There is nothing to "fix" inside the block. The fix is in the project configuration or in the partner application.

4. Root Cause Analysis: Why the Socket Will Not Open

Counting from the most common to the least common, the following causes account for nearly every TCON 7002 case on the S7-1500:

1. Direction mismatch (most common): PLC set as passive while the partner PC server is also passive, or PLC set as active while no PC is listening. Status 7002 persists because no one is calling.
2. Partner is not in LISTEN: Confirm the PC server is in listen() state with netstat -an | findstr :2000 on Windows or ss -tlnp | grep 2000 on Linux before calling TCON.
3. Firewall on the path: Per the official TIA Portal documentation for TCON/TDISCON/TSEND/TRCV, the instruction cannot establish the connection when firewalls on the connection path do not permit the required ports. Siemens TIA Portal: TCON/TDISCON/TSEND/TRCV TCP communication instructions.
4. Wrong port or partner address: Partner IP/port in the connection DB does not match the listening PC. Wireshark on the PLC port shows SYN retransmits with no response when the partner is unreachable.
5. Already-established connection reuse (error 80A3): The same connection ID is still bound from a prior session that was not closed with TDISCON. The CPU refuses to re-establish, returning 80A3 ("Attempt being made to re-establish an existing connection"). The PLC must run "Power off $L" or the program must call TDISCON before re-arming TCON.

5. Step-by-Step Resolution Procedure

Apply the following procedure in order. Each step has a binary pass/fail check you can perform online without leaving TIA Portal.

5.1 Prerequisites

• TIA Portal V15.1 or later recommended; the configuration screens for TCON are functionally identical from V15 onward, and V16/V17/SP2 are equally valid.
• S7-1500 CPU with PROFINET interface, e.g. CPU 1515-2 PN (article number 6ES7515-2AM02-0AB0) or any current S7-1500 variant running the firmware release bundled with the project.
• Online connection to the PLC (Ethernet, accessible from the programming PG).
• Watch table with the connection DB tags STATE, STATE_BYTE, STATUS, plus the TCON instance's BUSY, ERROR, STATUS, and CONNECT.

5.2 Verify the Active/Passive Direction

1. In the project tree, expand Devices & networks and open the Connections node.
2. Select the failing TCON connection. The properties dialog shows the General tab with the partners and addresses.
3. Click the Connection establishment field. Confirm which endpoint is set as Active and which is Passive.
4. Cross-check the role expected by the partner application:
   • If the PC is a server (listening), the PLC must be the Active endpoint that calls connect().
   • If the PC is a client (initiates), the PLC must be the Passive endpoint in listen().
5. If the setting is reversed, change the active endpoint in the dropdown and recompile + download only the connection data. A full hardware download is not required for connection DB changes.

5.3 Confirm the Partner Is Reachable

1. From the PG/PC connected to the same subnet, ping the PLC IP and ping the partner IP.
2. From the PG/PC, run telnet <plc_ip> <port>. A "Connecting to ..." message that resolves to a blank screen means the PLC is in listen() (passive). A "Could not open connection" message means the PLC is not listening at all, which is correct for active mode.
3. From the partner PC, run netstat -an (Windows) or ss -tln (Linux) and confirm the partner is in LISTEN state on the configured port.

5.4 Inspect the Online Status and Force a Fresh TCON

1. Open the watch table and force the TCON instance's REQ to FALSE, then back to TRUE. The instance must be triggered with a rising edge.
2. Monitor the instance's STATUS:
   • 7000 with CONNECT = TRUE: socket is open. The bug is in the partner side.
   • 7001 / 7002: state machine in progress; wait a few seconds.
   • 80A3: connection ID already in use. Run a one-shot "Reset connection" routine that calls TDISCON, waits one OB1 cycle, then TCON.
   • 80A1, 80A2, 80A4, 80A7, 80AA, 80B0, 80B1, 80C3, 80C4: classify with the table in section 8.
3. If 80A3 persists across power cycles, issue "Power off $L" in the online menu (CPU STOP/RUN does not clear TCON state on every firmware; only a real power cycle or STOP with memory reset clears the internal connection descriptor on older builds).

6. Configuration Parameters Reference

The connection DB (type TCON_Param) contains the fields that determine the runtime behavior. The wizard pre-fills most of them. The fields that must match the partner exactly are listed below.

For TCP connections, the remote port is stored in rem_tsap_id[0..1] as a big-endian 16-bit unsigned integer. Port 2000 is stored as bytes 0x07, 0xD0. Writing 0xD0, 0x07 makes the CPU dial port 0xD007 = 53255 instead of 2000 and produces 7002 followed by 80A4 or 80C3.

7. Firewall and Port Requirements

Per the official Siemens TIA Portal documentation for the TCON/TDISCON/TSEND/TRCV instruction group, the instruction cannot establish the connection when the firewalls on the connection path are not open for the required ports. Siemens TIA Portal: TCON/TDISCON/TSEND/TRCV TCP communication instructions. The minimum set of open ports for OUC on a CPU 1515-2 PN is:

• 102/TCP for ISO-on-TCP (default; configurable in connection DB).
• User port (e.g. 2000, 2001, 2002) for plain TCP OUC; one port per concurrent TCON.
• PROFINET discovery ports (UDP) used for engineering download and topology discovery.
• 102/TCP and the S7 communication port range (typically 50000..50100/TCP) on the PLC for S7 PUT/GET used by HMI panels (separate from TCON).

On Windows partners, create an inbound rule allowing the listening port of the server. On the PLC side, the integrated firewall (available on CPU 150xS / 1518 and licensed through the Security module) must allow the local port of the active connection or any port for the passive role. On a CPU 1515-2 PN without the Security module, the perimeter firewall is the controlling element and the integrated firewall is not in the path.

8. TCON Error Code Reference

The TCON instruction surfaces the following status codes. Error codes are returned in the STATUS output; transient codes (7xxx) indicate progress, permanent codes (8xxx) indicate faults.

9. Verification Procedure

Once the configuration is corrected, verify the socket is actually open using both PLC-side and PC-side methods.

1. Online in TIA Portal, open the watch table on the connection DB and confirm:
   • STATE = 3 (connected)
   • TCON instance CONNECT = TRUE
   • TCON instance STATUS = 16#7000
   • TCON instance BUSY = FALSE
   • TCON instance ERROR = FALSE
2. From the PC, run netstat -an | findstr <port> (Windows) or ss -tn 'sport = :<port>' (Linux). The connection must appear in ESTABLISHED state.
3. From the PC, send a probe payload with Sockettest / netcat. On the PLC, monitor LEN on the TRCV instance; it must rise to the payload length and NDR must pulse for one cycle.
4. Issue TSEND with a 4-byte identification string. The PC must receive the bytes in the correct order (the S7-1500 TCP is big-endian for INT/DWORD and little-endian for STRING; verify with a sentinel byte pattern such as 0xDE 0xAD 0xBE 0xEF).

If STATE = 3 is reached but the PC sees no SYN, the partner is blocking the response or a stateful firewall is dropping the return SYN-ACK. Enable Wireshark on the PLC port and confirm a full three-way handshake completes in under 1 second.

10. State Machine Diagram (Inline)

11. Field Commissioning Checklist

Use this checklist on every new TCON connection before declaring it production-ready.

1. Confirm partner IP reachable: ping from PLC engineering port returns <1 ms on the local subnet.
2. Confirm partner port in LISTEN: ss -tln | grep <port> on the partner host.
3. Confirm PLC interface is in RUN with no PROFINET diagnostics: Online & diagnostics > PROFINET interface > Port statistics.
4. Confirm connection direction matches the application: PLC active for a PC server, PLC passive for a PC client.
5. Trigger TCON with a one-shot rising edge; verify STATE = 3, STATUS = 7000, CONNECT = TRUE within 5 seconds on a local subnet.
6. Run a TSEND of 4 bytes 0xDE 0xAD 0xBE 0xEF; confirm arrival on the PC byte-for-byte.
7. Run a TRCV of the same 4 bytes echoed back; confirm LEN = 4 and NDR pulses for one cycle.
8. Run TDISCON; verify STATE = 4, STATUS = 7004, CONNECT = FALSE within 2 seconds.
9. Cycle the PLC to STOP and back to RUN; confirm the connection is re-established automatically if the program calls TCON on startup, or remains closed if gated on a handshake from the partner.
10. Document the connection ID, port, direction, and partner IP in the project handoff packet.

12. Frequently Asked Questions