Why can IEC 104 TCP connect but application data still fail?

TCP reachability only proves transport. IEC-104 also needs healthy STARTDT, I/S/U frame behavior, acknowledgements, timers, and sequence counters.

What frames should I check first?

Start with STARTDT act/con, then I-frame and S-frame acknowledgement behavior, TESTFR act/con, timer behavior, and reconnect evidence.

IEC-104 troubleshooting

TCP is connected, but IEC-104 sequence or session evidence is not healthy.

IEC-104 problems often look like networking problems, but the real issue can be STARTDT, TESTFR, I-frame/S-frame acknowledgement, sequence counter, timer, or reconnect behavior. ARIEC60870 exposes those frames directly in the evidence trail.

Download ARIEC60870 STARTDT / TESTFR basics

Normal evidence

IEC-104 requires more than an open TCP socket.

After TCP connects, the station should move into data transfer with STARTDT. I-frames carry ASDUs, S-frames acknowledge received I-frames, and TESTFR verifies the idle connection.

TXSTARTDT actClient requests data transfer

RXSTARTDT conServer confirms data transfer

TXI-frameASDU with send and receive sequence counters

RXS-frameAcknowledges received I-frames

TXTESTFR actIdle connection test

RXTESTFR conTest confirmation

What usually goes wrong

Session symptoms should be tied to frame proof.

STARTDT not confirmedTCP connects but the endpoint never enters data transfer.

S-frame missingI-frame acknowledgement is delayed, missing, or inconsistent.

Sequence driftSend/receive counters do not behave as expected across the session.

TESTFR timeoutThe idle connection test does not get a healthy confirmation.

Smart evidence

ARIEC60870 links IEC-104 frame interpretation to findings and reports.

Use Frame Trace to inspect U/I/S frames, Smart Findings to summarize likely issues, and PDF Report to package the evidence for commissioning, troubleshooting, homologation, or handover.

CA mismatch ACTCON missing PDF evidence report