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IEC 60870-5-104 Parameter Checker (k, w, t0–t3)

Check the link parameters of an IEC 104 connection between a gateway or RTU and the control centre: the window sizes k and w and the timers t0, t1, t2 and t3, against the rules of IEC 60870-5-104 — and, with the round-trip time of the link, whether t1 leaves enough time on slow, radio or satellite links.

Window

Timers

s
s
s
s

Link (optional)

ms
kbit/s
bytes
Result
Parameters are consistent

Checks

  • OKw = 8 ≤ 2/3 × k = 8.0.
  • OKt2 (10 s) < t1 (15 s).
  • NoteAll parameters are at the IEC 60870-5-104 defaults (k 12, w 8, t0 30 s, t1 15 s, t2 10 s, t3 20 s).
Tool guide · know before you use

What this tool is, how it works and how far to trust it

A checker for the link parameters of an IEC 60870-5-104 connection: the window sizes k and w and the timers t0, t1, t2 and t3.

Read this before relying on a result. Every method, formula, assumption and limit is stated here, so you can check the answer yourself instead of accepting it blindly.

What it can do

  • Check the standard's rules: w ≤ k, w ≤ 2/3 k, t2 < t1, timers within 1–255 s, and t3 relative to t1.
  • With the measured round-trip time and the link bit rate, estimate the worst case before an acknowledgement arrives and compare it with t1.
  • List every value that differs from the defaults, so both ends can be compared.

Why you need it

  • IEC 104 links that drop every few minutes or stall during an event burst are often caused by parameters that do not fit each other or the link — not by the network.
  • Radio, cellular and satellite links add delays that the default t1 may not cover.

Understand this first

  • k is how many I-frames the sender may have unacknowledged; w is after how many I-frames the receiver must acknowledge.
  • t1 is the sender's patience: no acknowledgement within t1 and the connection is closed. t2 is how long the receiver may wait before acknowledging when it has nothing to send. t3 is the idle time before a test frame.
  • Both ends must use matching values; a mismatch can work in quiet periods and fail under load.

What you need before you start

  • The k, w, t0–t3 settings of the gateway or RTU and of the control centre.
  • Optionally the round-trip time (ping between the ends) and the slowest bit rate on the path.
  • A current browser (Chrome, Edge, Firefox or Safari). Nothing is installed and your data is never uploaded; it is processed on your own computer.

How to use it

  • Enter k and w, then the four timers as set on the device.
  • If you know them, enter the round-trip time, the link bit rate and a typical frame size.
  • Read the result and the checks; compare the 'different from defaults' list with the other end's settings.

What you get

  • A consistent / needs correcting verdict, the checks with explanations, and the time needed to send k frames on the link.

How to read the results

  • Errors must be fixed on both ends. Warnings are risks that usually show up as dropped connections on slow links.
  • When in doubt, return to the defaults (k 12, w 8, t0 30 s, t1 15 s, t2 10 s, t3 20 s) and change only what the link requires.

Technical basis

  • IEC 60870-5-104 parameter definitions, defaults and ranges; the recommendation w ≤ 2/3 k.
  • Worst case before acknowledgement ≈ round-trip time + time to send k frames (APDU + about 40 bytes TCP/IP each) + t2.

Limits: when not to rely on it

  • Simple timing arithmetic: it does not model TCP retransmissions, packet loss or Nagle delays.
  • Vendor limits and the control centre's policies also apply.

For preliminary engineering use. Results must be checked by a qualified engineer against the project's standards. How results are checked · Terms & disclaimer · Build 424d8a2 · 2026-11-12