IEC 60870-5 Cause of Transmission Table

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Reasons, Not Just Data

IEC 60870-5 separates what the data is (the ASDU type) from why it was sent (the cause of transmission). The COT is a small field, but it changes how the master handles an ASDU. A measured value arriving spontaneously means the outstation reported a change on its own initiative. The same measured value arriving with an interrogation cause means it is part of the response to a general poll the master requested. The same value with a cyclic cause means it is a periodic background update. Same data, three different reasons, three different meanings to the receiving application.

The command lifecycle is where COT earns its keep. When a master sends a command, the exchange runs through activation, activation confirmation, and activation termination causes, plus a return-information cause when the point's state changes as a result. Following those causes is how the master knows a command was accepted, executed, and completed - or rejected. A negative confirmation flag on the COT signals the outstation refused the command.

Reading the Cause Table in Practice

In troubleshooting, the COT is often the fastest clue. Data arriving only under interrogation but never spontaneously points at event reporting not being configured; a command that gets activation but never a confirmation points at a rejection or a lost response. That is why the field is central to troubleshooting IEC 104 with cause of transmission. The COT pairs with the ASDU type and applies identically across serial 101 and networked 104; the field itself is detailed further in the cause of transmission reference.

Inside the Field: Cause Number, Flags, and Originator

The cause of transmission is more than the cause number. The field carries two flag bits alongside the six-bit cause value: a test bit, set when an ASDU is generated for test purposes so the receiver can process it without treating it as real process data, and a P/N bit that turns a confirmation negative - the outstation's way of saying it received a request and refuses it. In the IEC 60870-5-104 profile, and in IEC 60870-5-101 configurations that use the two-octet form, a second octet carries the originator address.

The originator address matters in systems with more than one controlling station or with a dispatcher hierarchy. It identifies which master initiated a request so that responses can be routed back to the source that asked, and so a control center can distinguish returns caused by its own command from those triggered elsewhere. When it is unused it is simply zero, which is why many field integrations never notice it exists - until a second master appears and responses start needing an addressee.

The Numbers Behind the Names

Interoperability lists and analyzer traces mostly show causes as raw numbers, so the common values are worth recognizing on sight:

COT value	Name
1	Periodic / cyclic
3	Spontaneous
6 / 7	Activation / activation confirmation
20	Interrogated by station interrogation
44-47	Unknown type / cause / common address / IOA

Causes 21 through 36 extend cause 20 for the group interrogations, cause 10 closes a command exchange as activation termination, and the counter-interrogation family mirrors the same pattern for integrated totals. The point of memorizing the handful above is speed at the analyzer: a trace full of cause 20 with no cause 3 tells a story before you decode a single information object.

The Error Causes: 44 to 47

Causes 44 through 47 are the protocol's built-in diagnostics. When an outstation receives an ASDU it cannot act on, it mirrors the ASDU back with the cause replaced: 44 for an unknown type identification, 45 for an unknown cause of transmission, 46 for an unknown common address of ASDU, and 47 for an unknown information object address. Each one names the exact field the outstation objected to, which makes them the fastest commissioning feedback the standard offers.

Reading them is a checklist in itself. A 47 during command testing means the point mapping in the master does not match the outstation's database - the IOA is wrong, a common outcome when planning ASDU and IOA addressing was skipped or two documents disagree. A 44 means the outstation does not implement the type you sent, so check its interoperability list. A 46 usually means the common address is wrong for that link, and a 45 that you used a cause the device does not accept in that direction, such as sending monitor-direction causes toward the controlled station.

A Command Exchange Read Cause by Cause

Take a symbolic single command to close a breaker. The master sends the command ASDU with cause 6, activation. The outstation answers with cause 7, activation confirmation, P/N positive, meaning the command is accepted and executing. As the breaker moves, the double-point status for the switch reports with the return-information cause, telling the master the field effect has actually occurred. Finally the outstation sends cause 10, activation termination, closing the exchange.

Every gap in that sequence is a diagnosis. Cause 6 with no 7: the command never arrived or the outstation is not processing commands - look at the link layer or the select-before-operate state. A 7 with the P/N bit negative: the outstation refused, often an interlock, a wrong qualifier, or a point not configured for control. A 7 but no return information: the command executed but the status point is not mapped or not reporting spontaneously. A missing 10 alone is the mildest symptom, but it can leave a master that waits for termination hanging on an exchange that has already finished in the field.

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