In IEC 60870-5-104, a single measured value can arrive at the master for several completely different reasons: because it changed on its own, because the master asked for a full snapshot, or because it is confirming that a command was carried out. The cause of transmission field, usually shortened to COT, is the part of the message that says which of those it is. Without it the master would receive a stream of values with no idea whether each one is fresh news, a routine refresh, or a reply to something it just sent. This guide explains the COT field, the common causes it carries, and why it is central to the command handshake and to mapping points into a cloud system.
Cause of Transmission (COT) in one line: The cause of transmission (COT) is a field inside every IEC 60870-5-104 ASDU that tells the master why that data unit was sent - for example spontaneously because a value changed, cyclically as a routine refresh, as a response to a general interrogation, or as part of a command sequence (activation, activation confirmation, activation termination). It lets the master interpret each message correctly and drives the confirmation handshake for commands.
IEC 60870-5-104 packages data into structures called ASDUs, application service data units. Each ASDU has a header that identifies the type of data, the address of the station, and - crucially - the cause of transmission. The COT is a small numeric field, and its value maps to a defined meaning: the standard assigns codes for spontaneous transmission, cyclic transmission, background scan, responses to interrogation, and the several phases of a command. So the same measured-value type can travel with different COT codes depending on the situation that produced it.
This matters because the payload alone is ambiguous. A voltage reading of a given magnitude is just a number; the COT is what gives it context. A master reading spontaneous COT treats the value as a change worth acting on immediately. Reading interrogation-response COT, it understands the value as part of a bulk snapshot it deliberately requested and can use to initialize or refresh its whole picture. Reading a command-phase COT, it knows the message is not process data at all but feedback about a control action. The COT is effectively the grammar that tells the master how to read each sentence.
The most common causes fall into a few families. Spontaneous transmission means the outstation sent the value on its own initiative because something changed - a status flipped or an analog crossed its reporting threshold. Cyclic transmission means the value is a routine, time-driven refresh sent whether or not it changed. Interrogation-response causes mark values that the outstation is sending because the master issued a general interrogation and asked for a complete current snapshot of all points, which is how a master synchronizes its database after connecting or reconnecting.
Commands use COT to run a small state machine. When the master issues a command, it sends it with an activation cause. The outstation replies with activation confirmation to say it accepted and is acting on the request, and later with activation termination to say the action has finished. If the command is rejected, the confirmation carries a flag indicating the negative outcome. This activation, confirmation, termination sequence is how the master knows a breaker-open or setpoint-write actually took effect rather than merely being sent. Following the COT through that handshake is exactly how an engineer verifies during commissioning that controls are landing and being acknowledged.
When a gateway or cloud SCADA platform ingests IEC 60870-5-104 data, two fields work together to make sense of each value. The information object address, IOA, identifies which specific point the value belongs to - which breaker, which measurement - and the COT explains the circumstance under which that point reported. The IOA answers what, the COT answers why. A robust integration reads both: it routes the value to the right cloud tag using the IOA and interprets it correctly using the COT.
That combination shapes good behavior on the platform side. A value arriving with spontaneous COT is a genuine change and can drive an event, a trend point, or an alarm evaluation immediately. Values arriving as an interrogation response are a bulk refresh and let the platform reconcile its whole point list against the outstation's current state, which is especially valuable right after a reconnect when the platform needs to rebuild an accurate picture. Command-phase COTs let the platform present control feedback to operators - confirming that an issued command was accepted and completed rather than lost. A cloud SCADA platform such as Merobix uses the IOA-plus-COT pairing so that each incoming value lands on the correct tag with the right meaning attached, whether it is a spontaneous change, a routine refresh, an interrogation snapshot, or a control confirmation.
Spontaneous COT means the outstation sent the value on its own initiative because something changed - a status point flipped or an analog crossed its reporting threshold - rather than because the master asked for it. A master treats spontaneous values as fresh news worth acting on immediately, driving events, trends, and alarm checks. It is the opposite of cyclic transmission, which is a routine time-driven refresh.
Commands use COT to run a small handshake. The master sends the command with an activation cause, the outstation replies with activation confirmation to say it accepted and is acting, and later sends activation termination to say the action finished. A rejected command comes back with a negative flag. Following that sequence is how the master confirms a control action actually took effect.
The information object address (IOA) identifies which specific point a value belongs to - the what - while the cause of transmission (COT) explains why that point reported - spontaneous change, cyclic refresh, interrogation response, or command phase. A good integration reads both: the IOA routes the value to the correct cloud tag and the COT tells the system how to interpret it.
Primary references from the standards bodies and regulators that define this topic:
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