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Function Codes |
A function code identifies the purpose of a message. There are two groups of functions, one for requests and another one for responses. There are several types of functions for requests, as shown on the next table. Response functions are used internally and not open to users.
Function Codes
Code |
Function |
Description |
|---|---|---|
1 |
Read |
Requests objects specified by a remote station and responds with the requested objects available |
2 |
Write |
Stores the objects specified by a remote station and responds with an operation status |
3 |
Select |
Selects or operates output points but does not configure or produce any action (controls, SetPoints, or analog outputs) and responds with an operation status. The Operate function must be used to activate these outputs. |
4 |
Operate |
Configures or produces actions on outputs or points previously selected with the Select function |
5 |
Direct Operate |
Selects and operates the specified outputs and responds with the status of control points |
6 |
Direct Operate NO ACK |
Selects and operates the specified outputs but does not send a response, but takes less time |
7 |
Immediate Freeze |
Copies the specified objects to a freezing buffer |
9 |
Freeze and Clear |
Copies the specified objects to a freezing buffer and then zeroes these objects |
10 |
Freeze and Clear No ACK |
Copies the specified objects to a freezing buffer and then zeroes these objects, but does not send a confirmation response |
11 |
Freeze with Time |
Copies the selected objects to a freezing buffer at the specified moment, responding with an operation status |
13 |
Cold Restart |
Performs a reset sequence |
14 |
Warm Restart |
Performs a partial reset sequence |
20 |
Enable Unsolicited Messages |
Enables a spontaneous notification for the specified objects |
21 |
Disable Unsolicited Messages |
Disables a spontaneous notification for the specified objects |
22 |
Assign Classes |
Links the specified objects to a class |
23 |
Delay Measurement |
Allows an application to calculate a delay, or propagation time, to a specific station |
After specifying the purpose of a message (APCI), there is a second part of that message, if needed, called ASDU. Every ASDU consists of one or more data identifiers (DUI), object headers, object information, or data fields.