Commands in IEC 101 and 104: Direct or Select-Execute

Short answer

IEC 60870-5-101 and 104 send commands with four families of types: single command, double command, regulating step command and setpoint. Each can run in direct mode, with a single order, or in two steps, select and execute. The device confirms that it received the order and, later, that it finished executing it; either reply can be negative.

What a command is

In IEC 60870-5-101 and 104, a command is a control-direction ASDU that asks the controlled station to perform an action: open a breaker, change a transformer tap, set a setpoint. Commands are more delicate than monitoring data, because they act on the process. That is why the standard gives them their own confirmation and control procedure.

Command types

TI	Name	What it is for
45	C_SC_NA_1, single command	A two-state value: on or off, activate or deactivate
46	C_DC_NA_1, double command	Two bits expressing open or close; avoids an unintended command with the all-zeros combination
47	C_RC_NA_1, regulating step command	Raise or lower one position, such as transformer taps
48	C_SE_NA_1, setpoint, normalized value	A reference value as a fraction of a full scale
49	C_SE_NB_1, setpoint, scaled value	An integer value with an agreed scale factor
50	C_SE_NC_1, setpoint, short floating point	A real value, for example a power or a voltage
51	C_BO_NA_1, 32-bit bitstring	Several bits as a single order

The 104 adds the same orders with a CP56Time2a time tag (types 58 to 64) and a test command with time (107). The device accepts them only if the time of the order and its own differ by less than a configured window. The 101 does not recommend those types.

The double command is the one used on breakers and disconnectors. With two bits, the device tells «open» from «close» and discards meaningless values. A single bit cannot do that.

Command qualifier

Each command carries a qualifier that describes how to execute it. The standard lists four options, each of which a device declares whether it supports:

No additional definition.
Short pulse: the output is held for a short time, set in the device.
Long pulse: the same, with a long time.
Persistent output: the output is held until a new order.

The pulse duration is not defined by the order: it is defined by a device parameter. For the same command, one relay may close the contact for 100 ms and another for 2 s, depending on its settings.

The qualifier also indicates whether the order is a select or an execute.

Direct mode and select-and-execute mode

The standard’s interoperability list separates two procedures, for both commands and setpoints.

Direct. The center sends a single order. The device executes it at once if it can.

Select and execute. Two consecutive orders:

Command with select and execute

The center sends the select. Command activation with the select bit set
The device confirms the select. Activation confirmation; the device checks that it is ready
The center sends the execute. Activation of the same command with the select bit cleared
The device confirms the execute. Activation confirmation, positive or negative
The device terminates the action. Activation termination when the operation ends

The select moves nothing. It only reserves the device and checks that it is in a condition to operate. If the execute does not arrive within the maximum time set by the project, the selection is cancelled. This procedure reduces the risk that a wrongly sent order, or one sent to the wrong device, causes an operation. It is the equivalent in the 101 and 104 of the mechanism IEC 61850 calls select before operate.

Confirmation and termination

The standard requires the device to answer each command in two stages:

Activation confirmation (cause 7). The device received the order and accepted it (positive) or rejected it (negative).
Activation termination (cause 10). The action has ended. It is positive if it completed and negative if it failed.

There are exceptions: select orders, the test command and reset process are only confirmed, not terminated. A setpoint can declare in the interoperability list whether it uses activation termination.

The device can reject a command before trying to execute it if it does not recognize one of its fields. It answers with a negative confirmation and one of these causes:

44: unknown information type.
45: unknown cause of transmission.
46: unknown ASDU common address.
47: unknown information object address.

That cause is the first clue when diagnosing a rejected command. The other reasons (interlocks, local mode, expired select) are in Command rejected from SCADA.

Maximum execution time

The interoperability list of the 104 includes maximum delay supervision of commands and setpoints: the device checks that the order is not too old. With commands with time, the comparison is made against the device clock, so an offset clock can cause rejections. Check time synchronization before blaming the command.

Good practices

Use the double command with select and execute on every high-voltage switching device.
Agree in writing the qualifier and the pulse duration of each output.
Test the complete sequence on each command: select, confirmation, execute, termination and state return.
Also test the negative cases: an order to an address that does not exist, execute without select and execute after the time has expired.
Verify that the final state reaches the center as return information caused by a remote command (cause 11) and not only as a spontaneous change.

We can help you test your commands

At Tiza Group we do the end-to-end tests of commands from SCADA to the switchyard device, and the SCADA and gateway integration that makes them possible. Contact us for a quote.

References

IEC 60870-5-101: command procedures; interoperability list (direct and select-and-execute command transmission, pulse qualifiers, activation confirmation and termination).
IEC 60870-5-104: command ASDUs with time tag (TI 58 to 64) and maximum command delay supervision.

Frequently asked questions

When should I use select-before-operate instead of direct command?

For equipment where a wrong operation has serious consequences, such as breakers and disconnectors. The selection checks that the device is ready and is the right one before executing. Direct is faster and is kept for low-risk operations, such as resetting a signal or adjusting a setpoint.

What happens if the execute does not arrive after the select?

The device cancels the selection when a maximum time expires and the order is not executed. That time limit is defined by the project. If an execute order arrives after it has expired, the device rejects it.

What is the difference between confirmation and termination?

The activation confirmation says the device received the order and accepted or rejected it. The termination says the action has completed or failed. An order can be confirmed positively and end badly, for example if the breaker does not close.