In IEC 60870-5-101 the link layer can work in unbalanced mode, where only the master initiates and each station answers when polled, or in balanced mode, where both stations may initiate. Unbalanced mode supports a shared bus with several stations; balanced mode is used on point-to-point links. Both ends must use the same mode.
Two ways to organize the conversation
The link layer of IEC 60870-5-101 has two transmission procedures. The standard calls them unbalanced and balanced. Both move the same application-layer ASDU, but they change who has the right to start an exchange. That decision determines the link topology, the response time to an event and the way faults are diagnosed.
The stations are called primary, the one that starts a transfer, and secondary, the one that responds. In the standard’s terminology the controlling station is the master and the controlled one is the slave, although in balanced mode the roles are not fixed.
| Feature | Unbalanced | Balanced |
|---|---|---|
| Who initiates | Only the master | Either station |
| What the slave does | Answers when polled | Initiates and answers |
| Spontaneous events | Wait for the next poll | Sent as soon as they occur |
| Shared bus (multipoint) | Yes | No |
| Link address | Required: identifies each slave | May be omitted on point-to-point |
Unbalanced mode: the master polls
The master is the primary station. It polls each secondary station in turn, and each one answers only when polled. No station may transmit on its own initiative.
Because only the master talks, several slaves can share the same wire pair or the same RS-485 bus. The master identifies each one with the link address. The standard lets this address take one or two octets and leaves it as a system parameter.
Data classes. The data a slave must send is split into two queues:
- Class 1: high-priority data, such as state changes and events with a time tag.
- Class 2: low-priority data, such as cyclic measurements.
The master polls for class 1 or class 2, and the slave replies with what it has in that queue. The standard assignment puts measured values (types 9, 11, 13 and 21) in class 2 when their cause of transmission is cyclic. If the interoperability list defines another assignment, both ends must know it. When the slave has no data of the requested class, it may answer with data of the other class.
The drawback is the wait. An event that occurs just after the poll has to wait for the next turn. With more stations on the bus or low baud rates, that wait grows.
Balanced mode: both stations initiate
In balanced mode each station can act as primary and as secondary. The controlled station sends its events at once, without waiting for a poll. The master sends its commands when it needs to.
For this to work, the link is initialized in both directions: each station requests the status of the other’s link and resets its own link. Until the link is initialized in both directions, the data exchange does not begin in the normal way.
The standard restricts it to configurations with no contention for the medium: point-to-point and multiple point-to-point. It is not used on a shared bus, because two stations could start at the same time and their frames would be mixed.
- One master and several stations on a bus. Multipoint: unbalanced mode only
- One master and one station per dedicated link. Point-to-point: unbalanced or balanced
- Several stations, each with its own link. Multiple point-to-point: unbalanced or balanced
- Immediate event reporting is needed. Choose balanced on a dedicated link
How the interoperability list declares it
The standard requires an interoperability list, a table where each manufacturer ticks the options its device supports. It declares:
- The network configuration: point-to-point, multipoint party line, multipoint-star or multiple point-to-point.
- The link transmission procedure: balanced, unbalanced or both.
- The link address: no address (balanced only), one octet or two.
- The transmission speed, per interface type.
If the devices come from different manufacturers, the parties must agree on the same parameters. Two devices that support both modes only understand each other if both are set to the same one.
How to choose in a project
The choice depends on three factors. First, the topology: if several stations share the medium, only unbalanced mode is possible. Second, the urgency of events: a dedicated balanced link delivers the change at once, while polling adds the wait for the turn. Third, what the devices support: both ends must declare the same procedure in their interoperability list.
If the project will migrate to IEC 60870-5-104 later, the chosen mode does not constrain the migration: the application layer is kept and the serial link is replaced by TCP/IP.
Practical effects in the field
- An RTU that «does not respond». On a shared bus, first check that the master is in unbalanced mode and that each slave has a different link address. Two stations with the same address answer at the same time and corrupt the replies.
- A balanced link that does not start. Check that both ends are in balanced mode and that link initialization completes in both directions. One end waiting to be polled and another waiting to initiate stay silent.
- Delayed events. In unbalanced mode the delay depends on the polling cycle. Reducing it means fewer stations per bus, a higher baud rate or moving to a dedicated balanced link.
The protocol overview and the migration to 104 are in IEC 60870-5-101: serial telecontrol protocol and move to 104. With the 104 the link layer no longer exists and balanced or unbalanced mode is no longer chosen: TCP connections replace it.
We can help you define the topology
At Tiza Group we do the SCADA and gateway integration of serial links and their migration to IP. We define the link mode, the addresses and the signal table, and test the whole against the control center. Contact us for a quote.
Related guides
- IEC 60870-5-101: serial telecontrol protocol and move to 104
- IEC 101 serial link errors: parity and checksum
- IEC 60870-5-101: Layers, Frame and ASDU Explained
- Addresses in IEC 101 and 104: link, ASDU and object
- Redundant gateway and dual link to the control center
References
- IEC 60870-5-101: companion standard for basic telecontrol tasks; link transmission procedures and interoperability list (network configuration, link layer).
- IEC 60870-5-2: link transmission services.
Frequently asked questions
Can I connect several RTUs on the same RS-485 bus with IEC 101?
Yes, but only in unbalanced mode. A shared (multipoint) bus needs a single station, the master, to decide who talks at any moment. In balanced mode every station may initiate, and on a shared bus the transmissions would collide.
What happens if one end is balanced and the other unbalanced?
The link does not come up. The two ends follow different link procedures: one waits to be polled and the other waits to initiate. That is why the mode is declared in the interoperability list and must match on both devices.
What is the advantage of balanced mode?
The controlled station can report a change at once, without waiting for the master's next poll. In unbalanced mode the report waits its turn in the polling cycle, which depends on the number of stations and the link speed.