CANopen Communication for PLCs: Network Interconnection for Various Industrial Devices

It is common to encounter situations in industrial settings where devices have dozens of sensors and actuators, each requiring individual wiring to the PLC. This not only takes a long time to wire but also increases the chances of errors, making troubleshooting particularly challenging later on. Today, let’s discuss AS-Interface bus technology, a fieldbus solution that can significantly simplify on-site wiring.

What is AS-i?

AS-Interface (shortened to AS-i) can be understood as a “smart power strip.” Traditionally, each sensor needs to be wired individually to the PLC. With AS-i, all devices can connect to the PLC using a single yellow flat cable, which transmits both data and power, making it very convenient.

Just like using a power strip at home to supply power to multiple appliances, the AS-i network can connect up to 62 slave devices with just one cable. Each slave can be a sensor, button, indicator light, solenoid valve, etc.

Hardware Components

1. Master Module: Installed on the PLC, responsible for managing the entire AS-i network.
2. Slave Devices: The sensors and actuators in the field.
3. Yellow Flat Cable: A specially designed two-core cable that transmits data and 24V power.
4. Power Supply: A dedicated AS-i power supply with an output voltage of 30.5V.

Communication Principle

AS-i uses a master-slave communication method, making data transmission very simple:

• Each slave has 4 input bits and 4 output bits.
• The master polls each slave, with a cycle time as fast as 5ms.
• It uses a special encoding method that is highly resistant to interference.

For example, the master is like a homeroom teacher taking attendance, asking each student, “How are things on your end?” The students (slaves) respond, “Here is my status.”

Wiring Method

The biggest feature is the use of piercing technology:

1. Press the slave module onto the flat cable.
2. The internal metal spikes pierce the cable’s outer skin.
3. Automatically make contact with the internal wires.
4. No need to strip wires or use terminal blocks.

This is similar to using a stapler to bind paper; just a press completes the wiring, making it very convenient.

Fault Diagnosis

The AS-i system provides excellent fault diagnosis capabilities:

• LED indicators show communication status.
• Can detect duplicate slave addresses.
• Can identify cable short circuits or disconnections.
• The master can display which specific slave has a fault.

Usage Precautions

1. Do not use a regular power supply: Must use a dedicated AS-i power supply, or the equipment may be damaged.
2. The maximum cable length is 100 meters; if exceeded, a repeater must be added.
3. Slave addresses must not be duplicated.
4. Avoid placing cables near high-power devices.

Practical Application Case

In a packaging production line:

• 20 photoelectric sensors detect products.
• 15 cylinders control mechanisms.
• 10 indicator lights.
• 5 emergency stop buttons.

Using traditional methods would require at least 100 wires, but with AS-i, only:

1. One yellow flat cable runs through the entire line.
2. 50 slaves can handle all devices.
3. Wiring time is reduced from 2 days to 2 hours.
4. Subsequent maintenance becomes much simpler.

Practice Recommendations:

1. Purchase an AS-i experimental kit, including a master station and 3-4 slave devices.
2. Practice setting slave addresses and configuring communication parameters.
3. Write simple control programs to achieve interlocking between devices.
4. Simulate common faults and practice the fault diagnosis process.