#41 - Introduction to AS-i modules

The Actuator Sensor Interface (ASi) protocol has existed since 1999 and is still widely used in industrial environments, logistics centers and others. Some of the main advantages are the relatively low cost of the systems and the facility to replace devices.

In this post we will make a brief introduction to this protocol with some practical examples. The  communication is the serial type, between an AS-i Master module and AS-i Slaves. Using a Siemens ET200SP CPU a possible hardware configuration is shown below. In this case, the AS-i modules are part of the CM (Communication Module) series product group.

In the field, the AS-i slave standard and safety modules (normally identified with yellow color) are installed. The connectors are usually M12. The addressing is done with an external device that is inserted in the ADDR port. The address of a standard module is divided into two (A and B) and the safety module has a single number. For example the standard module could have address # 14A (or # 14B) and the safety module # 12. When replacing a defective module, AS-i Master recognises the new module and automatically reconfigures it.

The two cables responsible for exchanging data between the AS-i modules are inside the yellow protection / insulation below, one for AS-i + positive control signals and one for AS-i- negative signals . In addition, these two cables inside the yellow protection also make the power supply of the slave modules and IOs in the field, normally the positive power is + 30VDC or + 24VDC and the negative 0V. The two cables inside the black protection, make an auxiliary power supply to the actuators. The teeth behind the modules ensure the connection between the modules and the cables.

Typical applications are digital or analog inputs and outputs and safety inputs as emergency pushbuttons. The cable distance above can be up to 100m for 30VDC systems, and 50m for 24VDC systems. There are repeaters that can be added for greater distances. In addition, good practices should be applied for the installation of AS-i network cables in the field, such as respecting the distances between them and the factory power cables, avoiding cable loops, installing terminations correctly and module grounding and trays in the field. These points are particularly critical for safety signs.

In the first image of the post, the AS-i Master is connected to the same "backplane" as the PLC. However, there are also options for AS-i Master modules that are Gateway and communicate with the networked PLC as Profinet or Ethernet-IP.