AS-Interface.

Polexa, Radu ; Moraru, Sorin Aurel ; Grigorescu, Costin Marius 等

1. INTRODUCTION

The Actuator/Sensor Interface or AS-Interface, normally abbreviated to AS-i, is a connection system for the lowest process level in automation systems. The cable harnesses previously found at this level are replaced by a single electrical cable, the AS-i cable. Using the AS-i cable and the AS-i master, the simplest binary sensors and actuators can be connected to the control devices at the field level via AS-i modules.

The AS-interface is distinguished by several main characteristics:

* AS-interface is optimized for connecting binary sensors and actuators. The AS-i cable is used both for data exchange between the sensors/actuators (AS-i slaves) and the AS-i master as well as for power supply to the sensors/actuators.

* Simple and cost-effective wiring: simple installation with the "penetration" technique, high flexibility with tree-like wiring.

* Fast reaction times: the AS-i master requires a maximum of 5 ms for cyclic data exchange with up to 31 nodes.

* Nodes (AS-i slaves) on the AS-i cable can be either sensors/actuators with an integrated AS-i connector or AS-i modules to which, up to four conventional binary sensors/actuators can be connected.

* With standard AS-i modules, up to 124 actuators/sensors can be operated on the AS-i cable.

* If you use AS-i modules with the extended addressing mode, up to 186 actuators and 248 sensors can be operated with one extended master.

* Extended AS-i masters from SIMATIC NET support extremely simple access to analog sensors/actuators or modules operating in compliance with the AS-Interface slave profile.

2. SYSTEM COMPONENTS IN THE AS-I NETWORK

The system components in the AS-i network are: AS-i master; AS-i slaves, distinguished according to their design as AS-i modules or sensors/actuators with an integrated AS-i connection; AS-i cable, AS-i power supply unit, addressing unit; SCOPE for AS-Interface.

The following diagram illustrates how the described components can be interconnected. The tree structure is particularly clear (Fig. 1.).

[FIGURE 1 OMITTED]

There are two AS-i master categories: standard AS-i master and extended AS-i master. Standard AS-i master could be: PLC, PCs IBM compatible, I/O distribute dispositive. Up to 31 standard slaves or slaves with the extended addressing mode (A slaves only) can be attached to standard AS-I masters. The extended AS-i masters support 31 addresses that can be used for standard AS-i slaves with the extended addressing mode. AS-i slaves with the extended addressing mode can be connected in pairs (programmed as A or B slaves) to an extended AS-i master and can use the same address. This increases the number of addressable AS-i slaves to a maximum of 62. (Margineanu, 2005)

All the nodes that can be addressed by an AS-i master are known as AS-i slaves. There are two categories of AS-i slaves: AS-i modules--are AS-i slaves to which up to 4 conventional sensors and up to 4 conventional actuators can be connected; sensors/actuators with an integrated AS-i connection--can be connected directly to the AS-Interface.

The AS-I cable, designed as an unshielded two-wire cable, transfers signals and provides the power supply for the sensors and actuators connected using AS-I modules. Networking is not restricted to one type of cable. If necessary, appropriate modules or "T pieces" can be used to change to a simple two-wire cable.

The AS-I power supply unit supplies power for the AS-i nodes connected to the AS-i cable. For actuators with particularly high power requirements, the connection of an additional load power supply may be necessary (for example using special application modules).

3. AS-I NETWORK PROGRAM CONTROLLED

An AS-i network is fully managed by a specialized communications processor that plays the role of master. The communications processor is attached to an special controller that extension is embedded in the slot.

The communications processor is seen by the special controller like an set of registers:

* a status register, where to find information about the functionality of the processor (protected mode or in configuration) and that it is or not operational. This register is an input register for the controller;

* a command register, through which is possible to perform commands to communication processor. This register is an output register for the controller;

* more input-output memory banks which can be selected by the command register in or from which it can be possible to retrieve information for AS-i networks data.

3.1 Using CP242-8 module to access the AS-i network

This module is designed by the Siemens company in order to connect the controllers from S7-200 class to AS-i network and PROFIBUS. The place where the status registers and the control registers could be seen depends on controller which is attached to CP242-8 module. To examine how it is seen the extension module to S7-200 controllers, it is necessary to use the special memory bytes from SMB8 to SMB21. These bytes are organized in pairs: one byte is for the identification register and the other one is an error register. (Simatic Net, 2006)

The status register is seen as an input register and is located in the first byte of free entry. The control register is seen as an output register and is located in the first free byte output.

The communication processor is represented like a series of 16 memory banks for input or output, where it is possible to transfer input data or output data for slave modules and other response data or commands. Each input and output memory bank is of length equal to 8 double words. (Simatic Net, 2006)

Accessing the program data from the slave is done by reading or writing in the memory banks. To do this, it must set the PLC_RUN bit from the command register and select the 0 memory bank. Data are available to be read if the CP_READY bit is equal to 1. For each slave unit in the memory bank are 4 bits reserved. The program for S7-214 controller which is reading the inputs from a slave, which has the address equal to 2 and transfer them to the QBO outputs from S7-211 controller. The program is running a procedure which reads the corresponding memory bank and transfer it to the output, only in case CP_READY=1. (Simatic Net, 2006)

From a program that runs on a controller it can be seen commands to the CP242-8. There are a lot of commands which can be sent. The most important ones are the reading and writing of the various parameters and data, change of addresses, status reading, etc. Each command has a number of parameters. First the user must specify the number and the parameters of the command into a buffer. This buffer is located in the memory bank number 2 and each byte is described in the specific documentation. (Simatic Net, 2006)

To receive a command the PLC_RUN bit must be set in the control register from communication processor. Afterwards the answer from CP242-8 must be expected, and this will set the CP_READY bit in the status register. Then the ASI_COM bit from control register will be set, and the takeover response will be made only after the communication processor will set the AS_RESP in the status register. Depending on the type of command will find the response to the bank 2 to 5 for the output memory from the communication module.

I will exemplify one command and the program sequence which should be run for its success. The chosen command has 1DH code. This command has no parameters and returns the input data from slave devices and a diagnostic list for all the slave devices named delta list. The command structure contains a single byte to be loaded into the byte 0 of bank 2. This byte shall contain the command code: 1DH. The response takes the banks 2 and 3 of the buffer and contains in the first byte the command echo, then the status (if the byte is 0 then the command was executed without errors, otherwise returns an error code which can be found in the company documentation). Next in the first part are the slave data and then, in the second part, a delta list for all the slave devices (Fig. 2.).

[FIGURE 2 OMITTED]

The program starts with selecting the memory bank 2, for loading the command code, setting the RUN_PLC bit from the control register and the ASI_COM bit resetting. If the communication processor is ready (CP_READY=1) the command code is loading in the memory bank. The command sending (ASI_COM=1) will be achieved only on the ascending front from AP input 0. When the communication processor sends the response (ASI_RESP=1) the procedure which processes the response information is called. Processing the data in my example is simple and consists in transferring the data to the controller's outputs. Obviously, the slave devices address must be known. The main program is presented below and also the routine.

4. CONCLUSION

AS-i is a monomaster system. This means that there is only one master in the AS-i network, which controls the exchange of data. The AS-i master supervises all the AS-i slave one after another and waiting for a response. AS-I uses constant length messages and a single-speed for communications (167 kbit/sec). There are no complicated procedures required for transmission control, messages length identification or data format. This makes the master to examine all the slave AS-i devices in maximum 5 ms and update data to AS-i master and to AS-i slave devices. Into an AS-i network is possible to connect maximum 31 standard AS-i slave devices.

The main advantages for AS-i interface are:

* all the data communication and power cables in one special cable

* less work and components

* higher safety in functionality.

5. REFERENCES

Margineanu, I. (2005). Use of computer for process control, Course for automatics engineering, Transilvania University of Brasov, Romania

*** (2006), As-Interface--Introduction and Basic Information, SIMATIC NET (electronic format)

*** (2008) www.ad.siemens.de/net, Accesed on:2008-02-25

*** (2008) www.as-interface.com, Accesed on:2008-04-22

*** (2009) www.turk.com, Accesed on:2009-03-05