AS-Interface... Let the Sensors Do the Talking |

Tuesday, February 16, 2010

AS-Interface... Let the Sensors Do the Talking

AS-Interface is a convenient bus for us... when we don't need to pass a lot of data back and forth. It's primary intention is for your plc to be used to with simple sensors and actuators... Hence the
meaning of AS-Interface is actually 'Actuator Sensor-Interface'.

ASI came into the world back in 1994... the good old days where many (most?) plc vendors were pushing out their buses. All felt they should be world standards and (to make a long story short) to this day
we have no 'world standard'. But ASI is in the mix.

ASI was introduced by Siemens and based around ProfiSafe (which was in  itself from Profibus DP). It is an 'open standard' which means there's an -unbiased- trade group that maintains the standard. Anyone can
get (buy) a copy of the standard and develop a product based on it. IEC 62026-2 and EN 50295 are the standards numbers.

It is a Main/Sub type of network meaning we can have one main unit (usually our plc) connect to many sub units. Actually, depending upon the version of ASI we're using, we can have up to 31 or 62 sub units.

Generally that is large enough for all but the biggest networks. Remember, we're just connecting our plc to sensors or actuators so 62 sensors/actuators is quite large. But again, large to some is small to others...

ASI is a two wire network and very simple to use. Power and data both flow over the cables. The cables are Ungrounded, Unterminated and Unshielded (That's a lot of Un's...). We can connect our network in
either ring, bus, star or tree fashion.

Distance is also quite good. We can go all the way out to 100 meters  (over 300 ft). And if we use a repeater we can go out to about 600 meters... hopefully that's far enough for us to go. We can also  connect a maximum of 124 inputs(31 subs x 4 inputs each = 124) and 124 outputs (31 subs x 4 outputs each = 124) according to the original spec (2.0) or 248 inputs (62 subs x 4 inputs each = 248) and 186
outputs (62 subs x 3 outputs each = 186) according to the 2.1 spec.

The cables can be connected to a power supply to transmit power to the devices on the network. Maximum power can be 8 amps at 24 VDC. Devices are simply 'tapped' into the cables at any point making installation a very simple process. The cables are typically called 'yellow cables'
because they are typically... 'yellow'.

We use 3 operating modes to communicate in. they are:
I/O data transfer mode- The inputs and outputs of connected sub units are read from and written to in this mode. So, we transfer the status of the input/output devices to/from the plc.

Analog Value transfer mode- version 2.1 allows us to work with analog  data. Here, that data is transferred and processed.

Command mode- Here we can send/receive various commands. Some examples are address setting of the sub units, diagnostic information reading from the subs and transferring various parameters and setting

How do we get/send the actual data from the sub units to the main unit?
Well that can be accomplished in one of two ways. The first way is arguably more popular. It is cyclic polling. This simply means that at a given point in time we have the main unit ask the subs for some  specific data. In other words, the main unit polls the subs for their data. The subs get the request and send back their data.

The second way is by having the sub units send their data at specific  time cycles. This is simply called cyclic data reception because we receive the data during specific cycles.

Maximum cycle time is ~5 ms when 31 subs are connected or 10 ms when  62 subs are connected. It's based on the fact that each cycle is 150 us and there are 2 cycle required for administration... so we get:
(2x150) + (31x150) = 300 + 4650 = 4950 us or 4.95 ms.

Well, hopefully that's enough of the basics to get you thinking as to  whether or not this bus is right for your needs. Next time we'll see how it actually works...

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