Pepperl+Fuchs Blog

AS-Interface - What is Bit Time?

Posted by Helge Hornis on Tue, Jan 21, 2014

In communicating systems, bit time is the time it takes for one data bit to be injected into a given network.  The main advantage of the bit time concept is that it makes discussions concerning network response times and delays quite easy; in fact, time independent.

AS-Interface Bit Time
As an example, let's take a look at AS-Interface.  AS-Interface uses a simple and efficient master-slave communication method where the master starts a communication cycle by sending data to a slave.  The slave responds only when spoken to. Since the master is sending 14 bits of data it will take 14 bit times for this packet to get injected into the wire.  How long that really is depends on the operating frequency of the network and can be determined at the very end without having to reconsider the details.

Once the master has finished sending its data, master-pause starts.  The master-pause is at least 2 bit times but can be up to 10.5 bit times long.  During the master-pause, no slave response data is received by the master.  In other words, the master expects 2 bit times of quiet time on the network but must start receiving the slave response after 10.5 bit times.  This rather wide margin of 8.5 bit times allows signals to travel along the network and includes the time it takes a module (the slave) to formulate its response.
Once the data from the master is traveling along the wire, it will eventually reach the intended slave.  The AS-Interface slave must then respond and, per specification, it must take at least 2 bit times -- the master-pause -- but no more than 5 bit times before responding with its data.  Since the slave responds with 7 bits of data, it takes 7 bit times to transmit this information.

When the master has received the 7 bits of data, the slave pause starts.  The slave pause is 1.5 to 2 bit times.  During this time, the slaves expect the network to be quiet.  With AS-Interface, the master starts sending data to the next slave after a send pause.  The send pause is at least as long as the slave pause but is permitted to be longer under some conditions. Figure A shows a simplified AS-Interface transaction timing diagram.

Let's add up the various bit times.  At the very minimum, a full communication cycle using this master-slave setup takes 24.5 bit times (14 + 2 +7 + 1.5).  The maximum is 36.5 bit times (14 + 10.5 + 7 + 5).  Again, up until now, we do not know how long this is in terms of milliseconds.  As soon as the designers of the communication system specify the length of the bit time (and thus the operating frequency) we know the expected performance.  For AS-Interface, a bit time is 6 microseconds such that the length of a communication cycle can vary between 147 microseconds and 219 microseconds.

AS-Interface transaction timing diagram

Figure A: AS-Interface transaction timing diagram from the point of view of the master and slave.  The offset between the master issuing the Master Request and the time when this packet arrives at the slave is due to the propagation delays.  This includes delays due to cable length and repeaters.  From a master's point of view, it matters only that the slave response arrives 1.5 to 10.5 bit times after the Master Request has been sent.  Consequently, the number of repeaters and their performance plus the delays due to cable length are important.  

Since the maximum time allowed is 10.5 bit times, the number of repeaters any message can cross is limited to two.  When using an advanced repeater, the maximum cable length is 1500 m for three segments.  This is strictly a theoretical limit as capacitive effects will render communication unreliable, if not entirely impossible at this length.

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