Sensor systems 2 AS-Interface

AS-Interface - Introduction
Master call
Slave response
18
The AS-Interface bus protocol
The AS-Interface message is constructed in a very compact way. For the
user, this all occurs in the background, and it is presented here only to give
you a better understanding of data transmission.
The slave response, always 7 bits long after the 14-bit master query, takes
the following form:
SB CB A4 A3 A2 A1 A0 I4 SEL I2 I1 I0 PB EB
1 start bit (SB, always "0")
1 control bit (CB)
5 address bits (A0 ... A4) plus SEL bit for A/B address for expansion to 62
slaves
5 data bits (I0, I1, I2, I4)
1 parity bit (PB)
1 end bit (EB, always "1")
The AS-Interface master uses the control bit to tell the slave whether this
message is a command (i.e. read I/O configuration, read ID code, etc.) or a
data or parameter call.
Together with the SEL bit, the 5 address bits denote the slave being addressed.
With 5+1 Bits, 64 nodes can be addressed. Address 0 must always
be unoccupied, since it is used for automatic addressing after an exchange.
Thus up to 62 slaves can be addressed.
In a data or parameter call (CB = "0"), the I4 bit is used to distinguish between
I4 = "0" ==> parameter call
I4 = "1" ==> data call
.
The bits I0 ... I2 contain the output data, by which up to 3 outputs can be
controlled or 3 parameter bits transmitted. The entire message is secured
with a parity bit and is concluded with an end bit. In specification 2.0 (standard
mode) the I3 bit was available as a fourth output bit, which is now used
as select bit (SEL) when extended-mode addressing is in operation. If
slaves built to specification 2.1 can be operated in standard mode, the I3 bit
can also be used as an output bit.
In the slave response, besides the 4 bits of input data, only the start bit (SB),
the parity bit (PB), and the end bit (EB) are transmitted.
SB I3 I2 I1 I0 PB EB
The master cyclically interrogates the unique addresses (1A, 1B, 2A, 2B,
3A...31A, 31B) of all slaves in turn in a polling process. The addresses in
use are stored in the master's configuration. It only queries these existing
addresses, which shortens the cycle time correspondingly.
Subject to reasonable modifications due to technical advances. Copyright Pepperl+Fuchs, Printed in Germany
Pepperl+Fuchs Group Tel.: Germany +49 621 776-0 USA +1 330 4253555 Singapore +65 67799091 Internet http://www.pepperl-fuchs.com
Issue date 2004-11-19 - Catalogue AS-Interface 2004/2005