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Design and Functions
Segment Length in bits
Content
B
4
Status bits
C
20
Verified position, coarse resolution (1 LSB
≙
1 mm)
D
32
CRC check sum for segments A to C
E
16
Additional information
F
8
Non-verified, high-resolution position offset (1 LSB
≙
62.5 µm)
NOTE!
In single-channel operation, channel B is switched off and segments D-
F are not transmitted. This corresponds to a non-verified position
transmission.
9.5.4.1.3
Message Counter
The message counter identifies the correct message sequence and is raised by one with every message. It is
according to this index that the additional information (
9.5.4.1.7) is attributed.
The range of values for the message counter is from 0 to 255. When the counter has reached the maximum
value, there will be an overflow and with the following message, counting starts again from 0.
9.5.4.1.4
Status Bits
The status bits provide additional information to the position. They always refer to the position with which they
are transmitted.
Table 5: Meaning of the status bits in an RS-485 message
Bit
1
Meaning of the signalled value
4
Warning – extrapolated position:
0 = Position was determined normally in both channels
1 = Due to a reading error in one of the channels, the position was extrapolated
5
Signalling faulty tape:
0 = Tape correct
1 = Tape not present or faulty
6
Signalling unauthorized extrapolation:
0 = Extrapolation active
1 = Extrapolation was deactivated because there were too many reading errors. Further operation is
not allowed. (
7
Single-channel operation
0 = Sensor is in dual-channel operation
1 = Due to under voltage, the sensor is in single-channel operation. Caution: only segments A to C
are transmitted!
Since the described status bits usually never change, they are dynamised, i.e. they change their status depending
on the message counter. If the LSB of the message counter is set (odd counter value), all four status bits are
inverted bitwise before transmission; if the LSB is cleared (even counter value), the transmitted value matches the
signalled value.
NOTE!
If the position is extrapolated due to reading errors unusually often, this
might point to an extremely dirty or damaged magnetic tape. In this
case, proceed as is described in chapter
9.5.4.1.5
Position
1
Please note that the status bits do not require an entire byte for themselves. The bit position stated here corresponds to the
actual position in the second byte of the message.
