EPC120 EPC [Espros Photonics corp], EPC120 Datasheet - Page 24

• Manufacturer Part Number: EPC120
• Description: Fully integrated Light-Barrier Chips with 2-Wire Bus Interface
• Manufacturer: EPC [Espros Photonics corp]

Data Integrity on the 2-Wire Power-Bus
Physical layer
Data link layer
Residual Error Rate
Error Cases
Error Codes
© 2011 ESPROS Photonics Corporation
Characteristics subject to change without notice
12. Considerations for Safety Applications
Since the epc120-family chips can be used in safety related products, like machine safety light products, certain data integrity mechanisms
have been integrated. The safety concept on chip and communication level are described in this chapter.
Several mechanisms on different layers are implemented to guarantee a low residual error rate on the 2-wire power-bus.
Although no explicit calculations have been done yet, the residual error rate of the 2-wire power-bus is at least as good as in the ASi. The ASi
has an residual error probability of a system with Hamming distance 5 (HD5) and belongs to the DIN 19244 data integrity class I2 for an error
probability p=1e-2, and to class I3 for p=1e-3.
Each sensor device has its unique address. The microprocessor addresses each device individually and fetches the result some scan periods
later. The result includes also the address of the answering device.
Different error states are monitored:
*) The last three error bits contain the number of detected errors.
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1
2
4
5
6
Modulation and medium: Current modulation on a twisted pair line is highly immune to interference.
Start bit detection: The start pulse must have the correct orientation. Otherwise, the pulse is discarded.
Pulse alternation: Since Manchester coding is used, the pulses need to alternate. An error is detected, if this is not the case.
Pulse timing: The timing of the information pulses is fixed. A missing bit (too long pause) is detected as an error.
End bit detection: Since current modulation is used and the current is switched off when the message is completed, the last pulse has a
specified orientation.
Sequence length: The message length is well known. A too short or too long message is detected as an error.
Error control coding: If no errors have been detected on the physical layer, the received pulse sequence is processed by an error
control algorithm. Depending on the application, either 2 errors can be corrected, or 4 errors can be detected. A higher number of errors
can be detected with a reliability of 1000:1.
Strict master-slave system: A sensor may only respond, if a request from the microcontroller was correctly received.
Explicit addressing: Each message (master > slave and slave > master) contains the address of the sensor element. Even if the wrong
sensor replies to the microcontroller call, the error will be detected.
Device
Sensor
Sensor
Interface
Interface
Interface
Error Cases
2 sensor answer on the same
address
Error during the scan command
Error during result transmission
Error
Non-correctable error in the received telegram
Command to fetch the result too early
No answer from the sensor device
Non-correctable error in the received telegram
Return telegram not complete
Consequences
Collision during the transmission of the result. → Error detection in the interface
device → Error state.
No device answers → no data (all zero).
Error detection in the interface device →
Table 11: Error states
24
Action
Device doesn't response
Normal answer
Result data zero
Error reported
Error handling procedure
Error state.
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1xxx *)
0100
Datasheet epc12x - V2.1
Error Code
E0 … E3
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