PROFIBUS Manual

UART modules are able to detect some errors: frame errors (when a character's stop bit has not been detected) and overrun errors (when the receiver has overwritten one received character with the next before the "old" character has been stored).

 

In the case of a UART character, every byte is saved with even parity. For an error not to be detected, a byte must have an even number of changed bits, i.e. at least two bits. This corresponds to a Hamming distance of 2 and allows the detection of 50% of all transmission errors.

 

However, to allow reliable error detection, a Hamming distance of at least 4 is required. With PROFIBUS this is achieved by an additional checksum. For a combination of errors not to be detectable, two bit errors are necessary in two bytes, so that the checksum is correct again. This means that at least 4 bits must be changed, which corresponds to the required Hamming distance of 4.

 

Since start and end characters are disregarded when forming the checksum, these characters are mutually secured with a Hamming distance of 4.

 

The standard IEC 870-5-1 defines different integrity classes for  telecontrol systems:

•Integrity class 1 (I1) is viewed as sufficient for cyclic data

•Integrity class 2 (I2) is suitable for event-controlled data, and

•Integrity class 3 (I3) for remote control.

 

With the help of error detection, bit error probability is reduced to residual error probability.

 

PROFIBUS should achieve integrity class 2. The checksum procedure used in PROFIBUS is called FT2.1 in IEC 870-5-1. The residual error rate can therefore be calculated with

p = bit error probability

i = number of bytes in a telegram

RFT1.2 = residual error probability for an FT1.2 telegram

 

The combination with even parity produces a further reduction to

 

R = residual error probability for a PROFIBUS telegram

 

The graph below charts the ratio between bit error probability and residual error probability for the three integrity classes and PROFIBUS telegrams with 1, 30 and 240 bytes of payload data.

This shows that, even for a long telegram with 240 bytes of payload data, a bit error probability of 10-4 can be reduced to a residual error probability of 10-10 and so meet the requirements of integrity class 2.