Massive occurrence of diagnostic messages 'Transmission P-card/CMX disturbed (OK)'
- The hardware problems must be localized and eliminated.
As the cause for the messages are hardware problems, these problems must be localized and eliminated using the messages, even when only OK messages are concerned.
If the transmission quality of a P-bus is so poor that only a few telegrams come through occasionally, the diagnostic messages ‘Transmission P-card/CMX disturbed’ and ‘Transmission P-card/CMX OK’ may be ‘bounced’. Due to a built-in hysteresis for error protection, it is not ensured in such a case that an error message has been received for each OK message.
Depending on the type and severity of the disturbance, the diagnosis will be more or less quickly filled with the messages ‘Transmission P-card/CMX disturbed (OK)’ or ‘P-card failed (OK again)’, whereby the messages may refer to a specific or to several P-cards, or may also come from the P-cards themselves (‘Transmission P-card/CMX disturbed (OK)’). Here the balance of the messages (same number of error and OK messages) can also be disturbed, whereby in most cases the number of OK messages will be greater (extreme case: only OK messages).
The imbalance of the messages has no further effects on the system, while the poor P-bus quality naturally impairs measured value traffic, and communication between the PU and the service station may be impaired by the very massive occurrence of error messages.
- The cause for the messages are generally hardware problems.
In general, any condition which causes a permanent reduction of the P-bus transmission quality can produce the described effect. Typical causes are:
- An open circuit in the differential voltage line of the P-bus A defective bus driver on a P-card or in the P-bus master A missing terminating resistor of the P-bus or of a P-bus repeater.
The different number of error and OK messages results from a hysteresis between error messages for line and slave failure which is practical in most cases, but which has the described side effects in a few cases.