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If errors occur when the Manchester receiver is receiving, it can no longer reliably
detect the end delimiter and therefore the end of the frame. Reception could then
go on forever. To prevent this, reception is terminated by the microprogram
whenever an error is detected during reception. If a CRC error occurs (detected in
the last data byte), the two following CRC bytes are stored in the receive memory,
whereas if a data bit error occurs (Manchester violation), the bad data byte ends up
as the last or next to last in the receive memory (depending on how full the internal
receive FIFO is). In either case, the length bytes indicate which byte was stored
last in the receive buffer regardless of whether it was a CRC or data byte.
When reception is finished, the SPC 4-2 enters the received length in the length
bytes of the receive buffer and activates the "frame received error-free" interrupt if
no errors were detected during reception. If an error occurred, the "bad frame
received" interrupt is activated. In this case, the user can decide whether this is a
CRC error (station type = 1) or a Manchester error (station type = 0) based on bit
11 of the status register (station type).
If there was receive buffer overflow, and the received data were lost, the length
information contains the correct length information but the receive buffer is filled
only up to its defined end (there are less bytes in the receive buffer than indicated
by the length bytes!). In this case, the "receive buffer overflow" interrupt is activated
(when there is a receive buffer overflow, it is not possible to find out whether the
frame was received with or without errors).
The SPC 4-2 activates the relevant interrupt approximately 17 bit times after the
end of the frame. In the meantime, it empties the three-level FIFO of the
Manchester receiver and updates the length bytes.
PROFIBUS Controller SPC 4®-2 LF
90
C79000-G8976-C157-3
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