National CP3BT26 - Manual (Page 115)

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CP3
BT26
Figure 42. Interframe Space
19.2.4 Error Types
Bit Error
A CAN device which is currently transmitting also monitors
the bus. If the monitored bit value is different from the trans-
mitted bit value, a bit error is detected. However, the recep-
tion of a “dominant” bit instead of a “recessive” bit during the
transmission of a passive error flag, during the stuffed bit
stream of the arbitration field, or during the acknowledge
slot is not interpreted as a bit error.
Stuff Error
A stuff error is detected if 6 consecutive bits occur without a
state change in a message field encoded with bit stuffing.
Form Error
A form error is detected, if a fixed frame bit (e.g., CRC de-
limiter, ACK delimiter) does not have the specified value. For
a receiver, a “dominant” bit during the last bit of End of
Frame does not constitute a frame error.
Bit CRC Error
A CRC error is detected if the remainder from the CRC cal-
culation of a received CRC polynomial is non-zero.
Acknowledgment Error
An acknowledgment error is detected whenever a transmit-
ting node does not get an acknowledgment from any other
node (i.e., when the transmitter does not receive a “domi-
nant” bit during the ACK frame).
Error States
The device can be in one of five states with respect to error
handling (see Figure 43).
Figure 43. Bus States
Synchronize
Once the CAN module is enabled, it waits for 11 consecu-
tive recessive bits to synchronize with the bus. After that, the
CAN module becomes error active and can participate in
the bus communication. This state must also be entered af-
ter waking-up the device using the Multi-Input Wake-Up fea-
ture. See System Start-Up and Multi-Input Wake-Up on
page 140.
INTERFRAME SPACE
8
SUSPEND
TRANSMIT
ST
AR
T OF FRAME
INT
3
Bus Idle
ANY FRAME
INT = Intermission
Suspend Transmission is only for error passive nodes.
DATA FRAME OR
REMOTE FRAME
Note:
d = dominant
r = recessive
r r r r r r r r r d r r r r r r r r r r r r r
DS026
SYNC
External Reset or
Enable CR16CAN
11 consecutive 'recessive" bits
received
(TEC OR REC) > 95
128 occurrences of
11 consecutive 'recessive" bits
TEC > 255
ERROR
ACTIVE ERROR
WARNING
ERROR
PASSIVE (TEC AND REC) < 96
(TEC OR REC) > 127
(TEC AND REC) < 128
BUS
OFF
DS027