HI-3110
binary value of this 4-bit field specifies the number of data
bytes in the data payload (0 - 8 bytes).
Note:
All binary
combinations greater than or equal to <1 0 0 0> specify 8
bytes of data.
The remaining fields of the extended data frame (Data field,
CRC field, acknowledge field, EOF field and IFS field) are
constructed in the same way as the standard frame format.
REMOTE FRAME
The remote frame is shown in figure 4. The function of
remote frames is to allow a receiver which periodically
receives certain types of data to request that data from the
transmitting source. The identifier of the remote frame must
be identical to the identifier of the requested transmitting
node’s data frame and the data length code (DLC) should be
equal to the DLC of the requested data.
Simultaneous
transmission of remote frames with the same identifier
and different DLCs will lead to unresolvable collisions
For this reason, ARINC 825 strongly
on the bus.
discourages the use of remote frames.
The format of a remote frame is identical to the format of the
corresponding data frame except the remote frame has no
data payload. Remote frames and data frames are
distinguished by a recessive RTR bit in the remote frame.
This means if a receiver sends a remote frame and the
sending node transmits at the same time, the sending node
(with a dominant RTR bit) will win arbitration and the
requesting node will receive the desired data immediately.
ERROR FRAME
The error frame is shown in figure 5. Any node detecting an
error generates an error frame. The error frame consists of
two fields, the error flag field and the error delimiter. The
type of error flag field depends on the error status of the
node, error-active or error-passive (see below). An error-
active node generates an active error flag and an error-
passive node generates a passive error flag.
Active Error Flag:
An active error flag consists of 6
consecutive dominant bits. This condition violates the rule
of bit-stuffing and causes all other nodes on the bus to
generate error flags, known as echo error flags. Therefore,
the error flag field will consist of the superposition of different
error flags sent by individual nodes, resulting in a minimum
of 6 and maximum of 12 consecutive dominant bits. The
error flag field is followed by the error delimiter, consisting of
8 recessive bits.
Passive Error Flag:
A passive error flag consists of 6
recessive bits. This is followed by the 8 recessive bits of the
error delimiter. Therefore, an error frame sent by an error-
passive node consists of 14 consecutive recessive bits.
Since this will not disturb the bus, a transmitting node will
continue to transmit unless it detects the error itself, or
another error-active node detects the error.
HOLT INTEGRATED CIRCUITS
7
Notes:
If the passive error flag is generated by a receiver, it
cannot prevail over any other activity on the bus. Therefore,
it must wait for 6 consecutive bits of equal polarity before
completing the error flag. If the passive error flag is
generated by a transmitter, the bit stuffing rule is violated and
it will cause other nodes to generate error flags. Two
exceptions to this rule are
a) the passive error flag starts during arbitration and another
node prevails and begins transmitting, and
b) the error flag starts less than 6 bits before the end of the
CRC sequence and the last bits of the CRC sequence all
happen to be recessive.
OVERLOAD FRAME
The overload frame is shown in figure 6. It has the same
format as the active error frame, consisting of an overload
flag field and an overload delimiter. The overload flag
consists of 6 consecutive dominant bits. This condition
violates the rule of bit-stuffing and causes all other nodes on
the bus to generate echo flags, similar to the active error flag
echos. Therefore, the overload flag field will consist of the
superposition of different overload flags sent by individual
nodes, resulting in a minimum of 6 and maximum of 12
consecutive dominant bits. The overload flag is followed by
the overload delimiter, consisting of 8 recessive bits.
An overload frame, unlike an error frame, can only be
generated during the interframe space. There are two types
of overload frame:
1) Reactive Overload Frame
, resulting from
a) detection of a dominant bit during the first or second bit of
intermission,
b) detection of a dominant bit at the last (seventh) bit of EOF
in received frames, or
c) detection of a dominant bit at the last (eighth) bit of an error
delimiter or overload delimiter.
The reactive overload frame is started one bit after detecting
any of the above dominant bit conditions.
2) Requested Overload Frame.
A node which is unable to
begin reception of the next message due to internal
conditions may request a delay by transmitting a maximum
of two consecutive overload frames. The requested
overload frame must be started at the first bit of an expected
intermission.
Note 1):
The HI-3110 will never initiate an overload frame
unless reacting to one of the conditions in case 1) above.
Note 2):
Initiation of overload frames is prohibited by ARINC
825 since they increase the network loading.
HOLTIC [ HOLT INTEGRATED CIRCUITS ]
