Avionics Protocols
ARINC:
Introduction :
Aeronautical Radio, Incorporated (ARINC) is a major company thatdevelops and operates systems and services to ensure the efficiency,operation, and performance of the aviation and travel industries.
ARINC 429 is a specification, which defines how avionics equipment and systems should communicate with each other. They are interconnected by wires in twisted pairs. The specification defines the electrical and data characteristics and protocols, which are used. ARINC 429 employs a unidirectional data bus standard known as Mark 33 Digital Information Transfer System (DITS). Messages are transmitted at a bit rate of either 12.5 or 100 kilobits per second to other system elements, which are monitoring the bus messages. Transmission and reception is on separate
ARINC 429 is a very simple, point-to-point protocol. There can be only one transmitter on a wire pair. The transmitter is always transmitting either 32-bit data words or the NULL state. There is at least one receiver on a wire pair; there may be up to 20.
32
|
31
|
30
|
29 11
|
10
|
9
|
8 1
|
P
|
SSM
|
DATA
|
SDI
|
Label
| ||
Parity
The MSB is always the parity bit for ARINC 429. Parity is normally set to odd except for certain tests. Odd parity means that there must be an odd number of “1” bits in the 32-bit word that is insured by either setting or clearing the parity bit. For example if bits 1-31 contain an even number of “1” bits, bit 32 must be set to create ODD parity. On the other hand, if bits 1-31 contain an odd number of “1” bits, the parity bit must be clear.
SSM
Bits 31 and 30 contain the Sign/Status Matrix or SSM. This field contains hardware equipment condition, operational mode, or validity of data content.
Table 1 SSM Codes for BCD data
Bits
|
Means
| |
31
|
30
| |
0
|
0
|
Plus, North, East, Right, To, Above
|
0
|
1
|
No Computed Data
|
1
|
0
|
Functional Test
|
1
|
1
|
Minus, South, West, Left, From, Below
|
Table 2 SSM Codes for BNR data
Bits
|
Means
| |
31
|
30
| |
0
|
0
|
Failure Warning
|
0
|
1
|
No Computed Data
|
1
|
0
|
Functional Test
|
1
|
1
|
Normal Operation
|
Data
Bits 29 through 11 contain the data, which may be in a number of different formats. Some examples are provided later in the tutorial. There are also many non-standard formats that have been implemented by various manufacturers. In some cases, the data field overlaps down into the SDI
bits. In this case, the SDI field is not used.
SDI
Bits 10 and 9 provide a Source/Destination Identifier or SDI. This is used for multiple receivers to identify the receiver for which the data is destined. It can also be used in the case of multiple systems to identify the source of the transmission. In some cases, these bits are used for data. ARINC 429 can have only one transmitter on a pair of wires, but up to 20 receivers.
Label
Bits 8 through 1 contain a label identifying the data type and the parameters associated with it. The label is an important part of the message and is described in more detail below. It is used to determine the data type of the remainder of the word and, therefore, the method of data translation to use. The various data types are described in detail below. Labels are typically represented as octal numbers.
Key Attributes
· Unidirectional data bus standard MARK 33 Digital Information Transfer System (DITS)
· Bit rates: high speed 100 kbit/s, low speed 12.5 – 14.5 kbit/s
· Encoding: return to zero bipolar tri-state modulation
· Message length: 32-bit word, 255 word data block in block transfer mode
· Classes of service: periodic, sporadic and file transfer
· Media access: simplex single source multiple sink plus full duplex RTS/CTS handshake
· Topology: single source multiple sink
· Media: 78 Ohm unbalanced shielded twisted pair copper cable
· Number of nodes: 20 sinks, 1 source
CAN :
The CAN communication protocol is a carrier-sense, multiple-access protocol with collision detection and arbitration on message priority (CSMA/CD+AMP). CSMA means that each node on a bus must wait for a prescribed period of inactivity before attempting to send a message. CD+AMP means that collisions are resolved through a bit-wise arbitration, based on a preprogrammed priority of each message in the identifier field of a message. The higher priority identifier always wins bus access.
CAN has two formats
Standard
1.standard 11-bit identifier, provides for signaling rates from 125 kbps to 1 Mbps
2. The standard 11-bit identifier field in provides for 211, or 2048 different message identifiers
Extended
1. The standard was later amended with the “extended” 29-bit identifier .
2. the extended 29-bit identifier provides for 229, or 537 million identifiers.
The Bit Fields of Standard CAN and Extended CAN
Standard CAN :
1
|
11 bits
|
1
|
1
|
1
|
4
|
8 Bytes
|
16
|
2
|
7
|
7
|
S
O
F
|
11-bit
Identifier
|
R
T
R
|
I
D E |
R0
|
D
L C |
DATA
|
C
R
C |
A
C
K
|
E
O
F
|
IFS
|
SOF–The single dominant start of frame (SOF) bit marks the start of a message, and is used to
synchronize the nodes on a bus after being idle.
Identifier-The Standard CAN 11-bit identifier establishes the priority of the message. The lower the
binary value, the higher its priority.
RTR– The single remote transmission request (RTR) bit is dominant when information is required from another node. All nodes receive the request, but the identifier determines the specified node. The responding data is also received by all nodes and used by any node interested. In this way, all data being used in a system is uniform.
IDE– A dominant single identifier extension (IDE) bit means that a standard CAN identifier with no
extension is being transmitted.
r0– Reserved bit (for possible use by future standard amendment).
DLC– The 4-bit data length code (DLC) contains the number of bytes of data being transmitted.
Data– Up to 64 bits of application data may be transmitted.
CRC– The 16-bit (15 bits plus delimiter) cyclic redundancy check (CRC) contains the checksum
(number of bits transmitted) of the preceding application data for error detection.
ACK– Every node receiving an accurate message overwrites this recessive bit in the original message with a dominate bit, indicating an error-free message has been sent. Should a receiving node detect an error and leave this bit recessive, it discards the message and the sending node repeats the message after rearbitration. In this way, each node acknowledges (ACK) the integrity of its data. ACK is 2 bits, one is the acknowledgment bit and the second is a delimiter.
EOF– This end-of-frame (EOF), 7-bit field marks the end of a CAN frame (message) and disables
bit-stuffing, indicating a stuffing error when dominant. When 5 bits of the same logic level occur in
succession during normal operation, a bit of the opposite logic level is stuffed into the data.
IFS– This 7-bit interframe space (IFS) contains the time required by the controller to move a correctly
received frame to its proper position in a message buffer area.
Extended CAN :
1
|
11
|
1
|
1
|
18
|
1
|
1
|
1
|
4
|
8……Bytes
|
16
|
2
|
7
|
7
|
S
O
F |
11-bit
Identifier
|
S
R R |
I
D E |
18-bit
Identifier
|
R
T R |
R1
|
R0
|
D
L C |
DATA
|
C
R C |
A
C K |
E
O F |
I
F S |
SRR– The substitute remote request (SRR) bit replaces the RTR bit in the standard message location
as a placeholder in the extended format.
IDE– A recessive bit in the identifier extension (IDE) indicates that more identifier bits follow. The 18-bit extension follows IDE.
r1– Following the RTR and r0 bits, an additional reserve bit has been included ahead of the DLC bit.
Arbitration
If two nodes try to occupy the bus simultaneously,
access is implemented with a nondestructive, bit-wise arbitration. Nondestructive means that the node
winning arbitration just continues on with the message, without the message being destroyed or corrupted by another node. The allocation of priority to messages in the identifier is a feature of CAN that makes it particularly attractive for use within a real-time control environment. The lower the binary message identifier number, the higher its priority. An identifier consisting entirely of zeros is the highest priority message on a network because it holds the bus dominant the longest. Therefore, if two nodes begin to transmit simultaneously, the node that sends a last identifier bit as a zero (dominant) while the other nodes send a one (recessive) retains control of the CAN bus and goes on to complete its message. A dominant bit always overwrites a recessive bit on a CAN bus.
The four different message types, or frames that can be transmitted on a
CAN bus
DATA FRAME
The data frame is the most common message type, and comprises the Arbitration Field, the Data Field, the CRC Field, and the Acknowledgment Field. The Arbitration Field contains an 11-bit identifier and the RTR bit, which is dominant for data frames, in Extended Frame it contains the 29-bit identifier and the RTR bit. Next is the Data Field which contains zero to eight bytes of data, and the CRC Field which contains the 16-bit checksum used for error detection. Last is the Acknowledgment Field.
Remote Frame
The intended purpose of the remote frame is to solicit the transmission of data from another node. The remote frame is similar to the data frame, with two important differences. First, this type of message is explicitly marked as a remote frame by a recessive RTR bit in the arbitration field, and secondly, there is no data.
ERROR FRAME
The error frame is a special message that violates the formatting rules of a CAN message. It is transmitted when a node detects an error in a message, and causes all other nodes in the network to send an error frame as well. The original transmitter then automatically retransmits the message. An elaborate system of error counters in the CAN controller ensures that a node cannot tie up a bus by repeatedly transmitting error frames.
OVERLOAD FRAME
The overload frame is mentioned for completeness. It is similar to the error frame with regard to the
format, and it is transmitted by a node that becomes too busy. It is primarily used to provide for an extra delay between messages.
There are five error conditions that are defined in the CAN protocol
CRC Error
A 15-bit Cyclic Redundancy Check (CRC) value is calculated by the transmitting node and this 15-bit value is transmitted in the CRC field. All nodes on the network receive this message, calculate a CRC and verify that the CRC values match. If the values do not match, a CRC error occurs and an Error Frame is generated. Since at least one node did not properly receive the message, it is then resent after a proper intermission time.
Acknowledge Error
In the Acknowledge Field of a message, the transmitting node checks if the Acknowledge Slot (which it has
sent as a recessive bit) contains a dominant bit. This dominant bit would acknowledge that at least one
node correctly received the message. If this bit is recessive, then no node received the message properly.
An Acknowledge Error has occurred. An Error Frame is then generated and the original message will
be repeated after a proper intermission time.
Form Error
If any node detects a dominant bit in one of the following four segments of the message: End of Frame, Interframe Space, Acknowledge Delimiter or CRC Delimiter, the CAN protocol defines this to be a form violation and a Form Error is generated. The original message is then resent after a proper intermission time. (see Figure 2 and/or Figure 3 for where these segments lie in a CAN message).
Bit Error
A Bit Error occurs if a transmitter sends a dominant bit and detects a recessive bit, or if it sends a recessive
bit and detects a dominant bit when monitoring the actual bus level and comparing it to the bit that it
has just sent. In the case where the transmitter sends a recessive bit and a dominant bit is detected
during the Arbitration Field or Acknowledge Slot, no Bit Error is generated because normal arbitration or
acknowledgment is occurring. If a Bit Error is detected, an Error Frame is generated and the original
message is resent after a proper intermission time.
Stuff Error
CAN protocol uses a Non-Return–to-Zero (NRZ) transmission method. This means that the bit level is
placed on the bus for the entire bit time. CAN is also asynchronous, and bit stuffing is used to allow
receiving nodes to synchronize by recovering clock information from the data stream. Receiving nodes
synchronize on recessive to dominant transitions. If there are more than five bits of the same polarity in a
row, CAN will automatically stuff an opposite polarit y bit in the data stream. The receiving node(s) will use
it for synchronization, but will ignore the stuff bit for data purposes. If, between the Start of Frame and
the CRC Delimiter, six consecutive bits with the same polarity are detected, then the bit stuffing rule
has been violated. A Stuff Error then occurs, an Error Frame is sent, and the message is repeated.
Key Attributes :
· Multimaster priority-based serial communications protocol supporting distributed real-time control and multiplexing using non-destructive contention-based arbitration
· Bit rates: 1 Mbit/s (with 40m bus), 100 kbit/s (with 500m bus)
· Encoding: non return to zero (NRZ)
· Message length: 0 to 8 bytes
· Classes of service: periodic and sporadic
· Media access: carrier sense multiple access with collision avoidance (CSMA/CA)
· Topology: terminated differential two wire bus
· Media: screened or unscreened twisted pair or flat pair telephone cable
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