SEAbus Plus Packet
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SEAbus Plus Packet
Both SEAbus and SEAbus Plus use a simple byte-count
approach to realize communications between a master and its
slave devices. Information is arranged in message packets. All
packet fields are one byte in length except Data which can
vary between 0 and 255 bytes, depending on the particular
message and device type. Communications is standard asyn-
chronous, using one start bit, eight data bits, no parity, and one
stop bit. All values are expressed in hex, with multiple-byte val-
ues sent least significant byte (LSB) first, most significant byte
(MSB) last.
Each packet begins with a 4-byte header (Sync, Devt, Msgt,
Len) to indicate the message source, destination, type, and
length. The variable length data field (Data) follows the header
and contains any actual data. All packets end with the LRC
error checksum byte.
The SEAbus Plus protocol is very similar to the original SEAbus
protocol in its format and timing requirements. The primary dif-
ference is a 16-bit cyclical redundancy check (CRC16)
included in the Data field of the message packet to improve
error detection. This checksum is produced for each SEAbus
Plus packet by treating the data to be transmitted as a binary
polynomial and dividing it by a generating polynomial. This
technique assures 100% detection of all errors in data up to 16
bits in length and 99.9997% in data 17 bits or longer. SEAbus
Plus uses an additional error check byte equal to the inverted
Sync byte (~Sync). It follows immediately after the CRC16 at
the end of the Data field. A brief description of each message
field in a SEAbus Plus packet follows.
Synchronization (Sync) Byte
The Sync byte indicates both the start and direction of a new
message packet. The direction is either master-to-slave or
slave-to-master. A message sent from the ACCESS host to a
field device always begins with 14h(exadecimal). Any
response packet sent by a slave back to the ACCESS master
uses a value of 27h in the Sync byte. Any messages sent with-
out the proper Sync byte are ignored by all devices.
Device Type (Devt) Byte
Each device placed in an ACCESS system is assigned a unique
address to distinguish it from other slave devices on the bus. The
range of address values permitted for a particular device
depends on the device type’s addressing scheme. Slave devices
using direct addressing must use an address between 1 and 224,
inclusive. Indirectly addressed devices may use any value from 1
to 254. (0 and 255 are reserved by the ACCESS host for universal
and global packets, respectively.)
The meaning of the second byte in a SEAbus Plus packet,
Devt, changes depending on the addressing scheme used by
the field device. For directly addressed devices, such as the
SB Energy-Comm trip unit, the Devt byte is simply the device’s
unique slave address. For devices using indirect addressing,
Devt takes on the value of the single-byte “device-type” code
assigned to a specific type of device. The device’s unique
slave address is included later as the first byte in the Data field.
Message Type (Msgt) Byte
The message type byte, Msgt, identifies the particular mes-
sage being sent. This, in turn, determines the length and type
of data (if any) to follow in the Data field of the SEAbus Plus
packet. Whenever a slave device responds to a message of
type Msgt sent by the ACCESS host, it uses that same value as
the message type byte in its return packet.
In the SEAbus Plus protocol, Devt and Msgt define a unique
message. With the exception of global messages, different
device types can use the same message type value for differ-
ent purposes without conflict.
Length (Len) Byte
The fourth byte in a SEAbus Plus packet specifies the total
number of bytes to follow in the packet’s variable-length Data
field. The count given by the length byte, Len, includes any
actual data bytes, the 2-byte CRC16, and the inverted Sync
byte. In the case of indirectly addressed devices, Len also
includes the slave address byte appearing as the first byte in
the Data subpacket.
Data Subpacket
The Data subpacket contains the actual data to be passed
between the ACCESS host and slave device. The particular
message type (Msgt) determines both the type and amount of
data. For indirectly addressed devices, the first byte appearing
in the data field is the device’s unique slave address. This is
followed immediately by any data values to be sent. The SEA-
bus Plus protocol requires that a two-byte cyclical redundancy
check (CRC16) and another error check byte equal to the
inverted Sync byte (~Sync) be placed after the data to com-
plete the Data subpacket.
The receiving device also computes its own CRC16 and ~Sync
based on the incoming packet bytes. If these newly calculated
check bytes do not equal the received error check values, the
message is invalid.
Longitudinal Redundancy Check (LRC) Byte
Both SEAbus and SEAbus Plus terminate all packets with an LRC
byte for error checking. For SEAbus, the LRC is the one’s comple-
ment of the sum (no carry) of all preceding bytes in the packet
except the initial Sync byte. SEAbus Plus performs the same sum-
ming procedure, but does not take the one’s complement.
The LRC is calculated at both the transmitting device and
receiving device for each packet. If the receiving device calcu-
lates a different LRC based on the incoming bytes than that
received at the end of the packet, an error has occurred during
transmission and the packet is ignored.
Note: Refer to ACCESS SEAbus Protocol Reference Manual
IPIM-2209 and SG6213-00 for detailed information.
