GE Industrial Solutions EntelliGuard TU Trip Units - Manual (Page 60)

EntelliGuard TU Trip Units: UL/ANSI Models DEH-4567B
Serial Communication
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Figure 8-2: Wiring for Shield Grounding
Modbus RTU Message Format
The Modbus RTU protocol is strictly based upon a
transaction scheme in which a master device generates a
query and a slave device replies with a response. Each
query and response message transaction consists of the
following parts:
• Device address: 1 byte
• Function code: 1 byte
• Data: N bytes
• CRC: 2 bytes
• Dead tim
e: 3.5 bytes transmission time
These parts are as follows:
• Device address – This is the first byte of each Modbus
RTU transmission. The device address is a number
limited to the range of 0–247 and is associated with a
single device configured with a matching address. This
device receives and processes the transmission from
the master. Only the addressed slave device responds
to a transmission beginning with this address. A device
address of 0 indicates a broadcast command.
• Function code – This is the second byte of each
transmission and represents the commanded action to
the slave device (for queries from the master) or the
action that was taken by the slave device (for responses
from the slave). Codes between 1 and 127 are defined
as Modbus RTU functions. If a slave device responds
with a function code with the most significant bit (MSB)
equal to 1 (or equivalently a function code greater than
127), then the slave device did not perform the
commanded action and is signaling an error response.
• Data – This field contains a variable number of bytes,
depending on the function performed. Data may
contain addresses, actual values, or setpoints.
• CRC – This is a two-byte error-checking code, known as
a Cyclic Redundancy Check. The Modbus RTU standard
requires each message to have a two-byte CRC
(commonly known as CRC-16 for 16 bits of error
checking) to be appended to every transmission.
If the EntelliGuard Trip Unit detects a CRC error in a
received Modbus message, the EntelliGuard Trip Unit does
not respond to the message. An error in the CRC
calculation indicates that one or more bytes of the
transmission were received incorrectly, so the entire
transmission is ignored, preventing an unintended
operation.
The CRC-16 calculation is an industry standard method
used for error detection.
ENTELLIGUARD TRIP UNIT FUNCTION CODE
The following sections describe the Modbus function code
supported by the
EntelliGuard G Trip Unit.