Basler Electric BE1-11 Modbus Protocol - Manual (Page 8)

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Data Block Field
The query data block contains additional information needed by the slave to perform the requested
function. The response data block contains data collected by the slave for the queried function. An error
response will substitute an exception response code for the data block. The length of this field varies with
each query. See the paragraphs on Register Definitions in this manual for interpretation of data.
Error Check Field
The error check field provides a method for the slave to validate the integrity of the query message
contents and allows the master to confirm the validity of response message contents. This field is 2 bytes.
Modbus
™ Modes of Operation
A standard Modbus network offers the remote terminal unit (RTU) transmission mode for communication.
BE1-11 Protection Systems support the Modbus/TCP or RS-485 modes depending on communication
options for the protection system. For example, the Modbus/TCP mode is employed when Ethernet
Protocol Option “2” (Modbus/TCP with BESTnet
™Plus) or Ethernet Protocol Option “4” (Modbus/TCP and
DNP3 with BESTnetPlus) is ordered. See the style chart in the Introduction chapter of the appropriate
instruction manual for the BE1-11. The BE1-11 also supports the Modbus over RS-485 protocol when the
BE1-11 is ordered with the RS-485 Port Protocol Option “M”. The BE1-11 supports Modbus/TCP and RS-
485 at the same time. These two optional modes of operation are described below.
A master can query slaves individually or universally. A universal ("broadcast") query, when allowed,
evokes no response from any slave device. If a query to an individual slave device requests actions
unable to be performed by the slave, the slave response message contains an exception response code
defining the error detected. Exception response codes are quite often enhanced by the information found
in the "Error Details" block of holding registers.
The Modbus protocol defines a simple Protocol Data Unit (PDU) independent of the underlying
communication layers. The mapping of the Modbus protocol on specific buses or networks can introduce
some additional fields on the Application Data Unit (ADU). See Figure 1.
Figure 1. General Modbus Frame
The client that initiates a Modbus transaction builds the Modbus Application Data Unit. The function code
indicates to the server which kind of action to perform.
Modbus
™ Over Serial Line
Message Structure
Master initiated queries and BE1-11 responses share the same message structure. Each message is
comprised of four message fields. They are:
•
Device Address (1 byte)
•
Function Code (1 byte)
•
Data Block (n bytes)
•
Error Check field (2 bytes)
Each 8-bit byte in a message contains two 4-bit hexadecimal characters. The message is transmitted in a
continuous stream with the LSB of each byte of data transmitted first. Transmission of each 8-bit data
byte occurs with one start bit and either one or two stop bits. Parity checking is performed, when enabled,
and can be either odd or even. The transmission baud rate is user-selectable, and can be set at
PDU
ADU
Function code Data Additional address Error Check
General Information BE1-11