Operation Proline Promag 53 MODBUS RS485
68 Endress+Hauser
5.5.6 MODBUS error messages
If the MODBUS slave detects an error in the request telegram from the master, it sends a reply to
the master in the form of an error message consisting of the slave address, function code, exception
code and check sum. To indicate that this is an error message, the lead bit of the returned function
code is used. The reason for the error is transmitted to the master by means of the exception code.
The following exception codes are supported by the measuring device:
5.5.7 MODBUS auto-scan buffer
Function description
The MODBUS master uses the request telegram to access the device parameters (data) of the
measuring device. Depending on the function code, the master gains read or write access to a single
device parameter or a group of consecutive device parameters. If the desired device parameters
(registers) are not available as a group, the master has to send a request telegram to the slave for
each parameter.
The measuring device has a special storage area, known as the auto-scan buffer, for grouping
nonconsecutive device parameters. This can be used to flexibly group up to 16 device parameters
(registers). The master can talk to this complete data block by means of just one request telegram.
Structure of the auto-scan buffer
The auto-scan buffer consists of two data records, the configuration area and the data area. In the
configuration area, a list known as the scan list specifies which device parameters should be
grouped. For this purpose, the corresponding register address, e.g. the register address 2009 for
volume flow, is entered in the scan list. Up to 16 device parameters can be grouped.
The measuring device cyclically reads out the register addresses entered in the scan list and writes
the associated device data to the data area (buffer). The request cycle runs automatically. The cycle
starts again when the last entry in the scan list has been queried.
By means of MODBUS, the grouped device parameters in the data area can be read or written by
the master with just one request telegram (register address 5051 to 5081).
Configuration of the scan list
During configuration, the MODBUS register addresses of the device parameters to be grouped must
be entered in the scan list. The scan list can contain up to 16 entries. Float and Integer-type device
parameters with read and write access are supported.
The scan list can be configured by means of:
1. The local display or a configuration program (e.g. FieldCare).
The scan list is configured here by means of the function matrix:
BASIC FUNCTION → MODBUS RS485 → SCAN LIST REG. 1 to SCAN LIST REG. 16
Exception codes Description
01 ILLEGAL_FUNCTION
The function code sent by the master is not supported by the measuring device (slave).
!
Note!
Description of the function codes supported by the measuring device → ä 64.
02 ILLEGAL_DATA_ADDRESS
The register addressed by the master is not assigned (i.e. it does not exist) or the length of the
requested data is too big.
03 ILLEGAL_DATA_VALUE
• The master is attempting to write to a register which only allows read access.
• The value that appears in the data field is not permitted, e.g. range limits exceeded or
incorrect data format.
04 SLAVE DEVICE FAILURE
The slave did not respond to the request telegram from the master or an error occurred when
processing the request telegram.
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