KROHNE MFC 400 Modbus - Manual (Page 30)

7
TROUBLESHOOTING
30
MFC 400
www.krohne.com 05/2013 - 4002525102 - AD Modbus MFC 400 R02 en
7.1 No response to Modbus requests
There are a number of possibilities why no response would be received from the signal
converter. Here is a list of some of the more obvious things to check:
• Check that there is an appropriate voltage input on the V+ and V- terminals of the signal
converter.
• Ensure that there is continuity between the A and B input terminals and their associated
terminals at the master device. Check that A and B are connected correctly (details on page
6). Ensure that there is a proper "Common" connection between the master device and the
signal converter.
• The signal converter will ignore messages that are not addressed to it, or any message that
contains fundamental formatting errors. So, check that the Address ID that is being
requested is correct, the default value is 1. Check that the transmission rate (default = 19200
Baud) and format (default = 8 data bits, even parity and 1 stop bit) are correct.
7.2 Communication errors
Intermittent communication errors can have a number of causes, almost all of which can be
attributed to the quality of the connection between the master device and the signal converter,
such as:
• Low quality connections at the terminals of the signal converter or master device. Ensure
that good contact is being made and that the connections are not frayed or corroded.
• Cable lengths and/or cable capacitance are too great for the data rates being used.
• Powerful sources of electromagnetic interference in close proximity to the path of the cable
route.
• It is common to use converter devices to connect the Modbus RS485 output of the signal
converter to the serial RS232 port or USB port of a host PC using off-the-shelf protocol
converters. Many of these, especially USB based converters will have problems operating the
Modbus interface as it is a timing critical protocol. Where possible, a dedicated RS485
interface PC card should be used.
7.3 Responding with exception "Illegal Function"
There can be two reasons why this error response will be returned by the signal converter in
answer to a request:
• The function being requested is not valid for the signal converter; check the list of valid
Modbus functions. For further information refer to Supported Function Codes on page 11.
• An attempt is being made to write to a register that is protected by the Custody Transfer Lock.