ModSnmp Manual - Troubleshooting
This section provides information on various errors that can occur when using ModSnmp, and suggests ways of tackling them.
Entry headings in quotes are messages that are displayed in ModSnmp's status bar or log output. For other entries, the heading is a description of the symptoms.
The easiest way to find a message in this section is to click on the Error Help button in the status bar (or press the F4 key).
The host name that you entered could not be mapped to an IP address.
The address of the Host can be 'localhost' if ModSnmp and the SNMP manager are on the same machine, or can be left empty to accept requests via any network interface. Otherwise check with your System Administrator for the address of your machine.
Check that your DNS server is working and reachable.
"Can't listen on interface XXX port YYY"
Check that the Port is not in use by any other server.
If ModSnmp is run under a Unix/Linux operating system by an ordinary user (not super-user) then the Port needs to be above 1023 and the SNMP manager should be configured to use that port.
The address of the Host can be 'localhost' if ModSnmp and the SNMP manager are on the same machine, or can be left empty to accept requests via any network interface. Otherwise check with your System Administrator for the address of your machine.
Check that the slave is still running and listening.
"Error response: ..."
ModSnmp received an error response from the slave.
The rest of the message says what kind of error response. See the troubleshooting entry for the rest of the message for further explanation.
If tracing is turned on, you can view the received message in the log output.
"Invalid data received: ..."
Data was received by ModSnmp that was unexpected or not correctly formatted as a Modbus message. The received data is discarded without further processing.
The rest of the message says what was wrong with the received data. See the troubleshooting entry for the rest of the message for further explanation.
If tracing is turned on, you can view the received data in the log output.
"Discarded: ..."
This message is displayed in the log output when data is received by ModSnmp that is unexpected or not correctly formatted as a Modbus message. The received data is discarded without further processing.
The rest of the message says what was wrong with the received data. See the troubleshooting entry for the rest of the message for further explanation.
The received data is displayed in the log output preceding the "Discarded" message.
Unexpected data was received by ModSnmp when it was about to send a request message.
The data may be a delayed response to an earlier request sent by ModSnmp (this should be apparent in the trace output). You may need to increase the response timeout setting if the slave or the comms link is slow.
"PDU size (XXX bytes) exceeds maximum (XXX bytes)"
A received message exceeded the limit on PDU size. The message is discarded without further processing.
Turn on tracing to get more information.
"Error response: Illegal data address"
The slave has sent an exception response indicating that the address in the command sent by ModSnmp was incorrect.
Check that the addresses of the registers defined in ModSnmp are consistent with the slave.
Check that the slave allows the registers/coils to be written, if you are trying to write.
"Error response: Illegal data value"
The slave has sent an exception response indicating that data in the command sent by ModSnmp was incorrect.
Turn on tracing to get more information.
If you are using 32 or 64 bit values check that the Word Count setting is consistent with the slave. If ModSnmp and the slave disagree over how to interpret the count in the Modbus message, then the number of bytes written by ModSnmp may be different from what the slave is expecting.
Check that the Modbus Type is consistent with the slave.
"Error response: Illegal function"
The slave has sent an exception response indicating that the function code in the command sent by ModSnmp is not supported.
Turn on tracing to see which function code is not supported.
"Error response: Server failure"
"Error response: Memory parity error"
"Error response: Error code XXX"
The slave/bridge/server sent an unexpected error response.
Check that your slave/bridge/server is functioning correctly.
"Wrong transaction ID in response: XXX instead of XXX"
The slave sent a response containing a transaction ID that was different from the transaction ID that ModSnmp sent in the request. This may have occurred because the slave responded late to an earlier request.
Turn on tracing to get more information.
Increase the Response Timeout if necessary.
Check that your slave is functioning correctly.
"Wrong function code in response: XXX instead of XXX"
The slave sent a response containing a function code that was different from the function code that ModSnmp sent in the request. This may have occurred because the slave responded late to an earlier request.
Turn on tracing to get more information.
Increase the Response Timeout if necessary.
Check that your slave is functioning correctly.
"Wrong slave ID in response: XXX instead of XXX"
"Wrong address in response: XXX instead of XXX"
"Wrong count in response: XXX instead of XXX"
The slave sent a response that did not correctly echo the data that ModSnmp sent in the request.
Turn on tracing to get more information.
Check that your slave is functioning correctly.
"Response PDU size incorrect: XXX instead of XXX"
The PDU size of a received response was not correct for the function code specified in the response. If the response included a byte count (e.g. a Read Holding Registers response) then the PDU size is inconsistent with the byte count. If the response did not include a byte count (e.g. a Write Multiple Holding Registers response), then the PDU size was not the expected fixed size.
Turn on tracing to get more information.
Check that your slave is functioning correctly.
"Response PDU too short: XXX when it should be at least XXX"
The PDU size of a received response was not big enough to contain all the fields required for the function code specified in the response.
Turn on tracing to get more information.
Check that your slave is functioning correctly.
"Wrong byte count in response: XXX when expecting XXX"
The byte count in the response sent by the slave is not what was expected for the count that ModSnmp sent in the request.
Turn on tracing to get more information.
Check that the Modbus Size is consistent with the slave.
Check that your slave is functioning correctly.
The host name that you entered could not be mapped to an IP address.
Check that the Host is consistent with the slave.
Check that your DNS server is working and reachable.
"Can't connect to host 'XXX': No route to host"
The host did not respond to the TCP connection request
Check that the Host is consistent with the slave.
Check that the network is working.
"Can't connect to host 'XXX' port XXX: Connection refused"
The host appears to be reachable and responding, but would not accept the TCP connection request.
It was not possible to deliver a UDP message.
"Error response: No path available to target"
The server/bridge sent a response indicating that the required slave was not reachable.
Check that the Slave ID is consistent with the slave.
Check that your server/bridge is correctly configured for the slave you are trying to reach.
A request was sent to the slave, but no response was received.
Turn on tracing to get more information.
Check that the Packet Type is consistent with the slave. The packet type is usually TCP for a socket connection.
Check that the Slave ID is consistent with the slave.
Check that Host and Port are consistent with the slave. You may have sent the request to the wrong slave/server!
If using UDP, check that the slave is running/listening.
If you sent a broadcast request, check that the Always responds setting is consistent with the slave.
"Error response: Target device failed to respond"
The server/bridge sent a response indicating that the required slave did not respond.
Check that the Slave ID is consistent with the slave.
Check that your server/bridge is correctly configured for the slave you are trying to reach.
"Serial comms package not installed"
The Java Communications API package must be installed in order to use serial communications.
See the Wingpath website for information on getting and installing the comms package.
The communications package could not find any serial ports.
If you are using an external serial port (e.g. a USB to serial converter), check that it is plugged in.
If you are using the IBM implementation of the Java Communications API, check that the javax.comm.properties file is correctly configured for your system.
If a port has become available since you started ModSnmp you will have to restart ModSnmp in order to use the port.
The serial port that you selected could not be opened and configured correctly.
Check that you have selected the correct serial port. On some systems, the communications package allows you to select ports that are not genuine serial ports.
On Unix/Linux systems, check that you have read and write access permission to the device file ('/dev/...').
Check that the port is not in use by another program (you will not necessarily get the "Port in use" error message).
If the port has become available since you started ModSnmp you will have to restart ModSnmp in order to use the port.
You must select a port before you can open the connection.
The configuration file that you loaded refers to serial port XXX, which does not exist. You will still be able to use the configuration, but you will have to select a different port.
Your Java Communications API package is not functioning correctly.
Ask your system administrator to install a correctly functioning Java Communications API package.
A request was sent to the slave, but no response was received.
Turn on tracing to get more information.
Check that the Packet Type is consistent with the slave. The packet type is usually RTU or ASCII for a serial connection.
Check that the Slave ID is consistent with the slave.
If the master and slave are on same machine, check that the ports are different.
Check that the Speed, Parity, Data bits, Stop bits and RTS control settings are consistent with the slave.
Check that a cable is connecting master and slave interfaces.
If using RTS control: Flow control check that the cable is appropriately wired.
Check that no other software is using the same port.
If you sent a broadcast request, check that the Always responds setting is consistent with the slave.
Data received by ModSnmp failed the CRC check.
"Message too short (XXX bytes)"
An RTU message must be at least 4 bytes long (1 byte for the slave ID, 1 byte for the function code, and 2 bytes for the CRC).
"Message too long (> XXX bytes)"
A received message was too long to fit in ModSnmp's input buffer.
"Invalid message length (XXX bytes) - should be odd"
"Non-hex character in message"
Data received by ModSnmp was not correctly formatted as an ASCII message.
"Message too short (XXX bytes)"
An ASCII message must be at least 9 bytes long.
"Message too long (> XXX bytes)"
A received message was too long to fit in ModSnmp's input buffer.
"Incomplete message: only XXX bytes"
A TCP message must be at least 8 bytes long (7 bytes for the header and 1 byte for the function code).
Turn on tracing to get more information.
Try increasing the EOM Timeout setting - messages may be getting fragmented by long delays introduced by the comms link or the operating system.
"Incorrect protocol identifier: XXX"
The protocol identifier in the received message header was not zero.
The value in the length field of the received message header must be at least 2 (1 byte for the slave ID and 1 byte for the function code).
"Incomplete message: only XXX bytes when expecting XXX"
The received message was shorter than indicated by the length field of the header.
Turn on tracing to get more information.
Try increasing the EOM Timeout setting - messages may be getting fragmented by long delays introduced by the comms link or the operating system.
Value is read from or written to address in slave that is wrong but near to address in command
If you are using 32 or 64 bit registers, check that Word Registers setting is consistent with the slave.
Turn on tracing to get more information.
Wrong value is displayed when reading from slave, or wrong value is written to slave
If you are using 32 or 64 bit registers, check that Little-Endian and Word Registers settings are consistent with the slave.
Turn on tracing to get more information.
The 16-bit words of 32 or 64-bit values are in reverse order
Select/deselect Little Endian to reverse the order and make it consistent with the slave.
"Not enough data in message: XXX bytes when expecting XXX"
A received message contains too little data for the count and register sizes specified in the request.
Turn on tracing to get more information.
If you are using 32 or 64 bit registers, check that the Word Registers and Word Count settings are correct.
"Excess data in message: XXX bytes when expecting XXX"
A received message contains too much data for the count and register sizes specified in the request.
Turn on tracing to get more information.
If you are using 32 or 64 bit registers, check that the Word Registers and Word Count settings are correct.