User`s manual

MGate MB3000 User’s Manual Configuring the Modbus Gateway
4-12
Modbus TCP Exception
The MGate MB3000 is a protocol gateway that transparently passes requests and responses
between the Ethernet and serial interfaces. In some situations, it may be necessary for the gateway
to return an exception in response to a request from a Modbus TCP master. This is enabled or
disabled with the “Modbus TCP Exception” setting. When enabled, the unit can return two types
of exception:
Exception Conditions
Timeout
There is no response from the slave. Maybe the device is
off-line or the serial cable is broken.
Request dropped
There are two situations that will result in this exception:
y The request queue is full (32 request queue for each
master)
y The destination ID not included in the SID map.
Not all Modbus TCP masters require this exception, so it is up to you to determine if this setting
should be enabled.
Slave Channel and Response Timeout
According to the Modbus standard, the time that it takes for a slave device to respond to a request
is defined by the device manufacturer (please refer to Appendix A for details). Based on this
response time, a master can be configured to wait a certain amount of time for a slave’s response.
If no response is received within the specified time, the master will disregard the request and
continue operation. This allows the Modbus system to continue operation even if a slave device is
disconnected or faulty.
On the MGate MB3000, the “Response Time-out” field is used to configure how long the gateway
will wait for a response from a Modbus ASCII or RTU slave. This field is set independently for
each serial port, which is selected through the “Slave Channel” field. Please refer to your device
manufacturer’s documentation to manually set the response time-out.
The MGate MB3000 also provides automatic calibration of the response timeout. Instead of
manually figuring out the appropriate setting, you can click “Auto Detection” to have the MGate
figure out the setting for you. Once a value has been recommended, you can fine-tune it for best
performance.