MGate MB3000 Modbus Gateway User Manual
Exception
The master sends a request to the slave. The slave may not support the command or an error is detected,
so it sends an exception to the master.
Broadcast
The master sends a broadcast command, such as a reset command. Every slave on the network complies
with the command, and no response is sent to the master.
Requests Need a Time Limit
The original Modbus protocol was not designed for simultaneous requests or simultaneous masters, so only
one request on the network can be handled at a time. When a master sends a request to a slave, no other
communication may be initiated until after the slave responds. The Modbus protocol specifies that masters
use a response time-out function to identify when a slave is nonresponsive due to device or line failure. This
function allows a master to give up on a request if no response is received within a certain amount of time.
This is illustrated as follows:
Response Time-Out
The master sends a request. The slave is unresponsive for the amount of time specified by the response
time-out function. The master gives up on the request and resumes operation, allowing another request to
be initiated.
To allow for a wide range of devices, baudrates, and line conditions, actual response time-out values are left
open for manufacturers to determine. This allows the Modbus protocol to accommodate a wide range of
devices and systems. However, this also makes it difficult for system integrators to know what response
time-out value to use during configuration, especially with older or proprietary devices.
The MGate MB3000 provides a patent-protected function that tests all attached devices and recommends a
response time-out value. This function saves considerable time and effort for system integrators, and it
results in more accurate time-out settings.
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