4.5 Modbus Timeouts
A Modbus timeout is the amount of time a Modbus slave is given to return an acknowledgement of a message sent by the
Modbus master. If the Modbus master waits for the timeout period and no response is seen, the Modbus master considers
it a lost message and continues on to the next operation.
The timeout parameter is simple to set for Modbus devices directly connected to the DXM controller, if there are no
MultiHop wireless devices. If a MultiHop network a part of the DXM Controller, special considerations need to be made to
set the timeout parameter.
Wireless communications are inherently lossy networks, and controllers operating these networks must be configured to
allow for enough time for hardware transmission retries. Set the Communications Timeout parameter to cover the
expected time for messages to be sent throughout the wireless network. For the DXM Controller, the Communications
Timeout parameter is the maximum amount of time the DXM Controller should wait after a request is sent until the
response message is received from the Modbus slave device. Use the DXM Configuration Tool to set the timeout parameter
on the Settings > General screen.
The default setting for the timeout parameter is 5 seconds.
4.5 MultiHop Networks vs DX80 Star Networks
The MultiHop wireless architecture is much different from the DX80 star architecture. Although both are wireless networks,
the DX80 star Gateway collects all the data from the Nodes, which allows the host system to directly read the data from
the Gateway without sending messages across the wireless network. This allows for DX80 Gateway to be treated like any
other wired Modbus device.
In a MultiHop network, the data resides at each device, forcing the controller to send messages across the wireless
network to access the data. For this reason, carefully consider the value of the wireless timeout parameter.
4.5 Battery-Powered MultiHop Radios
Battery-powered MultiHop radios are configured to run efficiently to maximize battery life. In optimizing battery life, the
allowed communications window for receive messages is slow (once per 1.3 seconds) and sending message rates are
standard (once per 0.04 seconds). A MultiHop device is set from the factory with the retry parameter set to 8. This means
that under worst-case conditions, a message is sent from the DXM Controller to an end device a total of nine times (one
initial message and eight retry messages. The end device sends the acknowledgement message back to the DXM
Controller a maximum of nine times (one initial message and eight retries). A single Modbus transaction may send up to
two me 16 retry messages before the transaction is complete. In addition, the radios randomly wait 0–1 time
period before retransmitting a retry message. So to allow for the random wait time, add one extra time period for each in-
between time of retries.
For a Master radio to a slave radio (no repeaters):
•
Master to Slave Send time = (9 × 1.3 sec) + (8 retry wait × 1.3 sec) = 22 seconds
•
Slave to Master Send time = (9 × 0.04 sec) + (8 retry wait × 0.04 sec) = 1 second
•
Total Send/Receive time = 23 seconds
•
Minimum Timeout period = 23 seconds
This calculates the maximum timeout value for a wireless transaction. If the link quality of the network is poor, the
maximum transfer times may happen. Set the timeout parameter to accommodate the maximum number of retries that
may happen in your application.
When MultiHop repeaters are added into the wireless network, each additional level of hierarchical network increases the
required timeout period. Since MultiHop repeaters are running at the highest communications rate, the overall affect is not
as great.
•
Master to Repeater Send time = (9 × 0.04 sec) + (8 retry wait × 0.04 sec) = 1 second
•
Repeater to Master Send time = (9 × 0.04 sec) + (8 retry wait × 0.04 sec) = 1 second
•
Additional Timeout period for a repeater = 2 seconds
Using the timeout calculation above of 23 seconds, if a repeater is added to the network the timeout should be set to 25
seconds. For each additional MultiHop repeater device creating another level of network hierarchy, add an additional two
seconds to the timeout period.
4.5 Line-Powered MultiHop Devices
Line-powered (10–30 V dc) MultiHop devices operate at the maximum communication rate, resulting in a much lower
timeout parameter setting. For each repeater added to the network, increase the timeout parameter 2 seconds.
For a Master radio to a line powered slave radio (no repeaters):
DXM100 Controller Instruction Manual
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