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February 02, 2024

DXM1200-X2 Controller Instruction Manual

Modbus Communication Timeouts

A Modbus timeout is the amount of time a Modbus server is given to return an acknowledgment of a message sent by the
Modbus client. If the Modbus client waits for the timeout period and no response is seen, the Modbus client 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. Special considerations need to be made to set the timeout parameter when a MultiHop network uses the
DXM Controller as the client radio.

Configure controllers operating wireless networks 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 server device. Use the DXM
Configuration Software to set the timeout parameter on the Settings

›

System screen (select Show advanced settings).

The default setting for the timeout parameter is 5 seconds.

MultiHop Networks vs DX80 Star 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.

Calculating the Communications Timeout for Battery-Powered MultiHop Radios

Battery-powered MultiHop radios are configured to run efficiently to maximize battery life. By optimizing battery life, the
allowed communications window to 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 of 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 acknowledgment 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 up to one 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.

To calculate the communication timeout parameter for a client to a server radio (no repeaters):

Client to Server Send time = (9 × 1.3 sec) + (8 retry wait × 1.3 sec) = 22 seconds
Server to Client 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

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 to 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 effect is not as
great.

Client to Repeater Send time = (9 × 0.04 sec) + (8 retry wait × 0.04 sec) = 1 second
Repeater to Client 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.

Calculating the Communication Timeout for 10–30 VDC MultiHop Radios

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 client radio to a 10–30 V DC powered server radio (no repeaters):

Client to Server Send time = (9 × 0.04 sec) + (8 retry wait × 0.04 sec) = 1 second