request is sent until the response message is received from the Modbus slave 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.
7.4.1 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.
7.4.2 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 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 of 8. This means that under worst-case conditions, a
message is sent from the DXM700 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 DXM700 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 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
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.
7.4.3 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 Master radio to a 10–30 V dc powered slave radio (no repeaters):
Master to Slave Send time = (9 × 0.04 sec) + (8 retry wait × 0.04 sec) = 1 second
Slave to Master Send time = (9 ×* 0.04 sec) + (8 retry wait × 0.04 sec) = 1 second
Total Send/Receive time = 2 seconds
Minimum Timeout period = 2 seconds
Sure Cross
®
DXM700-Bx Wireless Controller
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