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 DXM 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 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.
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
Adjusting the Receive Slots and Retry Count Parameters
The number of receive slots governs how often a MultiHop device can communicate on the wireless network.
Battery-powered devices typically have DIP switches that allow the user to set the number of receive slots, which directly
affects the battery life of the radio. Adjusting the receive slots changes how often a message can be received. By default, the
receive slots are set to 4 (every 1.3 seconds). When the receive slots are set to 32, the radio listens for an incoming
message every 0.16 seconds.
Users may also leave the retry mechanism to the application that is accessing the wireless network, in this case the DXM.
Adjust the number of retries in the MultiHop devices by writing the number of retries desired to Modbus register 6012. The
factory default setting is 8.
Calculating the Communication Timeout for a DX80 Star Network
In the DX80 network, all Node data is automatically collected at the Gateway to be read. The DXM does not use the wireless
network to access the data, which allows for much faster messaging and much lower timeout values.
For a DXM with an internal DX80 Gateway, set the timeout value 0.5 seconds. If other Modbus slave devices are connected
to the RS-485 lines, the timeout parameter governs all communication transactions and must be set to accommodate all
devices on the bus.
Sure Cross
®
DXM150 and 1500-Bx Wireless Controllers
www.bannerengineering.com - Tel: + 1 888 373 6767
65
