Network configurations Basic communications
XBee®-PRO 900HP/XSC RF Modules
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of the original packet. To reduce collisions, both repeater and end node radios in a repeater network
delay transmission of data shifted in the serial port allowing any repeaters within range to complete
their retransmissions.
The time for this delay is computed by the followinformula:
Maximum Delay (ms) = L * DS
DS = (((41(-100))/10)*RN)+RN+1
Where:
n L is the length of the transmitted packet in milliseconds.
n DS is the number of delay slots to wait.
n RSSI is the received signal strength in dBm.
n RN is the value of the RN register.
Use case - broadcast repeater network
Consider devices R1 through R10, each communicating to a PLC using the ModBus protocol and
spaced evenly in a line. All ten nodes are configured as ‘destinations & repeaters’ within the scope of
Basic Broadcast Communications (MD=3, AM, DT=0xFFFF, PK=0x100, RO=0x03, RB=0x100, RN=1). The
Base Host (BH) shifts payload that is destined for R10 to R1. R1 initializes RF communication and
transmits payload to nodes R2 through R5 which are all within range of R1. Modules R2 through R5
receive the RF packet and retransmit the packet simultaneously; they also send data out the serial
ports to the PLCs.
The following table shows the commands used to configure repeater functions.
AT command
Binary
command
AT command
name Range
# Bytes
returned
Factory
default
AM 0x3A (58d) Auto-set MY - - -
DT 0x00 (0d) Destination
Address
0-0xFFFF 2 0
MD 0x3C (60d) RF Mode 3-4 1 0
MY 0x2A (42d) Source Address 0-0xFFFF 2 0xFFFF
RN 0x19 (25d) Delay Slots 0-0xFF
[slots]
1 0
WR 0x08 (8d) Write - - -
Bandwidth considerations
Using broadcast repeaters in a network reduces the overall network data throughput as each
repeater buffers an entire packet before retransmitting it. For example: if the destination is within
range of the transmitter and the packet is 32 bytes long, the transmission takes approximately 72 ms
on a 9600 baud XSC device. If that same packet has to propagate through two repeaters, it takes 72
ms to arrive at the first repeater, another 72 ms to get to the second and a final 72 ms to get to the
destination, for a total of 216 ms.
Considering UART transfer times (~1ms/byte at 9600 baud), a server to send a 32 byte query and
receive a 32 byte response is ~200 ms, which allows for 5 polls per second. With the two repeaters in
the path, the same query/response sequence would take about 500 ms for 2 polls per second.
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