9XTendNEMA™
RS
‐
232/485
RF
Modem
–
Product
Manual
v2.x4x
[2007.01.04]
©
2007
MaxStream,
Inc.,
Confidential
&
Proprietary
‐
All
Rights
Reserved
56
Response Packet Delay
As a packet propagates through the repeater network, if any node receives the data and generates
a quick response, the response needs to be delayed so as not to collide with subsequent retrans-
missions of the original packet. To reduce collisions, both repeater and end node radios in a
repeater network will delay transmission of data shifted in the serial port to allow any repeaters
within range to complete their retransmissions.
The time for this delay is computed by the formula:
Maximum Delay (ms) = L * DS
DS = ((-41-(-100))/10)*RN)+RN+1
Where L is the length of the transmitted packet in milliseconds, DS is the number of delay slots to
wait, RSSI is the received signal strength in dBm, and RN is the value of the RN register.
Use Case - Broadcast Repeater Network
Consider modems R1 through R10 each communicating to a PLC using the ModBus protocol and
spaced evenly in a line. All ten modems are configured as 'destinations & repeaters' within the
scope of Basic Broadcast Communications (MD=5, 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.
The modems R2 through R5 receive the RF packet and retransmit the packet simultaneously. They
also send the data out the serial ports, to the PLCs.
Bandwidth Considerations
Using broadcast repeaters in a network reduces the overall network data throughput as each
repeater must buffer 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 will take 12ms on
an XTend modem operating at 115,200 baud. If the same packet must propagate through two
repeaters, it will take 12ms to arrive at the first repeater, 12ms to get to the second and a final
12ms to reach the destination for a total of 36ms. Taking into account UART transfer times (~1ms/
byte at 9600 baud), a server to send a 32-byte query and receive a 32-byte response is about
200ms, allowing for 5 polls per second. With the two repeaters in the path, the same query/
response sequence would take about 500ms for 2 polls per second.
Generally, network throughput will decrease by a factor of 1/(R+1), with R representing the num-
ber of repeaters between the source and destination.
Table
5
‐
03. 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
0 - 6
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
-
-
-
