XTIB-R Digi International/Maxstream, XTIB-R Datasheet - Page 49

• Manufacturer Part Number: XTIB-R
• Description: BOARD INTERFACE 1W RS232/485
• Manufacturer: Digi International/Maxstream

Specifications of XTIB-R

• Accessory Type: Interface Board
• For Use With/related Products: XTend Radios
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Other names: Q2172356

9XTend™ OEM RF Module ‐ Product Manual v2.x4x [2007.01.04]
Use Case - Broadcast Repeater Network
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:
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.
Consider modules R1 through R10 each communicating to a PLC using the ModBus protocol and
spaced evenly in a line. All ten modules 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 modules R2 through R5 receive the RF packet and retransmit the packet simultaneously. They
also send the data out the serial ports, to the PLCs.
Table 4‐03. Commands used to configure repeater functions
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 module 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.
© 2007 MaxStream, Inc.
Command
Maximum Delay (ms) = L * DS
DS = ((-41-(-100))/10)*RN)+RN+1
WR
AM
MD
MY
RN
AT
DT
Command
0x3C (60d)
0x3A (58d)
0x2A (42d)
0x19 (25d)
0x00 (0d)
0x08 (8d)
Binary
Destination Address
AT Command
Source Address
Auto-set MY
Delay Slots
RF Mode
Name
Write
0 - 0xFF [slots]
0 - 0xFFFF
0 - 0xFFFF
Range
0 - 6
-
-
Chapter 4 ‐ RF Communication Modes
Returned
# Bytes
2
1
2
1
-
-
Factory
Default
0xFFFF
0
0
0
-
-
49