Spread Spectrum Wireless Data Transceiver User Manual
LUM0002AG Rev D
Version 7.0
42
transmitted 10 times). For networks with solid RF links, this parameter should be set to a low value such
as 1 or 2. If a network has some weak or marginal links it should be set with higher values. If a Slave
receives a good packet from a Master more than once it will discard the repeated packets. Similarly, once
a MultiPoint Repeater receives a good packet from the Master, it will discard any further repeated
packets. In turn, the Repeater will send the packet out to the next Repeater or Slaves the number of times
corresponding to its own Master Packet Repeat setting.
Increasing the Master Packet Repeat setting will increase the probability of a packet getting through, but
will also increase latency in the network because each packet from the Master or Repeater is being sent
multiple times. Therefore, it is important to find the optimal mix between network robustness, throughput,
and latency. In general, a setting of 2 to 3 will work well for most well designed networks.
Note:
The Master Packet Repeat may be set to 0 if the user software is capable of, or requires
acknowledgment. In this case if a packet sent by the Master and not received by the Slave, the
user software will control the retries as needed.
M
ASTER
P
ACKET
R
EPEAT IN
M
ULTI
P
OINT
N
ETWORKS WITH
R
EPEATERS
The Master Packet Repeat parameter must also be set in MultiPoint Repeaters since a Repeater will
appear as a Master to a Slave. Therefore, the Repeater will send the packet out the number of times
corresponding to its own Master Packet Repeat parameter. If this parameter is set improperly the
reliability of the overall network may be diminished. For example, if a Master's Master Packet Repeat
setting is 3, the link between the Master and Repeater should be robust. If the Repeater's Master Packet
Repeat is set to 0, this could cause marginal communications between the Repeater and the Slaves. The
Slaves communicating through this Repeater will only receive the initial packet from the Master with no
repeats. Therefore, if the packet is not received on the first try, the Slave will not respond as expected.
(2)
M
AX
S
LAVE
R
ETRY
The Max Slave Retry setting defines how many times (0 to 9) the Slave will attempt to retransmit a packet
to the Master before beginning to use a back-off algorithm (defined by the Retry Odds setting). Slave
retries will stop when an acknowledgement is received from the Master.
(3)
R
ETRY
O
DDS
While packets transmitted from the Master to the Slaves in a MultiPoint network are not acknowledged,
packets transmitted from Slaves to the Master are. It is possible, that more than one Slave will attempt to
transmit to the Master at the same time. Therefore, it is important that a protocol exists to resolve
contention for the Master between Slaves. This is addressed through parameters (2) Max Slave Retry
and (3) Retry Odds. Once the Slave has unsuccessfully attempted to transmit the packet the number of
times specified in Max Slave Retry, it will attempt to transmit to the Master on a random basis. The Retry
Odds parameter determines the probability that the Slave will attempt to retransmit the packet to the
Master; a low setting will assign low odds to the Slave attempting to transmit. Conversely, a high setting
will assign higher odds. An example of how this parameter might be used would be when considering two
different Slaves in a MultiPoint network, one with a strong RF link and the other with a weak RF link to the
Master. It may be desirable to assign higher Retry Odds to the Slave with the weaker link to give it a
better chance of competing with the closer Slave(s) for the Master's attention.
When Retry Odds = 0,
after the Slave has exhausted the number of retries set in the Max Slave Retry
parameter and still not gained the Master’s attention, the Slave’s data buffer will be purged. Retry Odds =
0 is the recommended setting for most networks.
(4)
DTR
C
ONNECT
With the setting of 0 in the Slave, the transceiver will transmit when RS232 data is received. A setting of
1 will form a Point-to-Point link with the Master when the DTR line is high. With a setting of 2, the
transceiver will transmit in bursts. This mode is valuable when a network has many low data rate devices
and it is desirable to increase overall network capacity.