Black Box MD3310-R2, User Manual

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CHAPTER 6: Multipoint Operation
In point-to-point operation, the modems acknowledge every data packet
transmitted. In a multipoint network, transmissions from a master to the slaves are
not acknowledged by the slaves. This is to prevent system overload. If the slaves
acknowledged all data transmissions from the master in a large multipoint system,
then all system capacity would be spent having the master listen for
acknowledgments from the slaves. Because the transmission is not acknowledged
by the slaves, 100% confidence does not exist that every slave has received every
message from the master. To address this issue, you can modify option (1) Master
Packet Repeat
, assigning a value between 0 (the packet is transmitted once) to 9
(the packet is repeated 9 times). For networks with solid RF links, set this
parameter at the lower end of the scale (0 or 1). If the network has some weak or
marginal links, set it toward the higher values. If a slave receives a packet from a
master more than once, it will discard the repeated packets received.
While packets transmitted from the master to the slaves in a multipoint network
are not acknowledged, packets transmitted from slaves to the master are. However,
more than one slave might attempt to transmit to the master at the same time, so a
protocol must exist to resolve contention for the master between slaves. This is
addressed through parameters (2) Max Slave Retry and (3) Retry Odds. 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.
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 and conversely a high setting will assign
high odds. An example of how this parameter might be used would be when
considering two different slaves in a multipoint network, one close in with a strong
RF link and the other far from the master with a weak link. You might want to
assign higher retry odds to the slave with the weaker link to give it a better chance
of competing with the closer slave for the master’s attention.
Another parameter in a multipoint network is (4) DTR Connect. When set at 1, the
slave will connect to the master if it is free when the DTR line goes high on the 9-
pin RS-232 connector. In setting 2, the transceiver will accumulate data in its buffer
and transmit in a burst when the buffer is full. This mode is valuable when a
network has many low-data-rate devices and you want to increase overall network
capacity. In setting 0, the transceiver will transmit when RS-232 data is received.