Polling mode (basic)
9XTend-PKG-R RS-232/485 RF Modem User Guide
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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 takes 12 ms on a device operating at 115,200 baud. If the same packet must
propagate through two repeaters, it takes 12 ms to arrive at the first repeater, 12 ms to get to the second and a final 12
ms to reach the destination for a total of 36 ms. Accounting for UART transfer times (~1ms/byte at 9600 baud), the
time for a server to send a 32-byte query and receive a 32-byte response is about 200 ms, allowing for 5 polls per
second. With the two repeaters in the path, the same query/response sequence would take about 500 ms for two polls
per second.
Generally, network throughput decreases by a factor of 1/(R+1), with R representing the number of repeaters between
the source and destination.
Polling mode (basic)
Polling mode (basic) and Polling mode (acknowledged) operate in the same way. The only difference between the
two modes is in their means of achieving reliable delivery of data. Polling mode (basic) uses multiple transmissions to
achieve reliable delivery.
Characteristics
Uses a high percentage of available network bandwidth.
Eliminates collisions.
Works with reliable delivery (RR or MT parameters).
Supports binary data transfers.
Base device requests packets from remote device by polling a sequential range of
addresses.
Base device is configured to specify the range of addresses being polled.
Uses inter-character delay to create RF packet lengths aligned with protocol packet
lengths up to 2048 bytes long.
Constraints
The minimum time interval between polling cycles is configurable. However, if the
remote devices cannot all be processed within that time interval, the polling cycle is
ineffective (i.e. it will impose no additional delay). In order to ensure a pause between
polling cycles, you must set PD to a value that is large enough to accommodate the
pause.
Recommended use
Use for point-to-multipoint applications that require Reliable Delivery of data. Use this
mode when it is critical that a base device be able to discern data coming from multiple
devices.
Required parameter
values (Base)
MD (RF Mode) = 3
PB (Polling Begin Address)
PE (Polling End Address)
Required parameter
values (Remote)
MD (RF Mode) = 4
Related commands
Networking: MT, PD, DT, MY, and AM