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Understanding RF-DLC
Note
RF-DLC Frame Types
RF-DLC frames contain an address field which defines the sub station address, broad-
cast address, or group address. RF-DLC frames also contain a control field containing
commands, responses, and counts used to maintain the link. The control field formats
are:
• Unnumbered - used to initialize the sub station and send information using an
unreliable mechanism.
• Supervisory - used to acknowledge the receipt of information frames, convey
ready or busy conditions, and report frame numbering errors.
• Information Transfer - used to transfer data using a reliable mechanism. Infor-
mation frames are numbered, meaning the control field contains Ns counts,
which ensure frames are received in their proper order, and Nr counts, which
confirm that information frames were received and accepted.
The RF-DLC frame also contains a CRC bit for error checking.
RF-DLC Commands and Responses
The base station sends a command to the sub station, which sends back a response.
Commands and Responses are of Unnumbered, Supervisory, or Information format.
The 2.5.1 Base Port Status screen provides the status of frame activity. For descriptions
of RF-DLC commands and responses, see 2.5.1 RF-DLC Base Port Status, described
in detail on page 140, and 2.5.2 RF-DLC Sub Port Status, described in detail on
page 143.
Polling
The base station polls the substations one at a time to see if they have packets to send
to the base station.The base station manages three poll lists to which it assigns devices,
based on their activity:
• Substations on the Fast Poll
order by the base. The polling frequency is contingent upon the number of
substations.
• A substation drops to Slow Poll
times with no response. A sub can also drop to Slow Poll if it responds with "n"
RRs and there is no data in either direction.
• A substation drops to Inactive Poll
•While being on Slow Poll, the sub does not respond to the base in 8 tries.
•It is the only sub station to respond to the base with RR responses and no
data in either direction.
Consider a base station with three sub stations on the fast poll list, two substations on
the slow poll list, and one on the inactive poll list. The slow poll timer is set to one
Fast Poll list are constantly being polled one at a time in
Fast Poll
Fast Poll
Slow Poll if it does not respond to the base station in 4
Slow Poll
Slow Poll
Inactive Poll if either of the following events occurs:
Inactive Poll
Inactive Poll
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