User`s guide

2: ADCCP NRM Protocol Summary
DC 900-1317J 41
Because normal response mode (NRM) operation is usually used in a multidrop unbal-
anced configuration, primary station polling is necessary. For the secondary station to
efficiently transmit information back to the primary station, the secondary station usu-
ally delays its F-bit response to the primary station’s P-bit poll. This lets the secondary
station send several I-frames to the primary station before terminating the transmission
with a frame containing an F-bit. In Figure 2–4, delayed F-bit response and primary sta-
tion polling operations are shown as unique to NRM operation.
Figure 2–4 shows that the T1 timer function for primary station (or balanced station
primary function) operation is shared by all modes (NRM, ARM, and ABM). A primary
station uses the T1 timer to detect the absence of poll responses or I-frame acknowledg-
ment.
In asynchronous response mode (ARM), the secondary station may send unsolicited I-
frames to the primary station. Therefore, a secondary timer function is required to ini-
tiate recovery from conditions in which a previous I-frame transmission is not
acknowledged. Figure 2–4 shows this secondary timer requirement as the T2 timer;
however, no T2 timer is required in ARM operation if ADCCP option 8 is enabled, pre-
venting the secondary station from sending response I-frames.
In asynchronous modes (ARM and ABM), the secondary station (or balanced station
secondary function in ABM mode) is allowed to send unsolicited frames to the primary
station. Because nothing is gained by delaying the F-bit response, the secondary station
sends an immediate F-bit response to any P-bit received. Figure 2–4 shows the immedi-
ate F-bit response as a shared attribute of ARM and ABM modes.
ADCCP defines checkpoint recovery operations in which the primary or secondary sta-
tion initiates retransmission of unacknowledged I-frames. These procedures ensure
data retransmission in the absence of acknowledgment or REJ/SREJ recovery proce-
dures.