6100 ADCCP Programming Manual

Station States and Transitions
ADCCP Link and Station Management
2–6 069225 Tandem Computers Incorporated
READY^IDLE State In the READY^IDLE state, a modem connection exists and the Level 1 Protocol is
ready to receive frames. If an application requests a disconnect function while the line
is in the READY^IDLE state, the state changes to DISC^IDLE.
If an application makes a request to send data to a specific station, the state changes to
the XMIT state. The Level 1 Protocol enters the XMIT state when either the Level 2
Protocol has a frame to send due to a SENDTEXT request or a protocol response is
required from the Level 1 Protocol. Similarly, the Level 2 Protocol interprets the input
frame and queues a response frame for transmission by the Level 1 Protocol. The
queuing of an output frame for Level 1 causes the transition to the XMIT state.
If no application requests are made to send data to a station but there are requests to
receive data (RECEIVETEXT requests), the state can still change from the
READY^IDLE state to the XMIT state. If the local station is a primary in the Normal
Response Mode (NRM) or if the POLL option is selected in some other mode, the
Level 2 software routinely polls the stations on a line. To poll a station, the Level 2
software queues a frame with the poll bit set for transmission by the Level 1 Protocol.
The queuing of this frame causes a transition to the XMIT state.
The expiration of the T1TIMER can also cause a state transition. When this timer
expires, the Level 2 Protocol queues each retry for transmission by the Level 1
Protocol, changing the link state to XMIT.
XMIT State In the XMIT state, one or more frames are queued for output on the line. If the
XFERTIMER expires in this state, the link stays in the XMIT state until the retries are
exhausted. The XMIT state persists while the Level 1 Protocol:
Prepares each output frame for transmission
Calls the driver
Waits for the driver to report that frames are on the line
Normally, when the XMIT state has completed a transmission, the link returns to the
READY^IDLE state.
Note The Level 1 Protocol can still receive frames while it is in the XMIT state.
Station States and
Transitions
The Level 2 Protocol (Station Manager) handles communication with remote stations.
The commands or responses that it issues:
Establish the link
Determine whether a station can be polled or have data sent to it
Disconnect a malfunctioning station
Level 2 maintains one control block and one state machine per station. If the NonStop
system is a primary station, Level 2 redefines that station once for each remote station
on the link. If the NonStop system is a group of secondary stations, Level 2 keeps a
separate control block and state machine for each secondary station. The (70)
CHANGELIST, (69) DEFINELIST, (67) MODE SET, and (71) SCAN LIST requests also