Texas Instruments 990 User Manual page 24

I ntroducti on
1. 4. 1 • 2 Le ve 1 2 ( HOL C Line Con t r
0
1 ) •
Level 2 is variously called the HOLC frame level, and the link or
line-control level. It controls the communication line according
to the HOLC protocol. In the primary station, it polls the
secondary stations and sequences data transmissions. It also.
looks for and recognizes addresses received from the secondary
stations with which it is communicating. Polling is the process
of checking to see if any stations in the network have data to
transmit. Polling is used by the primary station to monitor the
status of the secondary stations in the network in addition to
otherwise controlling the flow of information. At the secondary
stations, as well as the primary, level 2 software performs all
necessary verification of the data flowing between the stations.
If an error is detected, the line-control software automatically
initiates a retransmission of the data.
In the primary station, the line-control software is resident
partially on the FCCC board and partially in main memory. In the
s econdar y, most of the sof tware is in mai n memory and ver y 1 i ttl e
is resident on the LLM board.
Four ports are available at the primary station (on the FCCC
board) for the installation of lines to secondary stations in the
network. Each line can be used as a mul tipoint line for
connecting as many as 32 secondary stations into the network.
Each secondary station has a unique address, which is embedded in
all transmitted frames (see Figure 1-4) destined for that
station. The secondary software recogni zes its own address and
proceeds to interpret and process the message contained in the
frame.
1.4.1.3 Level 3 (Packet Level).
The primary packet-level software, although appearing logically
to communicate directly with the secondary packet-level software,
actually queues the tas k-generated data and messages to the line-
control level, which in turn queues them to level 1. These
messages are referred to as data "packets." If multiple HOLe
communications tasks are executing simultaneously in the system,
the packet level at the transmitting station interleaves
(multiplexes) and queues the data packets passed from the various
tasks into one data stream for level 2. Conversely, the packet
level demultiplexes received messages for passing to the
i ndi vi dual tas ks •
As a consequence of multiplexing and queuing, a packet-level
message to another station may not receive a response until after
several other packet-level messages have been sent. The packet-
level software writes a received message into an appropriate
buffer and it is the responsibility of the receiving task to read
that message.
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