6100 BSC Programming Manual

6100 BSC Concepts and Context
DATA TRANSFER. The general protocol for data transfer is
shown in Figure 1-10. The station that owns the line transmits
a message or a block, conforming to one of the formats described
earlier in this section. The other station responds, and the
dialogue proceeds. After transmitting all of its messages,
the station gives up the line (see "Relinquishing the Line").
In general, you use a WRITE request to send a message, a READ
request to receive one, and a WRITEREAD request to send a
message and receive a text reply.
Alternating Positive Acknowledgements. The figure shows the most
common form of positive acknowledgement. The responding station
alternates between ACK 0 and ACK 1; this convention helps the
transmitting station to tell which block was acknowledged.
The application does not supply ACK 0 or ACK 1. Rather, BSC
sends ACK 0 or ACK 1 when the application makes a READ request.
(A request to read the next block implies that the previous one
was received correctly.) Alternation begins with ACK 0, and is
reset to ACK 0 whenever the station sends or receives an EOT
character.
S5066-010
Transmitting Station:
Receiving Station:
PAD SYN...SYN ENQ PAD
PAD SYN...SYN ACK0 PAD PAD SYN...SYN ACK1 PAD
PAD SYN...SYN TEXTSTX ETB BCC PAD
Line Bid Data Transfer
Application-Supplied
BSC-Supplied
SYNPAD
Figure 1-10. Data Transfer
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