6100 BSC Programming Manual

6100 BSC Concepts and Context
NOTE
A programmer doesn't have to know the details of
BCC computation. There are two basic kinds of block
checking; you define in SYSGEN the one you want to use.
The part of this section that describes line parameters
tells you how to make the right choice.
• The trailing pad. This character is present at the end of
each transmission. It ensures that the modem transmits the
last character--in this instance, the block check
sequence--before it turns the transmitter off. The trailing
pad is FF hexadecimal (all ones) in ASCII or EBCDIC.
Remember that once the message is transmitted, the receiving
station has its turn to use the line, to acknowledge or reject
the received message. Only after receiving a reply can the
sender dispatch its next message.
THE MESSAGE HEADING. In some cases, stations exchange not
only messages, but also information about the messages. For
instance, a station that receives and routes messages typically
needs addressing information, and may also need other data like
message sequence numbers, priorities, and routing criteria.
For such applications, as well as to accommodate special
device requirements, BSC lets you precede a message with a
header, as shown in Figure 1-5.
1-10