6100 BSC Programming Manual

6100 BSC Concepts and Context
The exchange of data in 6100 BSC is half-duplex. At a given
time, one station can transmit; the other responds to each
transmission with an acknowledgement or reply. Thus a sending
station finds out right away whether a transmission was received
correctly, and whether the other station is still ready to accept
data.
These factors are important when you plan the messages for an
application. You can build messages to suit the routing and
transparency requirements of a link. You can also divide a
message into blocks, sacrificing some throughput for greater
error control. The description of message formats, below,
highlights the advantages and disadvantages of different message
formats. Of course, most BSC devices have strict message format
requirements--for example, a terminal might require blocks of a
certain size; such cases limit the designer's latitude but also
simplify his task.
BASIC BSC MESSAGE FORMAT. The simplest format for a message is
illustrated in Figure 1-4. The parts of the message visible
to your application are:
• The STX (Start of Text) character. This character signals the
beginning of the text part of the message. Its value is 02
hexadecimal in either ASCII or EBCDIC.
• The text, which is application-dependent. It usually contains
application data. It may also contain control information for
a target station or for processing by higher-level
communication software. The length of the text cannot exceed
4K bytes for 6100 BSC; specific devices may require much
smaller messages.
• The ETX (End of Text) character. This character signals the
end of the text part of the message. Its value is 03
hexadecimal in either ASCII or EBCDIC.
The sending application includes these characters in its output
buffer. The receiving application finds them in its input
buffer. In general, you use a WRITE request to send a message, a
READ request to receive a message, and a WRITEREAD request to
send a message and receive a text reply. See Section 3 for more
information.
1-8