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Message Restrictions
Terminal to Processor
1. The characters used in EOB and EOT must not
be the same.
2. A space character received by the 2702 will not
downshift (FIGS to LTRS) unless the Unshift on
Space option is installed.
3. Text immediately following FIGS, Character.
LTRS is lost. EOT must not be sent between
blocks of data.
Processor to Terminal
1. When transmitted, the characters CR, LF,
space, and blank must be in the same case as
the preceding character.
2. The terminal control automatically inserts the
proper shift character in outgoing data when a
change of case is encountered.
TELEGRAPH TERMINAL CONTROL TYPE I--
OPERATIONS
The Telegraph Terminal Control Type I is designed
to service the following terminals:
AT&T 83B2 Type Selective Calling Terminals
AT&T 83B3 Type Selective Calling Terminals
Western Union Plan 115A Terminals
Transmission Code
The shifted Baudot code is used between the Tele-
graph Terminal Control Type I and the remote ter-
minal. Figure 2-31 shows a character in the Baudot
code. One start bit and two stop bits are trans-
mitted by the 2702/03. The 2702/03 checks for one
start bit and at least one stop bit on a receive opera-
tion. The start and stop bits are added to or deleted
from the character by the 2702/03.
Transmission Code Converted to Interface Code
Figure 2-32 shows the conversion between the I/O
interface code and the transmission code. Interface
bit position 2 represents the shift bit of the character.
A one in the shift-bit position represents a Figures
shift, while a zero represents a Letters shift.
On a transmit operation (write-type operation),
the 2702/03 monitors the shift bit of the data bytes
received from the
I/o
interface. When the 2702/03
2-48
detects a change in the shift bit between two char-
acters, the terminal control inserts the proper shift
character (LTRS or FIGS) between the two charac-
ters as it sends the characters to the communica-
tions line.
On a receive operation (read-type operation), the
2702/03 keeps track of the last shift character
received from the communications line and inserts
zeros or ones into the
I/o
interface bit position 2
to give the interface character the proper shift.
The terminal control is automatically set to
lower case (LTRS) whenever the 2702/03 is reset.
Special Characters
The Telegraph Terminal Control Type I detects
certain character sequences as special characters.
This sequence (FIGS H, LTRS) is recognized as an
EOT character. Figure 2-33 shows the special
characters.
Transmit
The following characters are recognized during
write-type operations.
LTRS (Letters Shift). In FIGS mode, any character
with the shift bit set to zero (lower case) causes the
terminal control to send the LTRS character and
sets LTRS mode, before it is transmitted.
If
the Downshift on Space feature is present, the
space character sets LTRS mode. In this case, the
shift bit accompanying the space character is
ignored and a LTRS character is inserted into the
outgoing data stream.
FIGS (Figures Shift). In LTRS mode, any character
with the shift bit set to one (upper case) causes the
terminal control to send the FIGS character and
sets FIGS mode before it is transmitted.
Receive
The following characters are recognized during
read-type operations.
V (or
M). If
V (or M) is received as the first non-
shift character,
it
sets Channel End and Device End
status.
AZ. This two character processor address, if
meived as the first two non-shift characters of a
message under the Search command only, sets
Channel End and Device End status.
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