Editing And Cursor Control Logic - Raytheon DIDS-400 Series Technical Manual

DIDS-402-2AMI3
registers until the initial characters of a message are detected (STX and DA).
When STX is received and the next character is the proper DA, clock pulses are
applied to the remaining buffer registers and all the following characters are
permitted to enter D2 and D3. Data is accepted until an ETX is decoded in Dl,
at which time the receive mode is terminated.
As each character is received, the parity check and generation circuitry
is employed to check each character for the proper parity.
If an erroneous
character is received, a parity error signal jams a parity error SUB code into
buffer register Dl. This code appears on the CRT screen as a blank at the
erroneous character position.
In buffer register D3. one of three control actions occurs depending upon
the binary value of the character code held in the register. As shown in
figure 4-16, several decodes are connected to buffer register D3. These decodes
are used to detect special control characters received from the CPU. (The
control action performed when these special codes are received is explained in
table 4-2. )
Normally, the character code held in buffer register D3 is one of the
displayable characters. If this is the case, the character code enters the delay-
line electronics and is displayed on the CRT screen in the usual manner.
I
If the character held in buffer register D3 is one of the numerous control
codes, the applicable decode produces an output which is used to perform the
control action called for by the code. Generally, control characters are not
entered into the delay line from D3 unless the character in the register was'
immediately preceded by an ESC code. (The exception to this rule is LF which
is always stored.) Assuming the DC3 control code is received and shifted into
buffer register D3, a back-line function is initiated. The code is not stored in
the delay line and no visible effect (other than cursor movement) is noticeable
on the screen.
The third control action occurs when a control code (such as DC3) is
received immediately after an ESC code. When this occurs, the function is not
performed and the DC3 code is stored in the delay line. A blank space appears
on the CRT screen at the DC3 character position and a visible character (lower
case) appears on the screen when the CONTROL key is depressed.
To summarize the receive mode, the flow of data is from the received data
line, through the buffer registers, and into the delay-line refresh-memory loop.
The data circulating in memory appears on the screen as a visible message until
the delay line is cleared by the local operator or by the reception of a CAN control
code from the CPU.
4-17 EDITING AND CURSOR CONTROL LOGIC
The purpose of the editing and cursor control logic is to permit local
operators or CPU programs to edit and format messages displayed on the CRT
screen. These circuits are contained on Display Logic Board Al3 and consist
of the following principal parts:
4-38