DEC VT220 Technical Manual page 163

The lowest rate which can be implemented by the keyboard is 12 Hz.
Values as low as 1 can be transmitted, but are translated to 12
Hz.
The system
(11111191) •
command.
NOTE
module must not send 125
This code is the power-up
7.5.6 Special Considerations
This section describes
considerations.
the special codes and related
7.5.6.1 Error Handling -- There are four error codes. The first
two are sent at power-up
if
the self-test fails (refer to section
7.5.4.3). The other two codes are the INPUT ERROR code and the
OUTPUT ERROR code.
The OUTPUT ERROR (B5 hexadecimal) is sent after the keyboard
receives a resume transmission command if the output buffer
overflowed while the keyboard was locked.
The INPUT ERROR (B6 hexidecimal) is sent when the keyboard detects
noise (unidentified command or parameter) on the line. B6 is also
sent if the keyboard detects a delay of more than 199 ms when it
expects a parameter.
7.5.6.2 Keyboard Locked Condition -- When the keyboard receives
an inhibit transmission command, it lights the LOCKED LED and
transmits one more byte. This is a special code that indicates the
keyboard is locked (KEYBOARD LOCKED ACKNOWLEDGE). If the system
module receives this code without a request, the code indicates
that noise on the line was interpreted as an inhibit transmission
command. The system module should immediately send the resume
transmission command to unlock the keyboard.
The output first in the first out (FIFO) buffer in RAM is four
bytes. When the keyboard is locked it attempts to store characters
received from the keyboard. The keyboard stops scanning its
matrix. When the keyboard is unlocked by the system module, it
transmits all four bytes in the output buffer. If any keystrokes
have been missed due to buffer overflOW, the keyboard transmits an
error code as the fifth byte (OUTPUT ERROR). Any keys that were
not transmitted and are being pressed when the keyboard is
unlocked are processed as new keys. An error code upon unlocking
the keyboard indicates a possible loss of keystrokes to the system
module.
The keyboard stops scanning its matrix when its buffer is full.
However, it processes all incoming commands.
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