DEC VT220 Technical Manual page 162

Autorepeat Rate Buffer Values -- At keyboard power-up time, the
four autorepeat rate buffers contain default values (refer to
section 7.5.2.3 for autorepeat rates and section 7.5.7 for
defaults). The system module may change these values.
In the command byte, bit seven (PARAMS flag) should be clear, bits
six three are 1111 (to indicate that this is a rate set
command), bits two and one should be the buffer number (0 -- 3),
and bit zero (TYPE flag) is clear.
There should be two parameters carrying the rate set data.
Figure 7-17 shows change rates in buffer 3.
The first parameter specifies the timeout to the store in the
selected buffer. The second parameter specifies the interval.
(Refer to section 7.5.2.1 for definitions of these parameters.)
For example, to set the autorepeat rate in buffer one, the system
module firmware transmits 00000011 followed by two bytes of
numeric parameters.
The autorepeat timeout is the transmitted number times 5 ms. To
specify a rate of
5
ms delay, the first parameter received is
00000001. The maximum allowable time is 630 ms (01111110). The
system module must not send 635 (01111111).
NOTE
This code (635) is reserved for internal
keyboard use. 00 is an illegal value.
Autorepeat timeout is implemented as a multiple of 8.33 ms (the
keyboard's internal scan rate). Timeout rates can vary ~ 4.15 ms.
The autorepeat interval is the number of metronome codes per
second. in order to specify a speed of 16 HZ, the second parameter
received is 10010000. Note that the high order bit is set because
it is the last parameter. The highest value which may be sent is
124 (11111100).
RATE CHANGE
PARAM COMMAND BUFFER NO. TYPE
~,,....
_ _..JA _ _
- - . . , ~
07 06 05 04 03 02 01 00
0 1
I
1
I
1
I
1
I
1
I
1
I
0
0
PARAMETE R 1 (TIMEOUT)
1 PARAMETER 21INTERVAL!
Figure 7-17 Change Rates in
Buffer 3 Example
7-36