Atari 400 Technical Reference Manual page 448

Serial Mode Control: There are 6 useful modes (of the possible 8)
controlled by bits 4, 5, and 6 of SKCTL. These are described on the next
page.
Note that two tone output (bit 3 of SKCTL) may be used in any of these
modes except for the bottom pair. This is because channel 2 is used to set
the output transmit rate and is therefore not available for one of the two
tones.
Note that the output clock rate is identical to the output data rate.
Serial Mode Control (see also register description SKCTL):
Force Break
ID7 D6 D5 D4 D31D2IDIIDO I SKCTL REGISTER
Pot scan and keyboard CTRL
Two Tone Control
Mode Control Bits
....!-~_....Ioo---J.,Ou
t
D6 D51D4 Rate
I
o
0 10 ext
I
I
o
0
1
ext
o
o
1
1
1
1
1
o
1 1
o
0
o
1
1 0
I
1
I
1
I
I
chan
4
CH4
A
chan
4
CH4
A
Chan
2
Chan
2
Out
Clock
ext
ext
chan
4
CH4
A
chan
4
CH4
A
Chan
2
Chan
2
In
Rate
ext
chan
4
A
chan
4
CH4
A
ext
CH4
A
Chan
2
Chan
4
A
Bi-Dir
Clock
ext
input
ext
input
chan
4
output
input
ext
input
input
Chan
4
Output
Input
not
used
A=asynchronous
I
I Comments
ITrans.
&
Receive rates set by
lexternal clock. Also internal
Iclock phase reset to zero.
Trans. rate set by external
clock. Receive asynch. (ch. 4)
(CH3 and CH4)
Trans.
&
Receive rates set by
Chan. 4. Chan. 4 output on Bi-
Directional clock line.
INot Useful
I
ITrans. Rate Set by Chan. 4
IReceive Rate set by External
IClock.
INot Useful
I
ITrans. rate set by chan. 2
IRecieve rate set by chan. 4 Chan.
14 out on Bi-Direct. Clock line.
ITrans. Rate set by Chan. 2. Re-
Iceive async. (chan 3&4) Bi-Dir.
IClock not used (Tri-state condi-
Ition)
Two tone (bit3) not useable in these modes
11.27