10
conditions, if A video goes away a positive going pulse edge will be created at
the input to U27:B clock and the output will toggle to the B inputs. This can only
occur if the unit is in Auto mode. U32:B locks out edges that could change the
selected inputs due to loss of video on the active channel.
The Manual/Auto function that controls NAND gate U32:B is activated by either
the Remote Auto/Man input from the External Control connector or from the front
edge momentary switch S1 or by the position of J9 at power on. S1 triggers one
shot U26:A, which toggles U27:A between manual and auto modes. The Remote
Auto/Man input can have its logic polarity changed with S4:C. After being
buffered by U31:C, the remote input triggers one shot U26:B, which in turn
triggers U26:A. This then toggles the state of the Manual/Auto lines. At power on,
if the J9 jumper is positioned to AUTO then the action of C30 charging will preset
the Q output of U27:A high, which sets the system to the AUTO mode.
Alternately, if the J9 jumper is set to MANUAL, U27:A will be cleared at power on
and the system will set to MANUAL mode.
AUTO RETURN TO A INPUT operates only in the AUTO mode. As long as the B
input is selected and A video is not present U49 is held in its reset state. The
action of A video going away sets a high on the Q output of U52:A. This high is
fed to one of the inputs of NAND gate U50:D, which will allow a Force-A signal to
pass through to the output of U51:D. Once a Force-A signal has forced the
system back to the A input, U51:D will clear U52:A and the Force-A signal will
not have an effect until the A video input is not present again. The A signal must
be present for a time-out period set by R177, from approximately 1 to 5 seconds.
The 3-input NAND gate, U53:A, insures that no more pulses from the counter
U49 will be created for the Force-A signal. The delay created by R184 and C132
insures that a 4µsec pulse will be present for the Force-A signal.
Status Outputs:
Status Outputs:
Status Outputs:
Status Outputs:
Status is available for different functions on the External Control connector. Each
output is an open collector output with a jumper-selectable pull-up resistor of 1K
ohms to +5 volts. There are five such outputs. The first one is A input health and
A video present, which will go to a high state if input health is bad or video is not
present. Second output is the same as the first but for the B input health and B
video present. The third output indicates the health of the power supplies in the
frame. If either supply goes down then the output goes high. The fourth status
output is for switch selection status. A low on this output indicates the A is
selected and a high indicates B is selected. The final output indicates the number
of inputs, where a low indicates one input and a high indicates two inputs. This
indication is changed with a slide switch, S3.
