The 4 input buffers feed a MAX4052 MUX to select one of them to pass on to the next stage.
U17 selects the input according to the values of:
(1) PIC pin RA2: CH-A/B select and,
(2) PLD pin PG0: BNC/Pod select.
CH-A/B is under the control of the PIC which can be programmed to chop the source between the pair of BNC inputs or the pair of POD inputs during a trace.
BNC/Pod is set via an option bit in Spock and may only be changed when Spock is reloaded. That is, between traces.
This signal also selects the SRAM bank so we have a completely separate 16K buffer for both Analog and Digital samples for POD and BNC sources !
As a bonus, there is a spare 4:1 MUX in U17 used to activate 1 of 4 front panel LEDs
Channel sample indicators on the front panel such as these are an important feature of modern DSO :-)
They look good and tell you when BitScope is actually acquiring analog data.
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