Just tearing down an old Belkin USB KVM switch, and I just found something that has me scratching my head and thinking "Why the hell have they done that???"
The front panel has 2 buttons and 2 LEDs on it. The LEDs indicate which of the two channels is active. The panel is fed by 6 wires - +5V, ground, two for the buttons (very nicely with full hardware debouncing), and two for the LEDs.
Now, you'd expect the LEDs to just be fed directly by the 2 control lines, yes? Nope. For some reason they have decided to feed the two control lines into a 74HC139 dual 2-to-4 decoder. One control line is one of the "select" lines (A1), and the other the the enable (/G) line. The LEDs then are connected via a single resistor (yes, a single resistor, but then only one of them will ever be on at once) to +5V.
They redesigned their 8 channel KVM that has say 4 digital I/Os and 8 LEDs and 8 buttons to make your 2 channel KVM. Same board layout, same parts, less wires, no need to to start a new design just because now there are only two channels. Just thinking out loud.
liudr:
They redesigned their 8 channel KVM that has say 4 digital I/Os and 8 LEDs and 8 buttons to make your 2 channel KVM. Same board layout, same parts, less wires, no need to to start a new design just because now there are only two channels. Just thinking out loud.
Can't see that one myself... The main board is completely crammed. No room for more components. Two channels is all there has ever been on that board. The actual unit is made up of 3 boards stacked up (strange shaped ones to fit the case design ) plus this small front panel.
The main control board has 2 PIC16 microcontrollers, 2 Cypress USB microcontrollers (pretending to be a mouse and keyboard for each host), 3 4-port USB hub chips, a USB master chip, and various bits of glue logic. And it's completely full. No space wasted.
Then you have the audio switch board, which has 3 pairs of jacks - 2 in, 2x2 out, linked by relays. There's also 2 more USB ports linked to the main board via ribbon cable.
The third board is the VGA switch - 3 15-pin connectors, linked by simple analog switches. Nothing fancy. That board also has a good ol' 7805 on it (no heatsink).
So no, I can't see it being a down-sized 4-port or 8-port, as it's completely built and designed around 2 ports.
Given that they evidently wanted to light one of the LEDs when /G & A1, the other LED when /G & /A1, and neither if G, then if they didn't use the demux then they would have needed to use some logic. I guess they could have used a 7400, by configuring 2 of the gates as inverters.
It's quite common to see microcontrollers with the tops scratched off to hide what they are. It's also quite common to see them with black lacquer over them to hide what they are. However, for some reason they have left all the nice microcontrollers alone, and all the funky chips, but the analog muxes for the video they have painted black laquer on to hide the part numbers. And all they are are simple 4 channel analog MUX/DEMUX chips - IDT QS3257Q - nothing special.
They use the same MUX chips as part of the USB switching too, and they are also covered in laquer.
It's almost as if IDT said "Yes, you can use our chips, but we don't want to be seen to have our chips in your products."
) controlling 2 LEDs via a 2-to-4 decoder.
) plus this small front panel.