understanding the 4D systems programmer

[BiOzZ]

OK so i have a project on the list that involves a 4D systems PICASSO display driver

so i made a few prototypes and i only have one of there little USB programming dongles ... its awkward to use and when i send protos around im stuck without a programmer so i wanted to put on PCB the differences between a normal FTDI dongle and this one as im already getting NXP parts adding in a single transistor wont add any real cost to the PCB protos

so i took a look at it and to no surprise its an FTDI FT232RQ with 5V power and running at 3.3v UART with an PMSS3904 to make an open collector output on the DTR line

BUT looking at it there is a cap and a resistor ... the resistor clamps the base to the emitter but it appears as if the DTR line is AC COUPLED ... what?!

i probed it all around, looked at high quality scans and thought there was some sort of trickery but nope its a simple 2 sided PCB and it appears as if thats what its really like

my BEST guess is that there using this to create a really fast pulse from a longer input pulse

have any of you seen this before?

im not close to a breadboard currently to test it out and wont be for a few days (out of the state) but here are the scans and what it appears to be





SIDE NOTE: yes the pads on the bottom have the RTS and GND lines swapped im guessing for another model or backwards compatibility or something ...

[senso]

AC coupled DTR line is sort of common, the Arduinos also use that trick, so you get a fast pulse to reset your device, but not to long that the bootloader misses the first chars coming along the serial lines.