B737 AND SF340 HAD TCASII RA WITH EACH OTHER IN BOS CLASS B AIRSPACE.

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TCASII RA. DSNDING INTO BOS UNDER 7000 FT; WE WERE CLRED TO 6000 FT TALKING TO APCH CTL. AN ACR SAAB DEPARTING BOS WAS ON THE SAME FREQ. ATC CLRED THE ACR REF US CLB TO 12000 FT. THE ACR RPTED US IN SIGHT. THE WX WAS EXCELLENT. MY FO HAD VISUAL CONTACT WITH THE ACR. THEY WERE COMING FROM OUR 2 O'CLOCK LOW POS; MOVING L TO R. I COULD NOT SEE THEM VISUALLY AND MY TCASII WAS NOT SHOWING THE TARGET. FOUND OUT LATER I MUST HAVE MISTAKENLY TURNED IT OFF WHEN I WAS SETTING MY DECISION ALT FOR THE APCH. BUTTONS ARE IN A SIMILAR PLACE. TCASII ON THE B737 WILL STILL GIVE YOU WARNINGS TA AND RA IF SELECTED OFF. WE RECEIVED A TA; FOLLOWED SHORTLY AFTER BY AN RA. THE FO WAS FLYING. SHE IMMEDIATELY FOLLOWED THE COMMAND TO CLB; THEN FOLLOWED BY AN INCREASE CLB. I ADVISED ATC AND FOUND OUT LATER FROM THE TRACON SUPVR WHEN I CALLED ON THE TELEPHONE HE COULD HEAR OUR TCASII WHEN I XMITTED. APPARENTLY ATC THOUGHT THE ACR WOULD LEVEL AND ALLOW US TO PASS OVERHEAD; BUT THEY CONTINUED TO CLB UP INTO OUR PATH. IT WAS NIGHT; WHICH CAN BE DIFFICULT TO JUDGE DISTANCE. I'M SURE THINGS CAN BE DONE DIFFERENTLY TO AVOID HAZARDS SUCH AS THESE.