B190 FLT CREW WITH A90 EXPERIENCED CONFLICT AT 10000 FT DUE TO OPERROR.

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ACR X DEPARTED BOS RWY 4L. FO WAS THE PF AND I WAS THE NPF. WE WERE SUCCESSIVELY GIVEN CLBS OF 4000 FT; 5000 FT ON BOS DEP FREQ 133.0. WE THEN WERE SWITCHED TO FREQ 118.25 AND WERE GIVEN CLBS OF 7000 FT AND 14000 FT (OUR FILED CRUISE ALT WAS 15000 FT). FROM ABOUT 5000 FT; WE WERE IN MODERATE TURB; SO WE WERE CLBING AT 160 KIAS AND GETTING ABOUT 2000+ FPM. CLBING THROUGH ABOUT 9500 FT; THE FOLLOWING OCCURRED: APCH CTLR (NORMAL VOICE): 'ACR X; DSND AND MAINTAIN 8000 FT.' I THOUGHT I MIGHT HAVE HEARD THE CTLR INCORRECTLY; SINCE THE CALL DID NOT SEEM TO MAKE SENSE AND IT WAS SAID IN A NORMAL TONE OF VOICE. NO MENTION OF 'TFC' OR 'TFC ALERT' ON THE CTLR'S PART. CAPT SAID: 'WAS THAT FOR ACR X?' APCH CTLR (PANICKED URGENT VOICE): 'ACR X; DSND TO 8000 FT ! DSND NOW! DSND NOW!' I IMMEDIATELY GRABBED THE YOKE; PITCHED DOWN 20 DEGS; PULLED THE PWR TO IDLE AND SAID 'MY CTLS;' TAKING THE ACFT FROM FO (HE HAD STARTED THE DSCNT MANEUVER; BUT NOT AS RADICALLY AS I THOUGHT WAS APPROPRIATE). THOUGH THE CTLR HAD NOT ONCE SAID THE WORD 'TFC;' I GLANCED AT THE TCASII IMMEDIATELY AFTER I HAD ESTABLISHED THE DIVE AND THERE WAS A TARGET OVERLAPPING WITH OUR ACFT AT THE SAME ALT (00 INDICATED AS ALT DIFFERENCE). WE WERE IN A 4000+ FPM DSCNT AND I WAS PRAYING WE WOULDN'T COLLIDE WITH THE OTHER ACFT; WHO I DETERMINED ABOUT 10 SECONDS LATER (WHEN THE TCASII WAS SHOWING ABOUT 1000 FT OF VERT SEPARATION) WAS LEVEL AT 10000 FT ON AN IFR FLT PLAN. WE WERE IMC IN HVY RAIN AND MODERATE TURB; AND WE NEVER SAW THE OTHER ACFT. I WOULD ESTIMATE THERE WAS NO VERT SEPARATION AND HORIZ SEPARATION OF LESS THAN 1/4 MI. ACR X DOES NOT HAVE TCASII; AND THE TCASI NEVER SAID 'TFC; TFC' OR ANYTHING ELSE. THE UNIT TESTED OK DURING FIRST FLT CHKS AND SUBSEQUENT TESTS AFTER THE EVENT. DURING THE TIME THAT WE INITIATED OUR DSCNT UNTIL WE WERE LEVEL AT 8000 FT; THE FREQ WAS ABSOLUTELY SILENT. THE NEXT RADIO CALL WAS FROM A DIFFERENT APCH CTLR WHO HAD IMMEDIATELY STEPPED IN TO RELIEVE THE CTLR WHO HAD GIVEN US THE CLB TO 7000 FT AND 14000 FT AND ORDERED US TO DSND. THE NEW CTLR SUBSEQUENTLY CLBED US TO 14000 FT AND HANDED US OFF TO ZBW ON FREQ 128.2. WE WERE NOT WITH CTR FOR 5 MINS WHEN THE FOLLOWING CONVERSATION OCCURRED: BOSTON CTR: 'ACR Y; BOSTON TRACON WOULD LIKE TO TALK TO YOU WHEN YOU GET ON THE GND.' ME: 'I WOULD BE INTERESTED IN TALKING TO THEM; TOO; DO YOU HAVE A PHONE NUMBER?' THIS NEAR COLLISION COULD HAVE BEEN AVOIDED BY REQUIRING ALL PART 121 CARRIERS TO HAVE TCASII. IN ALL LIKELIHOOD; I WOULD HAVE GOTTEN A 'MONITOR VERT SPD' OR A 'DSND; DSND' COMMAND SHORTLY AFTER PASSING 8000 FT. BECAUSE THE ACFT ONLY HAD TCASI; THERE WAS NO COMMAND GIVEN. ADDITIONALLY; THE INSTALLED TCAS UNIT FAILED TO EVEN CALL 'TFC; 'TFC' EVEN THOUGHT IT PASSED THE SELF-TEST BEFORE AND AFTER THE INCIDENT. PRECISELY WHY IT FAILED TO ISSUE AN ALERT IS NOT KNOWN.