COMMUTER MDT ALT DEV EXCURSION FROM CLRNC ALT IN RESPONSE TO TCASII RA.

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THIS WAS A TCASII SITUATION WHERE OUR ACFT WAS AT 3000 FT MSL APCHING ROCKFORD. TCAS 'TFC ALERT' ACTIVATED WHICH SHOWED TFC AT 10 O'CLOCK AND 2 MI. ATC (RFD APCH) THEN ADVISED US OF TFC AT 9-10 O'CLOCK AND 3 MI AT 2500 FT. THE VISIBILITY WAS APPROX 1/4 MI (IMC) IN HAZE. WE RESPONDED WITH 'NEGATIVE CONTACT'. TCAS SYS THEM ISSUED A RESOLUTION ADVISORY INDICATION AN 1800 FPM CLB WAS NECESSARY TO AVOID A MID-AIR COLLISION. THE CAPT INITIATED A CLB AS I ADVISED ATC WE DID NOT SEE THE TFC AND WERE CLBING OUT OF 3000 FT TO AVOID IT. AS WE ASCENDED TO 3300 FT; TCAS SHOWED TFC PASSED BEHIND US BY 1/2 MI. THEN TCAS ISSUED 'CLR OF CONFLICT.' WE DSNDED BACK TO 3000 FT AND ADVISED ATC. THE TCASII SYS WAS REQUIRED TO BE IN OUR ACFT BY THE FAA AND WITHOUT VISUAL CONTACT WE MUST OBEY THE COMMANDS ISSUED BY TCASII. THERE BECOMES A DEFINITE CONFLICT OF INFO WHEN ATC HAD VERIFIED AN ACFT'S ALT BUT TCAS SEES IT AS A THREAT. NOT ALL ATC FACILITIES ARE INFORMED ON THE PARAMETERS WHICH CAUSE TCASII TO ACTIVATE AND THEREFORE CONFLICTS OCCUR. WE DID CALL ROCKFORD APCH FACILITY AND EXPLAIN THIS TO THEM IMMEDIATELY AFTER THE INCIDENT.