EA32 CREW HAD A TCASII RA IN ZMP AIRSPACE.

Narrative

ACR AIRLINES FLT ABC DEC/XA/99 FROM SAN TO MSP. DURING DSCNT FROM FL210 TO FL200; JUST PRIOR TO LEVEL OFF AT FL200; WE RECEIVED A TCASII TA. I WAS THE PNF SO WAS REDUCING THE RANGE ON THE NAV DISPLAY TO IDENT THE LOCATION OF THE TFC. AT THE SAME TIME; ATC GAVE A CLRNC TO US OF 30 DEGS L TURN. WE WERE IN ALT CAPTURE AT THE TIME. THE FO (PF) REACHED TO CHANGE HDG ON THE FCU TO COMPLY; AND INADVERTENTLY TURNED THE ALT SELECT KNOB AND SELECTED FL197 IN THE FCU WINDOW. THE ACFT REVERTED TO VERT SPD PRIOR TO LEVELOFF (WE WERE UNAWARE OF THIS). AT FL200 WE RECEIVED AN RA FOR THE FL190 TFC; AND AT THAT TIME I REALIZED THAT WE WERE DSNDING THROUGH FL200. AT THE SAME TIME; THE FO DISCONNECTED THE AUTOPLT TO COMPLY WITH THE RA OF 'MONITOR VERT SPD.' THE ACFT LEVELED AT FL190 AND WE PROMPTLY RETURNED TO FL200 AND RPTED THE RA TO ZMP. SUPPLEMENTAL INFO FROM ACN 456711: WHILE APCHING ONL VOR AND DSNDING TO FL200; WE RECEIVED A HDG ASSIGNMENT FROM CTR FOR TFC. I BELIEVE AT THAT TIME I MAY HAVE MISTAKENLY TURNED OR BUMPED THE ALT SELECTOR KNOB WHICH PUT THE ACFT IN VERT SPD MODE. WE DIPPED BELOW FL200 AND RECEIVED AN RA OF 'MONITOR VERT SPD.' I DISENGAGED THE AUTOPLT AND LEVELED THE ACFT AT FL200.