SENDCMD →     RPL: ACB field → ACB
              AREA field → header and command to be sent
              RECLEN field = length of header and command to be sent
              Note: ECB and EXIT are mutually exclusive.
              If ECB is used, only one is allowed.
              ECB field → external ECB
              ECB field = internal ECB
              EXIT field → RPL exit routine
              BRANCH field = YES|NO
              OPTCD field = SYN|ASY
 
     Register 0 and 15 = return codes
 
     RPL:   RTNCD field = recovery action return code
            FDB2 field = specific error return code
            REQ field = request code (39)