1. A computer program product for determining a state of an input/output (I/O) operation in an I/O processing system, comprising: a tangible storage medium readable by a processing circuit and storing instructions for executing by the processing circuit for performing a method comprising: receiving a request from a channel subsystem at a control unit for performing the I/O operation, the channel subsystem configured to send the request using a transport control word (TCW), the request having an associated time period for completion of the I/O operation; receiving an interrogation request from the channel subsystem at the control unit at a point in time prior to the expiration of the associated time period and absent completion of the I/O operation, the interrogation request for determining the state of the I/O operation, and sent from the channel subsystem within a threshold of the expiration of the associated time period, wherein the interrogation request includes an interrogate device command word (DCW), and the interrogate DCW is received via a transport command control block (TCCB); and sending a response from the control unit to the channel subsystem in response to the interrogation request, wherein the response includes information indicating the state of the I/O operation, a state of an I/O device performing the I/O operation and a state of the control unit.