The SCSI Test Unit Ready command (opcode 00h) is used to determine if a SCSI device is ready to transfer data (read/write). The SCSI target device will then return either good status or a “check condition”. Implementation of this command is mandatory for target device.
This command allows an SCSI initiator to poll a logical unit until it is ready. At the end of this command the SCSI target returns a Status Code byte which is usually 00h => success, 02h => Check Condition (error), or 08h => busy.
When the target returns a Check Condition in response to a command, the initiator usually then issues a SCSI Request Sense command (03h) in order to obtain more information/sense key and additional sense codes.
Below is 6 byte CDB structure for this command.
Reference: SPC-4 doc