Sense Command

The sense command is used to receive both the status of the device and any information about any unusual or error condition for the last operation of the device.

Although sense information is specific to each device usually the first six bits of sense data give information about the conditions detected in the last operation.

Command Reject (Bit 0) indicates the device has detected a programming error.  The device is unable to execute the command because it is invalid for the device or the device cannot currently execute the command (for example the device is write protected).  Command reject can also indicate an invalid sequence of commands.

Intervention Required (Bit 1) The operation could not be executed because the device requires some type of intervention (for example printer needs paper) or the device is in a not ready state.

Bus Out Check (Bit 2) The device has received data with an invalid parity.

Equipment Check (Bit 3) Indicates the device has detected an equipment malfunction.

Data Check (Bit 4) Indicates the device has detected a data error other than those causing a Bus Out Check.

Overrun (Bit 5) Indicates the channel has failed to respond on time to a request from the device.

The final type of CCW command code is the Transfer In Channel (TIC).  It is a branching instruction for the channel command.  Normally when  command chaining is active CCWs are executed sequentially.  A TIC will cause the next CCW to be fetched from the data location specified by the data address in the TIC CCW.  The first CCW pointed to by the CAW cannot be TIC nor can a TIC specify a branch to another TIC CCW.

[Next – Channel Status Word]

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