| COMMAND | DESCRIPTION | VALUE |
| TOUT | A timeout has occurred. The destination board has failed to respond. | 0x544F5554 |
| ROUT | The controller is currently reading out image data. The only timing board command accepted by the PCI board is ABORT_READOUT. | 0x524F5554 |
| DON | The command succeeded without error. NOTE: not all commands return 'DON'. Some commands respond with other replies, like TDL, which returns the argument sent by the user. Refer to the command documentation for details. | 0x00444F4E |
| ERR | Error. The command failed. | 0x00455252 |
| SYR | System reset. Sent in response to a power-cycle or the PCI RESET_CONTROLLER command. | 0x00535952 |
| RST | Reset. | 0x00525354 |
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