Dell 2135cn Service Manual page 110

Reference_2:
1. Make sure that the printer is connected to the computer via USB port (remove the network cable).
Then, try downloading as follows:
1) Power on the printer while pressing the <X Cancel> and < > buttons.
2) The printer goes into the Download Mode with a message "Download Mode Ready". Then,
activate firmware update tool and follow the instruction displayed.
While the firmware download is being executed, the printer displays a message
"Writing...USB F/W" and the computer displays a progress bar and may be
restarted during the downloading process. Never power off the printer or the
computer until the downloading process completes, and never interrupt the
downloading process.
Version 4.2 2009.08.25
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Chapter 1 Troubleshooting