HP 2500C Series - Manual (Page 146)

Functional Overview 6-26
Firmware
Firmware is the collective medium comprising of the computer, the printer mechanism
and the key panel that enables all three components to communicate and interact. The
printer by itself does not independent capability to process printing data (such as print
mode and byte stream), obtaining this data from the software-driven printer driver.
The printer is thus dependent on the host for the processing and sending of job data. The
HP 2500C, however, supports full duplex (bi-directional) printing and is able to give
extensive feedback on print status to the host computer.
This is an overview of the firmware, summarizing the flow of data:
(firmware data flow diagram)
The firmware operates on three layers: the physical layer, the data link layer and the
application layer.