USB Logging vs Logic Analyzer – Difference Between USB Logging Software and Logic Analyzer for Printer Signals

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In my recent pursuit in exploring the workings of an inkjet printing head, I need information like the default firing voltage and waveform set by the printer's manufacturer for different instructions sent to each pin on the printing head.

Assuming that the printer is connected to the computer via USB, can I use an USB Logging Software (eg. USBlyzer etc.) installed on the computer, in place of a Logic Analyzer or Oscilloscope(probing the pins of the printing head) to get the information for the purpose above?

What is the main difference between such a software and hardware approach of measurement, apart from the cost?

Best Answer

Assuming that the printer is connected to the computer via USB, can I use an USB Logging Software (eg. USBlyzer etc.) installed on the computer, in place of a Logic Analyzer or Oscilloscope(probing the pins of the printing head) to get the information for the purpose above?

In short, no.

The universal serial bus (USB) has one job: transferring commands and data from the computer to the printer. For printers, the printer commands and data formats are defined by one or more industry standards--e.g., Hewlett-Packard's Printer Command Language (PCL). As an example, these HP PCL commands inform the printer of the print job's paper size and orientation:

unsigned char  pcl_paper_size_us_letter[] = { '\x1b', '&', 'l', '2', 'A' };
unsigned char  pcl_paper_orientation_portrait[] = { '\x1b', '&', 'l', '0', 'O' };

The first character of each PCL command is the ASCII ESCAPE 'ESC' caracter (0x1b) which is a non-printing character. Therefore these commands are commonly referred to as "printer escape codes" or "printer escape sequences".

A microprocessor within the printer receives the incoming information from the USB. The microprocessor decodes the PCL commands and data and then generates the required logic signals that actuate the hardware that physically performs the commands.

COMPUTER <--USB--> [ <-> uP <-> (other_hardware) <-> PRINT_HEAD ]
                   \------------------ PRINTER -----------------/

The PCL commands the computer sends the printer via the USB do not directly define hardware logic signals. It is the microprocessor's job to decode the incoming PCL commands (to determine which specific PCL command was received) and to generate the required sequence of logic signals that actuate each PCL command on the printer's hardware.

If you want to reverse engineer how a printer's microcontroller controls the printer's print head you could start by mapping specific PCL commands to specific hardware signals as captured by the logic analyzer, or the oscilloscope, or both. You probably need to find or create a custom program that runs on the computer, that allows you to send individual/specific PCL commands from the computer to the printer so that you can then capture with a logic analyzer the resulting logic signals, or capture with an oscilloscope the resulting analog voltage levels, or both, at the print head.

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