HP PCL/PJL reference (PCL 5 Printer Language) - Technical Reference Manual Part I

16-26 Status Readback EN
If the application sends the Echo command and Free Space
command, the printer returns the response to the first Echo command
and Free Space command, along with the response to the second
request. The application will assume that the response received is the
response to its last request. If the application uses random numbers
in the Echo command, this type of mix-up will not occur.
Echo Response
The Echo command returns the following response:
PCL
ECHO ValueField
where “
ValueField” is the Echo command value field value that
was selected (within the range –32767 to 32767).
For example, if the Echo command,
E
C
*s–999X was sent the status
response would be:
PCL
ECHO –999