[cups] Usb simultaneous print at different printers. Concurrent printing via USB interface

[AK]

Hello!

I’ve got a problem in concurrent printing. I have two USB connected printers.
One is Canon printer, and other is ZPL printer.
ZPL printer prints much faster (3 seconds) then Canon printer (8 seconds per page).
Moreover Canon printer is in state «Sending data to printer» while it prints the page, and ZPL printer ends jobs less then in a second and doesn’t wait while label is completely printed.

I’m sending few jobs to Canon printer, and when it starts to process job (starts printing) I send few jobs to ZPL printer. And ZPL printer doesn’t start to print while job at Canon printer quits state «Sending data to printer».
If I send three job to Canon printer, and then I send three jobs to ZPL printer, ZPL printer will not do it’s jobs in a parallel, it will do it semi-parallel:
It waits while first job at Canon printer changes state from «Sending data to printer», and then will print, but then the next job from Canon queue will be started processing, and ZPL will wait while it changes state and so on.
So if I send 3 jobs to Canon, and then 3 jobs to ZPL sequence will be:
1) prints first page at Canon
2) prints first job at ZPL
3) prints second job at Canon
4) prints second job at ZPL
5) prints third job at Canon
6) prints this job at ZPL

And it is sequential. But I expect that queues are parallel and ZPL printer should not wait while job at another queue will change it’s state.

Moreover. If there is no paper at Canon and one will send it a job printer will be at state «Sending data to printer», and it will completely lock ZPL queue. No any job sent to ZPL printer will start to print.


Why do I experience this if queues are expected to be parallel? I have no network printers, I use USB, so maybe this is the point.
How can I solve this issue?