I have a Zebra GK420 printer connected with a serial cable. It works for few labels jobs but if I send many many labels I get a "Full buffer error". I tought it can be the host handshake than I checked the settings. It is set as "xon/off" in the printer and also in windows printer port configuration. What else can be? It is correct to suspect about the handshake?
It’s probably exactly what it says: a full buffer.
This puts limits on how much data you can send to it.
What should be happening is that when you send more data than it can fit in memory, it reports that it is full and Windows adds the rest to the print queue, and sends them as each job finishes, but for some reason it is sending them all at once and the printer is running out of room to store them.
The most likely explanation is that the printer driver is having a problem. Are you using the correct driver? Are you using the latest driver? Have you modified the settings?
If you said no to any of these, then you should try uninstalling the driver (and other printer software), then installing the latest version of them with the default settings.
Look in the advanced print settings and see if they gave you an option to "buffer single label" (or "buffer single card" or "buffer single page") - that keeps the computer from sending the next (label/card/page) until the current one is finished. I have a funky old printer that does strange things to print jobs of more than 3 items without that set (it has a whopping 4MB of memory on its own, and does not seem to do a great job communicating about that with the computer.)