ZPL ~HS not working on some printers

Question

I have several Zebra ZM400 printers connected to the network and am working on customizing our printing application. The program uses the ~HS command to check the status of the printer before printing. Most of the printers work but some of them just hang when I type ~HS I get no response. I've used telnet to access the printer via 9100 and confirmed that it is indeed not working properly. As far as I can tell both printers are configured identically. I'm looking any ideas to help figure out why it is not working on all printers.

Model: Zebra ZM400
Firmware: V53.17.13Z

Answer 1

Stumbled upon the answer the other day. The output port of the print server had to be change to bidirectional communication.