Hard limits¶

Some things cannot be simulated in software, or not on every operating system:

Limit	What it means for you
Virtual serial ports on Windows need a driver	Windows has no built-in virtual serial port pairs, so serial devices there need a port pair driver such as com0com, which Secure Boot blocks on current Windows 11, or two USB serial adapters (see windows-serial.md).
Simulator-created serial ports are not listed	The ports emupos creates on macOS and Linux do not appear in serial port lists or pickers, including the browser's Web Serial API. Open them by path. A browser POS that uses Web Serial cannot reach them.
USB devices are not emulated	USB printer-class devices and HID POS scanners cannot be emulated in software. emupos offers the same devices over TCP, serial and keyboard input.
No keyboard scanning on Wayland	Wayland does not let one program type into another's windows. Use a serial scanner, or an X11 session.
macOS needs the Accessibility permission for keyboard scans	Without it macOS silently drops the keystrokes, so emupos refuses the scan and tells you which app to allow.
Windows print queues are one-way	A POS that prints through a Windows print queue never receives status replies such as paper out. Test status over TCP. Windows also takes up to 10 minutes to show a new fault on the queue.
Windows scans into elevated windows are lost	Windows drops keystrokes sent to a window running as administrator from a normal-user emupos, and reports them as delivered. Run both at the same privilege level.
Glyph shapes are approximate	Receipt geometry is dot-accurate (paper width, columns, line breaks, images, barcode module sizes), but characters are drawn with open-licensed bitmap fonts, not the printer's own. The code pages the built-in profiles name have glyphs: PC437, PC850, PC852, PC858, PC860, PC863, PC865, PC866, WPC1252, PC720, PC864 and WPC1256. Another code page prints placeholders, while the text dump still shows the characters.
Right-to-left layout belongs to the POS	Like the printers it simulates, emupos prints the bytes it receives left to right, one character cell each: it never reorders them and never joins Arabic letters. A POS shapes and reverses its Arabic before sending it, so text that comes out backwards on paper comes out backwards here too.
Virtual serial pairs ignore baud rate and parity	Data passes whatever settings your POS chooses, while a real device would misread the bytes. Watch for emupos's framing warnings, which cover what the operating system exposes (macOS: baud rate, data bits, parity; Linux: baud rate only).