vt100 terminfo with padding for an actual vt100?

No, that's a matter of what next step modulation scheme had become feasible. It's possible to imagine incompetently designed UARTs that do odd things. But given a normal UART, the first step it performs is to frame the character and the bits. That depends on an adequate signal, and a sufficiently accurate clock. After that step, you have a received character which should be correct. Next, that character has to be buffered and delivered to the consuming system. That probably involves a FIFO, and either an interrupt or DMA. If there is no room to buffer the character, it will be discarded, hopefully with a status in the UART registers to indicate that. But either the character will be buffered as received, or not at all -- it would not be modified at this point. Finally, if an XOFF is received, it has to be seen by the element that acts on it (either something in the serial port hardware, or the driver, or the application software) which has to stop the output stream. If that takes too long, or the output buffers beyond the stop point are too large, too many characters may get out after the XOFF was sent. That would generally cause a buffer overflow, which will drop characters as described above but will not modify them. You mean at the USB level? Or in a computer serial port? If the latter, the only timing issue I can think of would be baud rate clock issues. BTW, a serial port baud rate generator is a programmable divider. The range of speeds supported has changed over time; early on it was 110 to 9600 (or sometimes lower, say 110 to 2400). The clock crystal was chosen to deliver "sufficiently accurate" frequencies, taking into account the way the bit framing was done in the particular UART design. Some of those generators could do 19200 baud as well but that was often not supported because there the frequency was off too much. If you used it anyway, and both ends use the same generator, all would be well. You might be doing 19,900 baud rather than 19,200, but that's ok so long as the ends agree. But newer serial ports have a much higher upper limit, which means they have a higher frequency crystal and the individual error values for the individual speeds will be different. For the ones that were in spec all along that should not be an issue; for ones that were out of spec, it probably would be. paul