Non-standard comport baudrates in windows

Do the windows built in com port drivers support non-standard baudrates? (actually does windows have a built in driver for com1 & 2?)

The reason I ask is I'm having trouble getting a reliable connection to a device that uses the unusual baudrate 5787. The device & PC talk briefly, then seem to loose the dialogue, and then get it again. Once a long message is sent, it gets lost at the other end, a short time later the dialogue is back. This sounds to me like the classic baudrate mismatch. Not quite close enough to be reliable though but close enough that some data gets throu开发者_运维问答gh.

If I use an inexpensive PCI serial board it works without problems. It's only computers that use on board serial I've found don't work properly.

Baudrates in a PC are controlled by a UART and a crystal. The crystal frequency determines what baudrates the serial port can generate. The baudrate is often generated by a divide by 16 counter. The crystal frequency for a standard PC is normally 1.8432 MHz. Dividing that by 16 gives you 115200 which is usually the maximum the com port can do.

Inside the UART is a DLAB register. This further divides the clock. So essentially, to get 5787 baud you're talking about dividing 115200 by 5787 which gives you 19.906687... It's close to 20 you'd load the DLAB register with 20. 115200 / 20 gives you 5760. Therefore you're probably getting 5760 baud out of the PC com port. That's probably enough of a difference to cause the issue that you're seeing.

No, the difference from 5760 to 5787 is nowhere near enough to explain any sort of problems. UARTs identify the start of a byte from the leading edge of the start bit, then sample the data in the middle of each bit. This means they are tolerant to errors in Baud rate up to the point where the predicted middle is an edge. That's a half bit error in one full byte, because each byte has a stop bit so there's a re-synchronise event per byte. On half bit in ten bits (8 data, one start, one stop) is 5%. The difference from 5760 to 5787 is only 0.5% so miles inside the safe region.