[RS232] is someone knows how emulate thru usb and if there is a loopback feature for tests ?

[techwiz03]

@ken_yap

BTW you are wrong about the 8251. You probably looked at the Intel data book and thought IBM should have specified an Intel 8251 UART. But in fact they specified a 8250 Natsemi UART, which was a bit odd because the control lines are a little different than the 8088 generates, but nothing a couple of gates can’t fix.

1. Only a true IBM shipped with 8250 Uart from national semiconductor, generic PC’s like the TI-PC, Comtac-PC, home made PC’s using multi-function cards to supply serial ports mostly used Intel 8251, or motorola ua8251a.
2. Either way, Standard serial uses CTS, RTS, DTR, DSR plus data +/- using 15volt+/- threshold. USB uses data+/- using 5volt Threshold through some magic in the USB to Serial doogle (which I have not taken apart to examine ). I presume they are level shifting +/- 5v to +/- 15volt on the data lines and just crossing the handshake lines in the doogle to make things work null-modem fashion with the old serial device.
3. When mbo’s started putting built-in serial ports as standard the motorola 16550 became the chip of choice.