[anonymousiam]

Glad you succeeded in getting your clone working. Here's some speculation/information that might have been helpful.

1) The old XT clones used 8250 UARTs which had no internal FIFO buffer and triggered an interrupt each time received data became available (RDA). (That is, assuming your software used interrupt-driven queues vs. polling.) Either way, the maximum usable baud rate was determined by the inter-character timing vs. the system latency + ISR/polling timing. Assuming the 8250 is socket-ed, replacing it with a 16550 series UART would greatly improve your ability to operate at higher baud rates.

2) You mentioned flow control, but there are many variants of that in both hardware and software (such as ENQ/ACK, XON/XOFF, RTS/CTS). A common problem when dealing with serial ports is that hardware handshaking is enabled by default so the OS will not send/receive data without asserting CD/RTS/CTS/DTR/DSR to the proper levels. So you can either use a cable that correctly connects the hardware handshake signals at each end (assuming the OS on each end properly uses them), or disable hardware handshaking (via software).