Went back and retested. COM3 is working- awesome! COM4 is not. I must have mistakenly tested COM4 twice. Since COM3 is functional now, I'm assuming the COM4 issue is no longer related to the slave PIC. Hopefully, I find a solution soon. Update: After much research and discussion with the manufacture...

Here's the results: PIC1 mask 11111000 PIC2 mask 10001100 Each zero (0) bit shows IRQ's that are already in use. PIC1 mask shows IRQ 3,4,5,6,7 not yet in use. PIC2 mask shows IRQ 10,11,15 not yet in use. An fb program will try to install it's own ISR with the OPEN statement and restore the PIC/IRQ/...

I see. I was hoping that someone might be able to get more hands on with the DOS port. --- AW, I made the minimal changes I think that are needed and you can test from binaries here: fbc-dos-1.10.0-20220417.zip Take care copying over your existing files. This binaries are not optimized like would b...

It's been years since I wrote the dos serial driver. Seems I left myself a note: master/src/rtlib/dos/io_serial.c /* TODO: Support for slave PIC? */ IIRC, the set-up of the second PIC is should not be too difficult (to remember / research how to do ). FYI, the driver should already support IRQ shar...

While attempting to get COM3 and COM4 working in DOS, I discovered my x86 board has IRQ(10) assigned to COM3 and IRQ(11) assigned to COM(4).
The OPEN COM command only allows setting the IRQ from 3-7. Is there anyway to increase this range?