Adding a 3rd serial port (help needed)

[Walter D|rr]

My current Microport installation is configured with two serial ports
(tty0/1) using the standard COM1:/COM2: serial interfaces.
Now, I would like to add a third serial port (tty2) to the system. 

The new serial board is a standard dumb board (similar to the one used for
tty0/1). The switches on the new board are set to use IO address 0x3e8 
and IRQ 4.
(I can access the board under DOS as COM3: using Telix).
I used ttypatch to patch the kernel and mknod the /dev/tty2 file.
(ttypatch -t2 -i4 -a1000)

Now for the problem:
"echo test > /dev/tty2" hangs until a CR is typed on the terminal
connected to /dev/tty0.
Output to tty2 occurs only when there is output to tty0:
"cat /etc/passwd > /dev/tty2" hangs.
Then, when I type "echo test > /dev/tty0", four characters from the password
file are displayed on the terminal connected to tty2.

The documentation in the runtime manual and the online man page for 
ttypatch give the impression that it is possible to use up to eight ports
on one interrupt (either IRQ 3 or 4), so my setup above should work.

Are there any other kernel location besides si_ints, ctl or mcr that must
be patched to correctly install a third serial port? 
I even tried several values for mcr (0,4,8,12) but without any success.

(I can access the new serial board without problems if I patch ctl to 0x3e8.
Then the new board becomes tty0 and works correctly. That would rule out
a hardware problem with the new board, I guess.)


Walter