Black Box Computer Hardware IC181C 用户手册

Jumper E5 selects the interrupt mode for each port. Each port must be set 
in the correct mode to ensure proper installation.

“N” indicates the normal, single interrupt per port mode (Figure 4-2). “S”
indicates the shared interrupt mode, which allows more than one port to
access a single IRQ (Figure 4-4). “M” indicates the inclusion of a 1K ohm 
pull-down resistor required on one port when sharing interrupts 
(Figure 4-3).

Set jumpers to “S” for shared interrupt mode on all blocks sharing an IRQ
except one. Set that port block for “M.” This provides the pull-down resistor
circuit that makes sharing of IRQs possible. If you are using more than one
Four Port RS-232 Serial Interface or a compatible card in a bus, you should
have only one port set to “M.” Figure 4-3 shows all four ports sharing a single
IRQ.