RS-422/485 SERIAL INTERFACE PLUS, 4-PORT
Set the 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 IRQs possible. If you are using more than one Serial Interface Plus Card or a compatible adapter in a bus you should only have one port set to “M.” Figure 3-4shows two ports sharing a single IRQ.
J1A 
N S M
J2A 
N S M
Figure 3-4. Header J1A & J2A, Shared IRQ Mode.
Set the jumper to “S” if you are using more than one Card in a bus or to completely remove the pull-down resistor for hardware compatibility. Setting the adapter in this configuration when it is not accompanied by a pull-down resistor will prevent the ports from triggering an interrupt.
