[comp.sys.ibm.pc.misc] Interrupt sharing

f89ba@rigel.efd.lth.se (04/09/91)

I'm planning on letting my modem (COM3) and the
LPT2 port share IRQ5. This should work since I don't use
the second parallel port but need an extra serial port.

However, is it possible to let two devices (in particular
a serial port and a parallel port) share an interrupt
line without the risk of damaging the computer's hardware?

Although not particularly well informed in this area, I could
imagine a scenario with one device raising the signal and
the other one lowering it, causing a "short circuit" between
+5 volts and 0 volts, or am I way off here?

In either case, I would appreciate some facts before I flip
the dipswitch!

Please email any responses since I cannot keep up with
comp.sys.ibm.pc.misc

  Bengt Ask, Lund Institute of Technology, Lund, Sweden  

ntaib@silver.ucs.indiana.edu (Nur Iskandar Taib) (04/11/91)

>However, is it possible to let two devices (in particular
>a serial port and a parallel port) share an interrupt
>line without the risk of damaging the computer's hardware?

>Although not particularly well informed in this area, I could
>imagine a scenario with one device raising the signal and
>the other one lowering it, causing a "short circuit" between
>+5 volts and 0 volts, or am I way off here?

I always thought the computer's bus uses assertion-
level logic, i.e. if one device causes a line to go 
high, the line will go high no matter what the other
devices on that line are doing. The equivalent of a 
logical multi input OR. Assertion level logic devices
require a terminator resistor between the line and 
ground. 
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Iskandar Taib                        | The only thing worse than Peach ala
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