SECOMX-945/N270
User Manual
- Rev. First Edition: 0.1 - Last Edition: 2.1 - Author: S.B. - Reviewed by G.M.
Copyright © 2011 SECO s.r.l.
SECOMX-945/N270
pag. 37
4.7.2 IRQ Mapping
IRQ
State
Note
0
Reserved
Timer tick
1
Reserved
8042
2
Reserved
IC
3
Available
Usually assigned to COM2, can be varied using BIOS setup
4
Available
Usually assigned to COM1, can be varied using BIOS setup
5
Available
6
Available
If LPT interface has to be used in Floppy Disk mode, leave
this IRQ line free
7
Available
8
Reserved
RTC
9
Reserved
10
Available
11
Available
12
Available
13
Reserved
Coprocessor
14
Reserved
IDE0
15
Reserved
IDE1
4.7.3 ISA IRQ mapping considerations
On ISA Bus there can be up to 8 IRQ signals, that are contemporaneously available to all devices
connected to the bus. These IRQ lines are IRQ 3, 4, 5, 6, 7, 10, 11, 12.
SECOMX-
945/N270 board doesn’t offer ISA bus, but this can be derived on the Carrier Board from
PCI bus using a PCI-to-ISA bridge.
Since it is not generally possible to know which devices are connected to ISA bus, usually each
PCI-to-ISA bridge performs a subtractive decoding, i.e., it makes available all resources that are
not already used from PCI bus.
This means that, to assign a specific IRQ free for ISA bus, it must not be used by COM ports,
Floppy Disk Controller (that always use IRQ line #6) and/or PCI INTx lines.
COM ports require a not-shared IRQ. Usually COM1 and COM2 use IRQ lines 4 and 3,
respectively, but this IRQs can be changed, as written in paragraph 4.5.2.
PCI INTx lines can be singularly assigned (see par. 4.4.4, 4.4.5, 4.4.6 and 4.4.7), but they can also
be grouped together.
So, if you need to assign a specific IRQ to ISA Bus, it is sufficient to assign IRQ lines to Super I/O
(COM ports and LPT or FDC port, see par. 4.5.6), to PCI lines and LAN section (see par. 4.4.8),
leaving at least one IRQ line free. That IRQ line will be then automatically assigned to ISA bus.
IRQ Table assignment present in par. 4.7.2 can be useful to choose which IRQ line leave free for
IRQ.
