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Advantech SOM-Express Design Guide
46
Chapter 5 Carrier Board Design Guidelines
5.1.4 Application Notes
5.1.4.1 REQ/GNT
These signals are used only by bus-mastering PCI devices. Most SOM-Express
modules do not have enough REQ/GNT pairs available to support a bus-mastering
device at every slot position. A PCI arbiter design is recommended when extra
REQ/GNT pairs are required. Figure 5-5 shows an example design for PCI arbiter.
33
FRAME#
1
STOP#
2
SYSREQ#
3
SYSGNT#
4
PCIREQ1#
5
VSS
6
PCIGNT1#
7
PCIREQ2#
8
VCC
9
PCIGNT2#
10
PCIREQ3#
11
PCIGNT3#
12
VC3A
13
VC5A
14
AVCC
28
PCICLKI
27
RESET#
26
AVSS
25
VSS
24
PCICLK0
23
PCICLK1
22
VCC
21
PCICLK2
20
PCICLK3
19
PCICLK4
18
VSS
17
VC3B
16
VC5B
15
MS-1
33
MS1PCICLK
PCIRST#
PCIREQ#1
STOP#
FRAME#
MS1PREQ#1
PCIGNT#1
MS1PREQ#3
MS1PGNT#2
MS1PREQ#2
33
MS1PGNT#3
MS1PGNT#1
33
33
PCICLK1
PCICLK2
PCICLK3
Figure 5-5 Design Example: PCI Arbiter
If there are less than four REQ/GNT pairs available for external devices, they will be
assigned starting with the REQ0#/GNT0# pair. Therefore, external bus-mastering
devices should be placed in the lowest numbered slot positions and non-bus
mastering devices should be placed in the highest-numbered slot positions. Refer to
Chapter 2.3.1 REQ/GNT for details.
5.1.4.2 PC104-Plus Connector
If a PC104-Plus connector is used, the same signals are attached to the connector
but the pin numbers differ because of the different connector type. See the PC/104-
Plus Specification Version 1.0, February 1997, PC/104 Consortium (www.pc104.org)
for details.
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