Specifications

PCIe/104 and PCI/104-Express Specification Revision 3.0 February 17, 2015 Page 57
APPENDIX A: PC/104 BRIDGE CARD
While advancing the PC/104 family of specifications, maintaining the stackable PCI bus was chosen over the stackable
ISA bus for two reasons. First, many current and most future modern chipsets support both PCI and PCI Express. None
support ISA. Second, backward compatibility to PC/104, PC/104-Plus, and PCI-104 is mechanically easier to achieve if
the stackable PCI bus is retained over the stackable ISA bus.
To realize the stackable ISA bus, one merely needs to create a single board PCI-to-ISA bridge module using off-the-shelf
PCI-to-ISA bridge chips or FPGA cores. This will get a basic ISA bus without DMA or IRQs. With the addition of three
signals (SDMA_REQ, SDMA_GNT, and SIRQ) which are present on many chipsets and can be cabled to a bridge board,
a full ISA bus can be realized for complete backward compatibility to all PC/104 specifications without any mechanical
or electrical interference or deficiency issues.
And since the ISA bus is created using a PCI-to-ISA bridge chip, it is a natural electrical and mechanical extension to
create the ISA bus off of the PCI expansion bus to support the number of ISA legacy cards already on the market. If the
ISA bus was retained then creating a PCI bus off of the PCI Express bus would be easy electrically, but mechanically you
have problems because the both the PCI expansion bus and the PCI Express expansion bus would reside in the same
location. This would then require a two board solution to support the number of PC/104-Plus and PCI-104 cards already
on the market.