This may conflict with a power supply in the stack. PCI Signaling Levels the PCI bus may operate at +3.3V or +5V signaling levels. Some boards may expect one voltage, others may expect a different voltage. PCI VIO Line the VIO signal on the PCI bus is supposed to be driven by the CPU board. However, some peripheral boards and power supplies drive this signal (in violation of the PCI specification). ISA Bus with modern chipsets the legacy ISA bus has been omitted from modern chipsets. Newer CPU boards that provide an ISA bus often do so by utilizing an ISA bridge chip (either PCI-to-ISA or LPC-to-ISA). In some cases, the ISA bus is not fully implemented, and compatibility issues may appear. 4 Bus Masters the PC/104-Plus and PCI-104 specifications did not originally allow for 4 sets of Grant/Request pairs on the PCI bus. This was resolved in newer revisions of the specification. However, older boards may have issue using DMA in the 3rd or 4th PCI slot position. Software development The majority of PC/104 CPU boards are x86 compatible, and are capable of running commercially available off-the-shelf PC software without modification. The standard PC I/O interfaces of a PC/104 system (Serial Ports, USB, Ethernet, VGA, etc.) are typically supported via the native drivers built into the operating system. Certain peripheral boards, such as data acquisition may require special drivers from the board manufacturer. From a software development perspective, there is little difference between compiling software for a desktop PC or compiling for an x86 PC/104 stack. Software can be developed using standard x86 compilers (e.g. Visual Studio if the PC/104 system is running Windows). There is typically no need for specialized development tools, such as cross compilers, Board Support Packages, or JTAG debuggers. This is a significant departure from non-x86 embedded system platforms, which often require a development toolchain from the board manufacturer. Non-x86 PC/104 CPU boards based on ARM or PowerPC are also commercially available. However, such boards are not capable of running off-the-shelf PC software. In these cases, a Board Support Package is usually provided by the manufacturer for the supported operating system(s). Spelling variations and abbreviations When printing "PC/104" or its variants, it is common for the forward slash or dashes to be omitted. PC/104 may be abbreviated as PC104, PCI-104 abbreviated as PCI104, etc. Additionally, it is common for PC/104-Plus to be abbreviated with a