Last week I surveyed PC/104 standards and reviewed the pros and cons of the various standards.  This week I’ll look at how PC/104 compares against another small form factor standard, COM Express*.   To get things started, let’s review a few basics about PC/104 and COM Express.  Both standards enable designers to build systems using commercial, off-the-shelf (COTS) boards and modules.  The key benefit of the COTS approach is that it reduces hardware design time.  Modular solutions also make it easier to evolve designs from generation to generation.


As illustrated in Figure 1, PC/104 and COM Express are available in similar form factors.  Specifically, the new COM Express Compact form factor is 95 x 95 mm, about the same size as a 90 x 96 mm PC/104 board.  Many other COM Express and PC/104-compatible form factors are available, but I’ll focus on the PC/104 COM Express Compact form factors because they make a good point of comparison. (COM Express Compact was approved last week in a somewhat vague announcement.)



Figure 1. PC/104 and COM Express footprints.


The main differences between PC/104 and COM Express relate to their connectors.  Each standard supports a variety of connectors, but we’ll focus on two common connector types: PCI/104-Express and COM Express type 2.  As I explained last week, PC/104-Express modules can be stacked together like building blocks, and it is common to see stacks with four or more modules.  Each PC/104-Express module incorporates two connectors: a PCI Express* connector for high-bandwidth communications, and a PCI* connector for backwards compatibility with older PC/104 variants.  Backwards-compatibility is a huge asset if you want to use a legacy module in the stack.  However, this compatibility has its drawbacks.  The legacy connector eats up valuable board space that could otherwise be used for additional features.


COM Express Compact modules also use a stacking approach, but with only two boards in the stack: the COM Express Compact module plus a custom I/O carrier card.  COM Express Compact modules employ a connector that is less constrained by legacy concerns and offers better space efficiency than the PCI/104-Express connectors.


Many PCI/104-Express boards use double-sided connectors for stacking purposes.  Double-sided connectors have the obvious downside of using up twice the board space as a single-sided connector—such as like those used in COM Express.  (With a single-side connector, components can be placed on the opposite side of the board.)


As shown in Figure 2, COM Express Compact also offers more I/O than PCI/104-Express.  Note that a PCI/104-Express board can support features like Ethernet or SATA.  For example, the Intel® Atom™ processor N450-based AAEON PFM-LNP offers Gigabit Ethernet, SATA, and many other features.  However, these interfaces do not have dedicated pins on the PCI/104-Express connectors, so the signals are not available across the stack.


PCI/104-Express Connectors

C OM Express Type 2 Connector

4 x1 PCI Express links

6 x1 PCI Express links

1 x16 PCI Express link

1 x16 PCI Express link (or SDVO, dedicated I²C)

PCI Bus 32bit 33/66MHz


Not available

PATA ATA100 (1 Port only)



Not available

I²C Bus

Not available

AC’97 / Intel® HD Audio

Not available


2 x USB 2.0

8 x USB 2.0

Not available

SATA 150-300

Not available

10/100 or Gigabit Ethernet

Not available

Digital Display Interfaces

Not available

VGA (dedicated DDC)

Not available

LVDS (A&B, dedicated I²C)

Not available

GPI[0:3] GPO[0:3]

Not available

Speaker out

Figure 2. PCI/104-Express and COM Express type 2 connectors.


Another factor to consider is space constraints.  The beauty of PC/104 is that is lets you stack together a number of boards.  Thus, it may be possible to build a complete system using nothing but COTS boards.  However, the height of the stack may be a problem for some applications.  COM Express Compact uses a two-board approach with fixed vertical dimensions, potentially resulting in a more compact solution.  The tradeoff with COM Express Compact is that you must design a custom carrier board, which may increase your hardware design time relative to a PC/104-based design.


So far we’ve focused on the connectors and form factors without discussing what the boards themselves can do.  Both PCI/104-Express and COM Express Compact boards are able to accommodate high-performance processors and rich feature sets.  For example, the Kontron microETXexpress®-PC offers the following features in the COM Express Compact form factor:

  • Intel® Core™2 Duo at up to 1.86 GHz
  • Intel® GS45 chipset with DirectX* 10 capability and Microsoft* Windows* Vista support
  • Up to 4 GB  of DDR3 SO-DIMM memory
  • Dual SDVO multiplexed with PEG; HDMI and DisplayPort support
  • 3 x SATA 300 (AHCI; RAID 0, 1), 1 x PATA
  • Gigabit Ethernet


So which form factor should you choose? In a recent interview, Allen Lee, CEO of American Portwell Technology, gave an excellent answer:


  “The highly ruggedized “pin and socket” stackable interface of PC/104 is ideal for applications that require a reliable I/O interface in an environment with a lot of vibration. Alternatively, Computers-On-Modules (COMs) enable a customer with a proprietary SBC design and customized I/O requirements to create a solution that is both scalable and future-proof, should the CPU platform reach obsolescence. Additionally, new, highly integrated 32 nm CPU technologies allow a much smaller SBC footprint, thus creating smaller form factors such as Qseven and [COM Express Compact]. This benefits customers who have tight mechanical constraints and low power requirements.”


For more information on this topic, I recommend the Radisys white paper COM Express Gains on PC/104 in Military.  In addition to providing more details on PC/104 and COM Express, the paper presents useful background information on application demands.


It is clear that small form factor standards are evolving rapidly, so I will come back to these topics again in future blogs.  In the meantime, feel free to send me your questions and suggestions in the comments section below.


Radisys and Kontron are Premier members of the Intel® Embedded Alliance. Aaeon and Portwell are Associate members of the Alliance.



Kenton Williston

Roving Reporter (Intel Contractor)

Intel® Embedded Alliance


Embedded Innovator magazine