Introducing PC/104-Plus
Introducing PC/104-Plus
Brief History
In February 1992, Ampro converted its "MiniModule" form-factor into an open standard called the PC/104 Standard, and created the non-profit PC/104 Consortium to serve as custodian of the new standard. PC/104 rapidly became the most popular embedded-PC standard, used almost universally as the modular basis of many types of embedded systems. Over the past four years, several hundred companies joined the PC/104 Consortium and together offer thousands of different PC/104 modules and support products and services targeted to embedded applications where compactness, ruggedness, reliability, and functional modularity are required.
As desktop-PC CPU speeds skyrocketed, the 5 MBytes/sec maximum throughput rate of the PC bus has driven desktop-PC manufacturers to add a second, higher speed bus to PCs. In this process, the Peripheral Component Interface (PCI) has become the add-on bus of choice, thanks to its high data throughput (133 MBytes/sec) and processor independence. Today, nearly every new desktop-PC comes equipped with PCI expansion slots.
Would PCI impact embedded-PCs in a similar way? Embedded applications span a very wide performance range. Many of today's PC/104-based embedded systems have evolved from previous implementations based on 8-bit microcontrollers (e.g. 68HC11, 8051, and Z80). These applications are well served by 16- and 32-bit PC-compatible CPUs (e.g. the Intel386) at clock rates from 16 to 33 MHz. For these systems, the 5 MBytes/sec bandwidth of the original PC/104 (ISA) bus is more than adequate.
However, with the arrival of the Intel Pentium* processor in PC/104-based embedded-PCs, the 5 MBytes/sec ISA bus throughput becomes an obstacle to taking full advantage of the CPU's processing power. Therefore, it became necessary to find a way to add PCI to PC/104.
Objectives in adding PCI to PC/104
First, Ampro explored the suitability of several existing PCI adaptations to other buses and form-factors (PMC, PICMG, CompactPCI, SPCI, and CardBus). None matched the key attributes of PC/104 that have made it so successful in embedded applications, namely:
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- Stackable and usable along with standard PC/104 modules (in the same stack)
Finding a PCI bus connector that met the above list of requirements was a significant challenge. After an exhaustive search, it was determined that nothing suitable was currently available. Ampro therefore entered into a business arrangement with Samtec, manufacturer of the original PC/104 (ISA) bus connectors, to create a new self-stacking 120-pin high-density (2mm) connector. The two companies also teamed up to develop a unique connector pin "shroud," that would: (1) guide the male portion of the PCI connector as it mates with the female portion of the next connector in the stack; and (2) protect the PCI connector pins, which are slightly thinner and therefore somewhat more vulnerable than those of the ISA connector.
The resulting combination of PC/104 with PCI, called "PC/104-Plus," meets all the objectives listed above. In addition, extensive electronic simulations have been used to validate its reliability for both current 33 MHz PCI bus chipsets and future 66 MHz PCI silicon.
PC/104-Plus overview
Figure 1 shows the basic mechanical dimensions of a PC/104-Plus module. The module's form-factor matches that of PC/104, so PC/104-Plus can be said to be "PC/104 form-factor compliant". The new self-stacking 120-pin high-density (2 mm) PCI bus connector fits between the standard PC/104 mounting holes along the edge opposite the regular PC/104 (ISA) bus, so it consumes minimal space (approximately 10%).
Figure 2 illustrates a typical PC/104-Plus stack. 8- and 16-bit PC/104 (ISA) modules can be stacked along with 32-bit PC/104-Plus (PCI) modules, provided that similar type modules are adjacent to each other. This results in maximum system expansion and configuration flexibility.
Here is a brief summary of the basic specs of PC/104-Plus modules:
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PC/104-Plus, like PC/104, is essentially a repackaged version of PCI that is optimized for the unique requirements of embedded systems — where space is scarce and ruggedness is paramount.
Although PC/104-Plus modules provide connectors for both buses, the "120-pin PCI bus" and the "104-pin ISA bus," only the PCI bus usually connects to circuitry on such a module. The ISA bus connectors are only there to pass the ISA bus on for possible use by the next module in a stack.
Current status of the PC/104-Plus standard
As with the original version of PC/104, Ampro announced its intention to make PC/104-Plus an open, public domain standard. Accordingly, Ampro offered the new standard to the PC/104 Consortium to serve as the basis of a next generation, PCI-enhanced version of the PC/104 standard. In response to Ampro's offer, the Consortium formed a PCI Extensions Working Group "to review, revise, and potentially adopt" the Ampro-proposed standard. In February 1997, the PC/104-Plus standard was formally adopted by the Consortium's voting members and is now available from the PC/104 Consortium (415-903-8304).
PC/104-Plus shows every indication of following in the footsteps of its predecessor. Numerous companies (including Motorola Computer Group) have announced support or begun development programs based on PC/104-Plus. Products being developed high performance single-board computers, full-motion video interfaces, high speed LANs, communications interfaces (100BaseT, USB, IEEE-1394, etc.), high performance data acquisition and control interfaces, and PCI bus adapters and bridges.
Comparison Of PCI Form-Factors
Desktop PCI |
Passive Backplane PCI |
PMC |
Compact- |
Card- |
Small- |
PC/104- |
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Dimensions |
Long: |
12.3 x 3.9 |
5.9 x 2.9 |
6.3 x 3.9 |
3.4 x 2.1 |
3.4 x 2.1 |
3.8 x 3.6 |
Area |
Long: 48 |
48 |
17 |
25 |
7 |
7 |
13 |
Bus Connector |
Edge-Card |
Edge-Card |
Pin & Socket |
Pin & Socket |
Pin & Socket |
Pin & Socket |
Pin & Socket |
Includes ISA bus |
No |
Yes |
No |
No |
No |
No |
Yes |
Installation Plane |
Perpen- dicular |
Perpen- dicular |
Parallel |
Perpen- dicular |
Parallel |
Parallel |
Parallel |
Expands Without Additional Slots (Self- |
No |
No |
No |
No |
No |
No |
Yes |
Positive retention |
No |
No |
Yes |
Yes |
No |
No |
Yes |
Standards Body |
PCI-SIG |
PICMG |
IEEE |
PICMG |
PCMCIA |
PCI-SIG |
PC/104 |
Primary Application Area |
Desktop: |
Industrial: |
Industrial: |
Industrial: |
Laptop: |
Laptop: |
Embedded: |