Wouldn’t it be nice if one could actually add mezzanine cards in the field to upgrade systems without powering down the system or removing the host card? This was the dream of the PICMG (PCI Industrial Computer Manufacturers Group) when they invented the AMC “Advanced Mezzanine Card.” This committee was actively working on the ATCA (Advanced Telecommunication and Computer Architecture) at the time. ATCA has a generous 30-mm card pitch with plenty of room for mezzanine cards. The committee saw the concept of being able to add or remove mezzanine cards into the front of the host boards as a significant market advantage. An ATCA system supplier could ship the box with all of these slots empty, and the end customer can add mezzanine cards as needed to add additional ports, to change port formats or upgrade functionality with higher capacity cards, all without shutting the system down and disturbing other customers served by these cards. Today’s systems can have data from many thousands of customers flowing over an individual card.
The obvious application of AMC cards is to enable lots of IO (Input-Output) flexibility, but other functions such as DSP (digital signal processing used to enhance voice quality in digital telecommunication systems,) quickly migrated to the AMC card format as well. Even complete processor modules are now available in this format. Solid State Memory or thin hard drives also fits very nicely in this format. The benefit to the end user is the flexibility to add cards when traffic demand requires it and to upgrade cards as customer demands change. To make the AMC hot plug concept work, a new generation of edge-card mezzanine connectors were developed that accept AMC cards with gold edge-card fingers. A lot of work went into achieving the signal integrity needed to make this format successful.
These gold fingers are not your grandfather’s gold fingers found in Personal Computer add-in cards for years. The gold pads on the AMC card are fine tuned to provide minimum electrical stubs and tuned for controlled impedance. Pairs of signals are flanked by ground contacts to achieve higher signal integrity performance. Edge card connectors still have the limitation that one half of the mating system is the PC board, with the inherent ±10% thickness tolerance, but on nominal 1.6-mm thick AMC cards, this is OK.
The AMC connectors and cards are used in many applications beyond ATCA. The best example is MicroTCA where edge card backplane connectors accept the same AMC cards as larger systems, dramatically increasing the market size for makers of the AMC cards. One of the major successes for the MicroTCA architectures has been in micro base stations that extend cellular networks with small base stations that can be on utility poles, or discretely positioned on the sides of buildings. This same architecture has been quite useful for smaller systems used in factory control and small telecommunication boxes providing aggregation, net security, encryption, voice quality improvement, etc.
As with other standards, the AMC card has created an industry of smaller companies who provide specialized cards that can be used in many different systems. Examples are IO cards in many different flavors from T-1 old technology hard wire telecommunication cards to 25 Gb Ethernet fiber transceiver cards all in the same form factor. Processor cards can offer the mainstream CPU chips, but also specialized graphics or network processing functionality. Storage cards can be hard disk drives, or solid state storage. This kind of mix and match capability allows system designers to use the maximum amount of off-the-shelf cards and to just focus on the special sauce they add to the system, speeding time-to-market and offering maximum flexibility to adapt to future updates in customer requirements, or available technology. This article has focused on the AMC as a mezzanine form factor. I encourage you to go to www.PICMG.com and study the AMC standards. The architecture has interesting features that enable these cards to communicate with the system as they are inserted to gracefully power up and to gracefully power down when unplugged. The control systems are quite sophisticated and are the result of many thousands of man hours thinking about all of the possibilities in order to create a control architecture robust enough for failsafe applications in telecom and computing systems.