New Hybrids Rock the Market

Larry Desjardin- October 11, 2012

No, we aren’t talking about cars- we’re talking about PXI chassis.  Three manufacturers, Agilent Technologies, Adlink, and recently National Instruments are now offering fully-hybrid PXI chassis.  You can read about hybrid slots in my recent article about PXI hardware integration here, but let’s review the basics.

When PXI was introduced, it was based on the PCI bus that was found in all personal computers.  It was a parallel memory-mapped bus that went to all slots of a PXI chassis.  It is referred to as PXI-1, and many modules and chassis still retain this architecture.  With 32 parallel lines clocking at 33MHz, a maximum 132MB/s data transfer rate was the theoretical maximum.  Not bad, but not much faster than Gigabit Ethernet.  An additional constraint was that the bus bandwidth was shared among all devices.

Along comes PCIe (PCI Express) in the mid-2000s.  It brought several benefits.  First, it retained the PCI memory map, so it acts logically as a PCI bus, and could be mixed and matched with it's predecessor. In its most common configuration of 4 lanes to each device, first generation PCIe delivered 1GB/s transfer rate in each direction, a gain of about eight over standard PCI.   However, since it is physically a point-to-point serial bus, any slot can deliver that speed simultaneously with any other. Multiply up by the number of slots, and you find a very impressive aggregate bandwidth.  And that’s just for Gen 1. Each generation doubles that bandwidth.  Also, simply add more lanes for more bandwidth.  It's like sipping a quadruple espresso.

You can see the benefits.  The PXISA (PXI System Alliance) adopted PCI Express, and those modules are called PXI Express. But there’s already the existing PXI-1 standard with many modules that don’t need the improved performance.  What can you do to take advantage of both?

This led to a second innovation by the PXI standard: the hybrid slot. Technical experts noted that while PXI-1 defined two connectors at the rear of any PXI-1 module, it was the bottom connector J1 that was always required. The most common functions of upper J2 connector could be done with half the pins. They also noted that PCIe, being a serial protocol, required many fewer pins as well. So in the space that J2 existed, engineers could place two smaller connectors, called XJ3 and XJ4. Essentially, XJ3 delivers the new PXIe functionality, and XJ4 connects specific PXI-1 J2 signals to the backplane. A hybrid slot is a chassis backplane slot that has all three of these connectors: P1, XP3, and XP4 for connecting to J1, XJ3, and XJ4 respectively.

This allowed makers of the older PXI-1 modules to upgrade to a PXI-1 Hybrid Compatible module simply by replacing the J2 connector with the XJ4 connector.  PXI-1 Hybrid Compatible modules are often referred to as PXI-H.  PXI-H modules and all PXIe modules can be plugged into the same slot.  Chassis can now be designed with a large number of hybrid slots that bring back the concept of “any module into any slot.”

Problem solved?  Not quite.  Making all slots hybrid-compatible is very difficult.  This is because the large PCIe switching ASICs take space on the backplane where the connectors have to be.  Hybrid connectors leave no room for other components.  Without disclosing details, Agilent, Adlink, and NI have all created some secret sauce that performs the switching without taking any backplane space.

Here’s what this means to you.  Any PXI module in any slot. Mix and match any PXI instruments, and place them in the chassis locations you want, not those restricted by the vendor.  Put the modules that are mutually linked next to each other, or optimized for your fixturing. Changing manufacturing mix? Swap modules without worrying whether your chassis will let you.  Hybrid chassis may cost a bit more, but they bring great option value.

So the next time your neighbors brag about their new hybrid, tell them you got one too. Not for fuel, but for flexibility.