DAWN.DEC–Dawn VME Products–SC
Selecting a VME backplane for optimal system performance
The right performance and physical attributes are key to good power
distribution
BY BARRY BURNSIDES Dawn VME Products, Inc. Fremont, CA
Many backplane applications today still involve the VMEbus. The bus'
specifications are both well known and well suited for many high-speed bus
applications. Specifying a VME backplane is no black art, although it does
require careful consideration of both performance and physical factors.
The designer must consider impedance, shielding, and crosstalk to ensure
that the backplane functions accurately. Physical aspects of the
backplane's design, such as layout of various sections and the connectors
used, also play a key role. A VME backplane actually consists of two
individual backplanes. The J1 is the upper portion and the J2 is the lower
portion. The J1/J2 configuration is achieved using either a J1 and a
separate J2 backplane, or, more commonly, a single J1/J2 monolithic
(common substrate) backplane (see Fig. 1). The monolithic approach will
usually be the choice when the designer wants a 32-bit backplane and has
the same number of slots for both J1 and J2. The monolithic backplane
guarantees a matched impedance between the J1 and J2 portions. Although
the VMEbus specification defines a great many backplane size combinations,
the most common choices are 5, 7, 10, 12, 20 and 21 slots.
Impedance is crucial Of the key electrical characteristics to look for
in a backplane, impedance is the most important. For VME transmission lines,
a populated backplane should provide a minimum of 50-ohm impedance from
one end to the other, to maximize quality of results. A good typical base
impedance is 55 to 57 ohms. Improper impedance causes an inefficient
signal transmission, with data either not getting through at all, data
getting through intermittently, or the wrong data being transmitted. The
designer should be aware that a 75-ohm backplane design does not guarantee
a finished product with a 75-ohm impedance. The impedance changes during
different phases of the design and build process. Classical physics
calculations indicate that when a design exceeds 75 Ω, various
tolerances may cause a bare board to exhibit 60 to 65 Ω impedance.
Later, the impedance may turns out to be lower once the board is populated
with connectors. Transmission line resistance is also important. A
backplane should have a transmission line resistance of less than l.5 ohms
from one end of the backplane to the other. This ensures that the traces
can handle the signal power without voltage shifts. A low-inductance
power feed system should distribute power uniformly.The capacitance
between power plane and ground should be high to provide filtering.
Reliable power distribution is particularly important as power demands and
speeds of systems increase. Higher speeds can cause power and ground
planes to become noisy, adversely affecting performance. Slot load boards,
which contain active and passive circuits, apply loads that can be used to
test and characterize power distribution, transmission line integrity, and
thermal properties of a VME board (see Fig. 2). Minimizing crosstalk is
also important. A good backplane can minimize crosstalk by shielding
between lines. Crosstalk is primarily a function of the edge rate of the
drivers being used. To minimize crosstalk, signals should be isolated from
one another and each transmission line should be the proper impedance and
should be properly terminated. Placing an exact figure on the amount of
acceptable crosstalk is difficult. The amount of crosstalk tolerable
depends on the types of components used to drive the bus. Some ICs used to
send and receive data on digital backplanes tolerate crosstalk better than
others. In measuring crosstalk, the designer should use test techniques
that closely model the actual circuitry being used.
Physical considerations Selecting a backplane also involves looking
closely at physical factors. For instance, the backplane material should
meet MIL-P-13949 specifications. The circuits should be impedance-matched.
They should provide the proper thickness (usually 0.125 in. for VME) for
the components being inserted into the backplane. And they should offer
high capacitance between their power and ground planes for filtering. The
designer should ask several questions. Is the layout user-friendly, with
sufficient provisions for feeding power into the unit? Does the backplane
incorporate structural integrity, with good clean workmanship and
high-quality materials? A good VMEbus backplane will have very tight
mechanical tolerances and dimensions that conform to the specification.
Typical VMEbus systems have two 96-pin connectors per card that must mate
with backplane connectors. If connectors or mounting holes are not exactly
where they should be, inserting or removing cards can be very difficult.
Even worse, connector contacts can be bent or broken. The quality of the
backplane connectors is very important. High-performance backplanes often
use gold, Class II, contacts good for 400 insertion/removal cycles. The
backplane's markings and legends should be clearly marked. Model numbers,
slot numbers, power feed voltage information, and component reference
designators should be clearly silk-screened in a color that contrasts with
its background so it is easy to read. Each pin should also be clearly
labeled. Otherwise, an integrator trying to connect to a pin with test
equipment, or trying to wire-wrap from the back, would have to count pins.
It's easy to be off by a pin or two in such situations.
How many layers? System designers sometimes mistakingly believe that the
number of layers in the backplane determines its performance. In reality,
a layer count statement alone is nothing to be excited about. It's the
total design and performance of the backplane that count. Additional
layers are not necessarily bad; they just may not be necessary. Designers
should be concerned, instead, about performance and reliability. The board
must, for instance, embody proper design, materials, and layer and trace
spacing. The board must also use enough copper. Five-layer monolithic
VMEbus backplanes have been tested, for instance, against others that have
8 to 11 layers. The tests indicate that five-layer boards provide better
performance. Figure 3 shows the five-layer backplane has a higher
impedance than the ten-layer backplane and exhibits less propagation
delay. More designers are developing “VME-like” architectures that use
either the VME mechanical or electrical specification (or both) as a
starting point. At that point, the designer tailors many of the
backplane's characteristics to fit their specific application. Here, a
good backplane supplier can offer the designer substantially more
flexibility than was available even a few years ago.The supplier can make
available many design options. These include a choice of press-fit or
soldered connectors, custom busing, custom power distribution, and signal
isolation. Designers should work with such a supplier early in the design
process so they can use as many off-the-shelf components as possible,
saving time and money. A single source can often provide in addition to
the backplane, complete subsystems that include the chassis, card cage,
and enclosure with power supply and fans. Then, all the designer has to do
is plug in the boards, run the required tests, and ship the product out
the door. In addition to supplying the hardware, the backplane supplier
should also be able to provide computer-assisted design help to the
designer.
CAPTIONS:
Fig. 1. Monolithic VME backplanes, such as these from Dawn VME Products,
combine J1 and J2 sections.
Fig. 2. Slot-load boards (bottom) apply a resistive load that can be used
to test performance characteristics of a VME backplane.
Fig. 3. This time-domain reflectometer plot shows that a well-designed
five-layer backplane (top) maintains a higher impedance than a ten-layer
backplane (bottom). In addition, the five-layer backplane has a shorter
transmission line, with less propagation delay than the ten-layer board.
Advertisement
