TECH TALK

A journal of bus architecture tips & techniques

System Slot Essentials: Is Yours Right- or Left-Handed?

Which is it? Does the system slot belong on the left side or right side of the backplane? That depends on the architecture. CompactPCI allows either. Other architectures specify a fixed location. For example, PXI (a derivative of CompactPCI ) and VMEbus specify a leftmost system slot. The table below illustrates the location of the system slot as specified by various architectures. CompactPCI is the most confusing because slot locations can vary. Let’s look at CompactPCI and PXI in detail.

System Slot Position (assuming a vertical card cage)

CompactPCI System Slot Essentials
The CompactPCI specification allows a system slot to be located anywhere relative to peripheral slots. It can be found in the leftmost slot, rightmost slot, or in a middle slot for bridged backplanes. This flexibility can create confusion or, even incompatibility.

The system slot provides such special features as arbitration, clock distribution, and reset functions for all adapters on the bus. It performs system initialization by managing each local adapter’s IDSEL signal. A CompactPCI system denotes the system slot two ways: a triangle symbol (Figure 1.) on the backplane and a red card guide.
	Figure 1. System Slot Identifier

When a CompactPCI controller (a.k.a. single board computer) requires an additional slot, it extends to the right. If a double - (8HP) or triple - (12HP) slot wide controller (Figure 2) is plugged into a backplane with a right hand system slot, it won’t hang over needed backplane slots. If it is plugged into a left hand system slot, it will consume usable backplane slots. Many CompactPCI enclosures, particularly 3U, have backplanes with right hand system slots to alleviate this problem.

Manufacturers of CompactPCI backplanes and enclosures have chosen to support both left -hand and right -hand system slot configurations. Fortunately, the majority of 6U single board computers are only one slot wide so it matters little where the system slot is positioned.

PXI System Slot
The PXI specification clearly defines the system slot to be the leftmost PXI backplane slot. PXI renames the system slot as the controller slot. To accommodate system controllers larger than one slot width, PXI stipulates that controllers extend to the LEFT of the controller slot into additional controller expansion slots. This is opposite of CompactPCI. In comparing Figure 2. and Figure 3., note the location of the handles.

Figure 2. 3U CompactPCI Controller.         Figure 3. 3U PXI Controller.
(ADLink cPCI-3700)                                        (ADLink PXI-3710)

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