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VMEbus Daisy Chains
By John Rynearson, Technical Director, VITA
Question: What are the VMEbus daisy-chains and how do they work?
Beginning VMEbus users not familiar with the concept of daisy chains signals in bus architectures often find backplane configuration a major headache. However, the concept of daisy chain signals is simple and once understood, system configuration becomes straight forward.
VMEbus Daisy Chain Signals
There are five daisy chain signals on the VMEbus. Four are used for bus arbitration and one is used for interrupt acknowledge. Unlike bussed VMEbus signals that are connected to the same pin on each connector, daisy chain bus signal propagates down a backplane starting from slot one by entering the connector on an input pin and exiting the connector on a separate output pin. A board plugged into a connector slot is responsible for providing continuity between the input pin and the output pin. If the slot is empty, then a jumper on the backplane can be used to provide continuity between input and output. When the jumper is installed the input pin is connected directly to the output pin. When the jumper is not installed the daisy chain is broken.
When a board is installed and the jumper is also installed unpredictable system operation will occur since the input and output pins will be shorted together defeating the on-board circuitry.
Most backplanes will group the jumpers so that it is easy to tell the four bus grant in/out jumpers from the single interrupt acknowledge in/out jumper. One set of jumpers is required for each connector except for the first one. Thus a 21 slot backplane would have 20 jumper groups.
One easy to follow rule without having to understand system requirements is the following. If a backplane slot has a board in it, then don't install jumpers. If the backplane slot doesn't have a board in it, then install the jumpers.
However, a system may work perfectly well while not following the above rule. A closer look at the operation of daisy chain lines will reveal why.
System Arbitration
System arbitration on the VMEbus uses four bus request lines and four bus grant in/out daisy chain lines. A board with master capability (the ability to put addresses on the bus) must first request the bus. It does this by asserting its bus request line which is sensed by the system controller in slot one. Since there are only four request lines and since a system could have up to 21 masters, the daisy chain lines are used to allow bus requests to occur on the same request level at the same time. The master closest to slot one will receive the bus because the grant signal will not propagate past the closest requesting master. If a slot is empty then the bus grant in/out daisy chain is broken. If there are masters in slots past the empty slot, then they will not receive bus grant signals unless continuity through the empty slot is provided.
However, if no masters exist past the empty slot, then daisy chain continuity is not required. Note that slave boards do not request the bus and therefore don't use the bus grant in/out lines. Although slave boards are required by the VMEbus specification to provide continuity, its always a good idea to confirm that bus grant in/out traces are shorted together on slave boards.
Interrupt Acknowledge Daisy Chain
The VMEbus interrupt subbus provides seven levels of interrupt requests with IRQ1* being the lowest and IRQ7* being the highest. In a 21 slot system with many interrupt sources, it is likely that two or more boards will interrupt on the same interrupt level at the same time. A daisy chain line, named IACKIN/OUT (Interrupt ACKnowledge IN/OUT) is used to grant the interrupt to the interrupter closest to slot one that requested an interrupt at the level being serviced. If the backplane has an empty slot that is not jumpered, then the interrupt daisy chain will be broken. If boards which assert interrupts are in slots past the empty slot, then they will not receive interrupt acknowledgments unless continuity for IACKIN/OUT is provided.
Based on a greater understanding of requirements our new rule might be the following. Jumper the daisy chain lines (bus grant in/out and iack in/out) for any empty slot if boards in the backplane past these slots require these daisy chain signal lines.
Automatic Daisy Chain Jumpering
Improperly installed daisy chain jumpers account for the large majority of system configuration problems simply because many system integrators don't understand jumpering requirements. Wouldn't it be great if manual jumpering weren't required? Well, a solution is at hand. Connector manufactures have developed a connector that automatically shorts both bus grant in/out pins and the interrupt acknowledge in/out pins together whenever a board is removed. Thus the jumpering issue is automated. Automatic daisy chaining is a feature that has been around for several years now and is available from many backplane manufacturers. Most reports from the field indicate that these connectors work well and prevents the daisy chaining configuration headache.
Summary
On new systems, install backplanes that provide automatic daisy chain jumpering. On older VMEbus systems that require manual jumpering understand each boards signal requirements and install or remove backplane jumpers accordingly.
This page last updated: Sep 15, 1999
Reprinted from the VITA Journal with permission from VITA.