1. GENERAL INFORMATION

This specification describes a methodology for the safe distribution of power from VME modules to J3 backplanes for supplying termination networks or activity indicators. This document does not cover the distribution of power to Transition Modules.

1.1 System Introduction

VME subracks often incorporate a custom J3 backplane which is used to implement a custom bus architecture for private communication between modules. High-speed private buses require proper termination, which requires the distribution of power to the custom J3 backplane.

1.2 Limitations of J3 Power

J3 power as provided for in this specification is not intended for use in Modules or Transition Modules. J3 power is supplied through the VME modules. Those modules that supply power to J3 do so by reducing the amount of power available to themselves by the amount routed to J3. Therefore, J3 power requirements should be kept to a minimum so as not to place undue hardships upon the designers of VME modules within the same system.

2. Rules for Distribution of Power to J3 Backplanes

Proper routing of power to a J3 backplane is, in the end, the system integrator’s responsibility. This document will delineate a series of rules, which if followed, simplify and standardize this task.

2.1 Power Routing in the J3 Custom Backplane

Rule 1:

All voltages and returns on the J3 Backplane shall be carried on solid copper layers. Individual traces shall not be used to route power or return currents.

Rule 2:

Any power to the J3 backplane shall be carried through VME module to J3 pins which supply J3 electronics.

Observation 1:

In general supplying power to J3 reduces the total amount of power available to the rest of the module as module total power dissipation limits still must be observed.

2.2 Power Routing in the VME module that supplies power to J3

Rule 3:

Power carried through VME modules shall be used exclusively for backplane functions such as terminators. Transition modules shall derive their power solely from a direct connecton to the J0, J2 or J4 connectors.

Rule 4:

Every VME module which is designed to supply power to the J3 backplane shall allocate sufficient J3 pins to drive the entire load of the J3 backplane.

Rule 5:

Every VME module that is designed to supply power to J3 shall provide that power via a separate power trace or plane which connects directly to J1/J0/J2 and does not share current with the main module power traces/planes.

Observation 2:

No additional transient suppression or decoupling is required on the main module for backplane power. Backplanes with active circuitry should incorporate their own decoupling at the noise source.

2.3 VME Module Connector and Front Panel Considerations

Rule 6:

The number of J1/J0/J2 pins used to supply power to the J3 backplane shall be equal to the number of pins of the J3 connector used to supply power to the J3 backplane.

Rule 7:

The number of J3 pins used for power return shall equal or exceed the number of J3 pins used for power supply.

Rule 8:

Any VME module which is designed to supply power to J3 shall have a red LED on the front panel which shall be illuminated whenever power is being applied to the J3 backplane. This LED shall be clearly labeled ‘J3 PWR’.

2.4 Fusing of J3 Power

Rule 9:

Every VME module that is designed to supply power to J3 shall provide an on-board, socketed fuse between the J1/J0/J2 source of power and the J3 connector, which fuse shall be rated at the lesser of

• 150% of the normal operating current of the J3 electronics or

• 80% of the temperature-derated amperage which can be sustained by the J3 pins allocated to supplying power.

Rule 10:

Opening of the J3 power fuse on a VME module shall not affect the power supply to the rest of the VME module. Similarly, opening of the main VME module fuse shall not affect the current supplied to J3.

2.5 J3 Power and Transition Modules

Rule 11:

Transition modules shall neither supply power to J3 nor draw power from J3.

2.6 System power distribution considerations

Rule 12:

The total power load of the J3 Backplane shall not exceed that which can be supplied by any single VME module in the system. No J3 electronics shall be dependent upon multiple VME modules for sufficient current.Identification and keying

Rule 13:

J3 total power dissipation shall not exceed that which can be safely removed by simple convection.

Recommendation 1:

Keep the total J3 power dissipation under 20 Watts.

2.7 Indications and Keying

Rule 14:

Any J3 backplane which is designed to have power supplied to it by a VME module shall be permanently marked with a legend, visible from the Transition Module side in letters no less than 0.2 inches high, indicating that this is an actively powered J3 Backplane. Pins of connectors which are connected to any power supply voltage shall be cut flush with the backplane so as to remove any possibility of accidental connection to Transition Module electronics.

Recommendation 2:

VME subracks that contain a J3 backplane designed with power should utilize keying mechanisms to insure that foreign VME modules are not inserted in to slots that connect to J3 power pins.

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