This section summarizes Plug and Play requirements for cable power distribution.
For Plug and Play, all devices—whether cable or self-powered—must comply with the applicable requirements in 1394 Specification for Power Management. These requirements enable a power management-capable bus manager to provide instant-on application support while reducing system-wide device power consumption.
In addition, all devices must comply with the 1394 Trade Association Power Specification Part 1: Cable Power Distribution. Although the requirements for devices that do not consume or produce cable power are minimal, all devices share responsibility for propagating the power bus as defined in the Cable Power Distribution specification.
A standard cable-power distribution model is necessary to reduce the likelihood of power-fault conditions, such as insufficient power for connection of a cable-powered device and surprise removal of a device power source. In addition, a bus manager that is power management-capable can allocate or de-allocate available power within diode-isolated power domains, accounting for the overall power budget and voltage drop.
Plug and Play requirements in this section highlight details specified in the applicable power specifications.
29. Devices power their PHY at all times
Required
All devices must perform the bus repeater function when powered down as specified in the IEEE 1394-1995 specification. Therefore, a device power switch must allow for local power to the PHY when switched off. Alternatively, a device can implement the standard protocol to request cable power (if available) from the power manager to power the PHY.
An exception to these requirements is necessary for PC add-on cards and system-board host connection devices that are subject to the power characteristics of the PCI bus.
A device that does not provide power to its PHY or consume power from the cable for its PHY will terminate the bus at the point of connection and must, therefore, terminate the pass-through of power.
30. Devices report power source and cable power consumption in Self_id packet
Required
Self-powered devices must report zero power consumed in the power class field of the Self_id packet. Alternately, if a device consumes cable power only to keep its PHY alive, it must report this consumption in the Self_id packet. This allows the power manager to reserve power for this occasion.
31. Devices implement link power control
Required
All cable-powered and self-powered devices must implement the Link_on packet and Link_off bit in the State_Clear register. These controls allow a power management-capable bus manager to control the node’s power state. Access to the device configuration ROM must be possible following a Link_on. A device cannot increment its power consumption by more than 3 watts following a Link_on. Self-powered devices can power up with Link_on. However, cable-powered devices must rely on the power manager to enable their link.
32. Device requiring power increments in excess of Link_on implements unit-power CSRs
Required
All cable-powered and self-powered devices that require power increments in excess of Link_on power must implement standard unit-power CSRs as specified in 1394 Specification for Power Management. This is necessary to allow for seamless integration of centralized power management capabilities when a device is connected to a mini-system.
In addition, all devices of a given device class must implement a standard set of unit power states as specified in the device class power management specification for that device class. For example, all VCRs must exhibit a consistent behavior with respect to power states and transitions between states. This is necessary to provide a consistent user experience.
Note: Please check with the 1394 Trade Association or send e-mail to 1394@microsoft.com to determine whether a power class specification exists for your device type. Alternatively, you are encouraged to draft a proposal for your device type and submit it to the 1394 Trade Association architecture working group for review and approval.
33. Devices that source cable power must report this capability
Required
This reporting is necessary to enable centralized power management. A device that sources 20 volts or more of cable power at 15 watts minimum must report that it provides power in its Self_id packet as specified in IEEE 1394-1995. Devices that provide less than 20 volts at 15 watts can be discovered using configuration ROM information as described in 1394 Specification for Power Management.
34. IEEE 1394-enabled PC sources cable power
Required
An AC-powered PC must source cable power to the bus. Cable power in turn enhances Plug and Play with a single connection for low-cost cable-powered devices. Battery-powered mobile and notebook devices are exempt from this requirement, whether or not the device is connected to an AC adapter.
Minimum power wattage is defined in the following item.
35. Power source supplies a minimum of 20 volts at 15 watts
Recommended
To minimize the cost of a power source, actual power output can be reduced to less than 40 volts at 1.5 amps as specified in IEEE 1394-1995. Also, a cable power source should supply enough power for at least one cable-powered device (15 watts) while also addressing voltage drop in the cable. Therefore, a minimum cable power source of 20 volts DC at a current limit of 1 amp is recommended. However, at the expense of higher component ratings, a 30-volt cable power source will reduce power loss in the cable.
In addition, if the power provider specifies a power capability greater than 15 watts, it must be capable of providing that power under full load. A power provider is required to always be able to provide its stated power under full load conditions.
For example, a minimum 20 watts output will ensure delivery of only 15 watts to a load some distance away from the source device. This is because of a cable voltage drop of 5 volts—that is, 1 amp × 0.66 ohm × 7 to 8 cable hops separating source node from sink node at a rated cable-hop resistance of 0.66 ohms. The voltage at the load will drop to 15 volts, with the source current limited at 1 amp. Therefore, a practical design target for a cable power source is a minimum of 20 volts with a current limit of 1 amp.
A device such as a notebook that wants to source less than 20 volts can do so if it reports in its Self_id packet that it does not source power, but does report in its configuration ROM the exact power it provides.
36. Devices notify the power manager of power change requests
Required
All devices that produce or consume cable power must use an electronic power switch to notify the power manager of requests from the front panel to change the power state. This function must be accomplished using the notification request protocol specified in 1394 Specification for Power Management. This protocol provides a time-out for defaulting to local control as is necessary for operation in non-power-managed environments.
This same mechanism is required for safe removal of a device (hot unplugging).