Hello:
Perhaps it's a bit offtopic, but is pci related. I want to make a
pci card but don't know how many watts can i take from the pci bus At
minimum i'll need 15-17, Is it too much?

TIA

The spec is for up to 25W max. I am at home, and the spec is at the
office, so I can't look it up at the moment but there are some
constraints on how that power is drawn.

Each voltage has a limit, but the sum of those limits exceeds 25W, so
you do have flexibility in how you use them. There are a few other
constraints, but I do not remember them at the moment.

Regards,

John McCaskill


a

Exactly, according to PCI Local Bus rev 2.3, sections 4.3.4.1 and 4.4.1:

4.3.4.1. Power Requirements
All PCI connectors require four power rails: +5V, +3.3V, +12V, and -12V.
Systems that
provide PCI connectors are required to provide all four rails in every
system with the
current budget specified in Table 4-10. Systems may optionally supply
3.3Vaux power,
as specified in the PCI Bus Power Management Interface Specification.
Systems that do
not support PCI bus power management must treat the 3.3Vaux pin as reserved.
Current requirements per connector for the two 12V rails are provided in
Table 4-10.
There are no specific system requirements for current per connector on the
3.3V and 5V
rails; this is system dependent. Note that Section 4.4.2.2. requires that an
add-in card
must limit its total power consumption to 25 watts (from all power rails).
The system
provides a total power budget for add-in cards that can be distributed
between connectors
in an arbitrary way. The PRSNTn# pins on the connector allow the system to
optionally
assess the power demand of each add-in card and determine if the installed
configuration
will run within the total power budget. Refer to Section 4.4.1. for further
details.
Table 4-10 specifies the tolerances of supply rails. Note that these
tolerances are to be
guaranteed at the components not the supply.

Table 4-10: Power Supply Rail Tolerances
Power Rail Add-in Cards (Short and Long)
3.3 V ±0.3 V 7.6 A max. (system dependent)
5 V ± 5 % 5 A max. (system dependent)
12 V ±5% 500 mA max.
-12 V ±10% 100 mA max.

[...]

4.4.1. Add-in Card Pin Assignment
The PCI connector contains all the signals defined for PCI components, plus
two pins
that are related to the connector only. These are PRSNT1# and PRSNT2#. They
are
used for two purposes: indicating that an add-in card is physically present
in the slot and
providing information about the total power requirements of the add-in card.
Table 4-12
defines the required setting of the PRSNT# pins for add-in cards.

Table 4-12: Present Signal Definitions
PRSNT1# PRSNT2# Add-in Card Configuration
Open Open No add-in card present
Ground Open Add-in card present, 25 W maximum
Open Ground Add-in card present, 15 W maximum
Ground Ground Add-in card present, 7.5 W maximum

In providing a power level indication, the add-in card must indicate total
maximum
power consumption for the add-in card, including all supply voltages. The
add-in cards
may optionally draw all this power from either the 3.3V or 5V power rail.
Furthermore,
if the add-in card is configurable (e.g., sockets for memory expansion,
etc.), the pin
strapping must indicate the total power consumed by a fully configured
add-in card,
which may be more than that consumed in its shipping configuration.

Regards

Eric