User's Manual

Turbo PMAC2 PCI Hardware Reference Manual

Hardware Setup 7

Pull-Up/Pull-Down Resistors

The differential input signal pairs to the PMAC2 PCI have user-configurable pull-up/pull-down resistor

networks to permit the acceptance of either single-ended or differential signals in one setting, or the

detection of lost differential signals in another setting.

The ‘+’ inputs of each differential pair each have a hard-wired 1 kohm pull-up resistor to +5V. This

cannot be changed.

The ‘-‘ inputs of each differential pair each have a hard-wired 2.2 kohm resistor to +5V; each also has

another 2.2 kohm resistor as part of a socketed resistor pack that can be configured as a pull-up resistor to

+5V, or a pull-down resistor to GND.

If this socketed resistor is configured as a pull-down resistor (the default configuration), the combination

of pull-up and pull-down resistors on this line acts as a voltage divider, holding the line at +2.5V in the

absence of an external signal. This configuration is required for single-ended inputs using the ‘+’ lines

alone; it is desirable for unconnected inputs to prevent the pick-up of spurious noise; it is permissible for

differential line-driver inputs.

If this socketed resistor is configured as a pull-up resistor (by reversing the SIP pack in the socket),

thetwo parallel 2.2 kohm resistors act as a single 1.1 kohm pull-up resistor, holding the line at +5V in the

absence of an external signal. This configuration is required if encoder-loss detection is desired; it is

required if complementary open-collector drivers are used; it is permissible for differential line-driver

inputs even without encoder loss detection.

If Pin 1 of the resistor pack, marked by a dot on the pack, matches Pin 1 of the socket, marked by a wide

white line on the front side of the board, and a square solder pin on the back side of the board, then the

pack is configured as a bank of pull-down resistors. If the pack is reversed in the socket, it is configured

as a bank of pull-up resistors.

The following table lists the pull-up/pull-down resistor pack for each input device:

Device Resistor Pack

Encoder 1 RP43 ADC/Fault1&2 RP58

Encoder 2 RP48 ADC/Fault3&4 RP59

Encoder 3 RP44 ADC/Fault5&6 RP119

Encoder 4 RP49 ADC/Fault7&8 RP120

Encoder 5 RP104 SCLK12&34 RP62

Encoder 6 RP109 SCLK56&78 RP123

Encoder 7 RP105

Encoder 8 RP110

Connections

Mounting

The Turbo PMAC2 PCI can be mounted in one of two ways: in the PCI bus, or using standoffs.

• PCI bus: To mount in the PCI bus, simply insert the P1 card-edge connector into PCI socket. If there

is a standard PC-style housing, a bracket at the end of the PMAC2 PCI board can be used to screw

into the housing to hold the board down firmly.

• Standoffs: At each of the four corners of the PMAC2 PCI board, there are mounting holes that can be

used to mount the board on standoffs.