PMAC2A PC104 Hardware Reference Manual
Resistor Packs Configuration
Differential or Single-Ended Handwheel Encoder Selection
The handwheel encoder differential input signal pairs to the PMAC 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 kΩ pull-up resistor to +5V. This cannot be changed.
The ‘-‘ inputs of each differential pair each have a hard-wired 2.2 kΩ resistor to +5V; each also has another 2.2 kΩ 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), the two parallel 2.2 kΩ resistors act as a single 1.1 kΩ pull-up resistor, holding the line at +5V in the absence of an external signal. This configuration is required if complementary open-collector drivers are used; it is permissible for differential line-driver inputs.
If Pin 1 of the resistor pack, marked by a dot on the pack, matches Pin 1 of the socket, labeled by a white square, 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.
RP22 is the 6-pin pull-up/pull-down resistor pack for the handwheel encoder input.
Handwheel Encoder Termination Resistors
The PMAC provides a socket for termination resistors on the handwheel encoder differential input pairs coming into the board. As shipped, there is no resistor pack in the RP23 socket. If these signals are brought long distances into the PMAC board and ringing at signal transitions is a problem, a SIP resistor pack may be mounted on the RP23 socket to reduce or eliminate the ringing. The 6-pin termination resistor pack is the type that has independent resistors (no common connection) with each resistor using two adjacent pins.
