Delta Tau PMAC2A-PC/104 - Manual (Page 18)

PMAC2A PC104 Hardware Reference Manual
Acc-2P Hardware Setup 12
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 k
Ω pull-up resistor to +5V. This cannot
be changed.
The ‘-‘ inputs of each differential pair each have a hard-wired 2.2 k
Ω resistor to +5V; each also has
another 2.2 k
Ω 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 k
Ω 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.