Rockwell Automation 1771-QC , D17716.5.25 SER/B SERVO POS - Manual (Page 47)

Hardware Description
Chapter 5
5Ć10
Loss of Power
The 1771-ES expander holds the velocity command output signal at zero
and disables the servo drive by turning off the drive disable circuit if it is
unable to sense the specified voltage as the following power-supply
terminals:
positive (+) terminal for the input power supply
common (-) terminal for the input power supply
positive (+) terminal for the analog power supply
negative (-) terminal for the analog power supply
Therefore, if one of these power supplies connected to the 1771-ES
expander terminal fails or if one of these connections from these power
supply breaks, the 1771-ES expander will disable the servo drive.
The drive disable circuit normally provides current to a sensing circuit on
the servo drive to enable it. However, if the 1771-ES expander detects a
fault, it cuts off the current in the drive disable circuit, thereby disabling
the servo drive. Therefore, if a connection in the drive disable circuit
breaks, this disconnection will disable the servo drive.
Auto Position Correction
Each time the 1771-ES expander receives a marker pulse, it checks the
value in the position register to see if it is an even multiple of the number
of feedback increments per revolution. If the value is off, the 1771-ES
expander will automatically adjust it to the closest even multiple.
This auto position correction feature corrects position errors caused by
noise on the channel A and B encoder feedback signals. However, the
function of this feature assumes a noise-free marker signal. Although this
feature may be able to prevent an accumulation of position error caused
by occasional noise on the channel A and B inputs, it cannot maintain
position accuracy if the environment is excessively noisy or if the cabling
and shielding is not proper.
If the environment is excessively noisy or if the cabling and shielding is
not proper, this feature will cause the axis to jump or jerk. This jump or
jerk indicates a problem.
Note that when the module detects a position error, it does not necessarily
disable the servo drive.