any externally applied command voltages connected to the COMMAND IN input BNC by factors
of x0.1, x0.01, and x0.001.
Internal or external commands are activated by moving the COMMAND SENSITIVITY toggle
into the on position. When switched off, these modifiers are disconnected from the COMMAND
pathway.
Test pulse
When internal command is selected and COMMAND SENSITIVITY is switched on, a 1V p-p
square wave test pulse (line frequency; 50/60 Hz) is generated by the PC-505B. The test
pulse is attenuated by the SENSITIVITY SELECTOR toggle and is available for adjustment of
capacitance compensation, measurement of pipet resistance, or for monitoring the formation
of a gigohm seal at the electrode tip.
When external command is selected, the internal TEST PULSE circuitry is disabled and
commands appearing at the COMMAND IN BNC are attenuated and available to be applied to
the headstage.
Junction Zero and Auto Zero
These controls set the pipette current to zero after the pipette is placed in the test
solution and prior to seal formation. This circuitry is used to compensate for electrode
potentials, liquid junction potentials and other offset voltages, and establishes a zero
baseline reference potential. The 10-turn JUNCTION ZERO control manually adjusts I
m
between
120 mV with immediate response.
With the AUTO ZERO toggle switch in the on position (auto zero LED on) depressing the
associated pushbutton zeros the current automatically. The settling time is rapid when used
with M pipette resistances, but can take several seconds if used with G resistances. For
complete compensation, hold the pushbutton in until I
m
on the METER reads zero.
NOTE: With AUTO ZERO turned on, the manual junction control remains active. In this mode,
depressing the auto zero pushbutton will zero any manual setting, holding potential, and
time-averaged test pulse or command voltage present. Switching AUTO ZERO off restores the
uncompensated current and voltages. To re-zero, repeat using either manual or auto zeroing.
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