Traditionally, Grass has designed instruments having a "-UP" output polarity, meaning that a negative signal (voltage) at the input causes an UPWARD pen deflection. This tradition is opposite all other fields where it is normal to expect an UPWARD deflection or trace with a POSITIVE signal voltage. This "NEGATIVE UP" tradition evolved from the early days of neurophysiology where researchers recording nerve action potentials preferred to have this "negative going potential" move upwards on the oscilloscope display, so the preference was for "NEGATIVE UP".

Most modern instrumentation provides "single-ended" outputs having a positive output voltage referenced to either ground or a common terminal. For example, pulse oximeters provide an analog output of 0 to +1 volt corresponding to 0 to 100% saturation values, for connection to various recording devices. In older Grass recorders, (Model 8A-D, Models 7&78A-D, and 79A-D, we provided a reversible oscillograph connector, to permit the pen deflection to be inverted. This connector is located under the ink well tray and is clearly labeled "NORMAL POLARITY THIS SIDE UP" or "REVERSE POLARITY THIS SIDE UP". The newer oscillographs, Model 711G and 811G do not allow this reversal.

The analog output, J6, follows the "-UP" tradition i.e., when the pen deflects up, the J6 goes negative. To provide a reversible polarity, the Model 7P122, 7DA, 7P511 amplifiers (all amplifiers having the pen driving circuit), have a PEN POLARITY switch on the rear panel. In all other Grass amplifiers, P511,CP511,etc, either output polarity can be obtained by simply reversing input leads to the G1/G2 differential input.

This information is in the corresponding instruction manuals.