Printronix L5000 Series Maintenance Manual page 173

Therefore, when the AC bias renders the developing roller surface potential positive,
the strength of an electric field resulting from the difference between that potential
and the drum surface potential is greater than the magnetic force exerted between the
toner and inner-pole magnet. As a result, the positively charged toner is attracted to
the neutralized area of the drum surface (electrostatic latent image).
On the other hand, when the developing roller surface potential is rendered negative,
due to the magnetic force exerted between the toner and inner-pole magnet and the
strength of the electric field generated between the drum and developing roller, the
toner attracted to the drum surface is returned to the developing roller.
As the roller surface potential is rendered negative and positive alternately as
mentioned above, the toner is properly attracted to the electrostatic latent image area
on the drum surface in a trapezoidal form during the travel within the developing
section, so that a visible image, free from density irregularities, is formed.
Principles of Operation 5–13