Drive circuit for thermal printer

Claim 1

For use in a printer of the kind that utilizes a thermal printing ribbon having a moderately conducting outer layer and a highly-conducting inner layer, said printer including a printhead with a set of electrodes that wipe over said moderately-conducting outer layer at a print zone to apply respective printing signals, timed in accordance with gating signals from a print controller, for causing localized heating within said ribbon and resultant mark formation at a receiving medium, printing signal circuitry comprising: collection contact means for establishing electrical contact with said moderately conducting layer of said ribbon at a collection zone spaced from said print zone; monitoring contact means for establishing electrical contact with said moderately conducting layer of said ribbon at a detection zone spaced from said print zone and said collection zone; printhead energization means including, a high impedance circuit, connected electrically to said monitoring contact means, for producing a buffered feedback signal representative of the electric potential assumed by said monitoring contact means; signal channels coupled to respective of said electrodes, signal processing means; responsive to said buffered feedback signal, for producing a set of printing signals, said signal processing means including means for adjusting a reference voltage level in accordance with said buffered feedback signal to produce an energization signal; and switching means for selectively connecting said energization signal to said signal channels in accordance with said gating signals to act as respective printing signals at said electrodes; and means defining a low impedance signal return path between said collector contact means and said printhead energization means whereby a return path for said printing signals is established.