Patent GB-1182215-A (GB)
Title: Thermal Printing System.
IPC: G06K15/00
Filed: 1968-09-17
Abstract: 1,182,215. Electric selective printing control. NATIONAL CASH REGISTER CO. 17 Sept., 1968 [2 Oct., 1967], No. 44066/68. Heading G4H. In a thermal printing system comprising a number of selectively energizable resistive thermal printing elements 24, Fig. 2, these elements are each connected in series with a triggerable switching element 22 and connected together in groups G1-G4, there being a plurality of selection lines 58 each connected to a first terminal 28 of one of the switching elements in each group, a second terminal 30 of each switching element in a group being connected to a common point and to group switching means, normally applying a first potential to the point but being capable of applying a second potential concurrently with the appication of a triggering signal to the first terminal of a selected switching element, this serving to trigger the element, the arrangement being such that a printing operation may be effected by selecting each group in turn and selected switching elements in the group. The elements 24 are resistive, made, for example of tin oxide which heats when a current flows through it. The heat marks a sensitized record material, certain ones of the elements 24 being selected to form the desired character as a pattern of dots. The cathodes 30 of each group are connected to the collector of a group selection transistor 20. Zener diodes 32 are connected across the transistors and a positive source is connected via resistor 34 to each diode to pass sufficient current through the Zener diode, when the transistor is cut off, to develop the Zener voltage. Initially all the transistors are non-conducting and the supply line 56 is energized by a positive voltage to bias all the silicon-controlled rectifiers. If a triggering voltage is applied on a selection line 58 the associated SCR will be further biased, but will still not conduct. However, if a selection signal is applied to a transistor 20 of Group 1 to cause it to conduct, the collector 38 is brought substantially to earth potential. The cathodes of the SCRs therefore first receive the Zener voltage, and then earth potential. A triggering current then applied to selected gate electrodes 28 causes those SCRs to switch to their low-impedance state allowing a large current to flow through the associated heaters 24. After the removal of the two selection signals, currents continue to flow in these heaters, and heaters of Groups 2, 3 and 4 are successively selected in the same way. When all the Groups have been energized, the power supply is removed from line 56 and the currents flowing in the SCRs decrease below the minimum holding current and they revert to their high-impedance state. In another form (Fig. 4, not shown) the silicon-controlled rectifiers are replaced by two-terminal semi-conductive triggerable switching devices e.g. as described in Specification 1,021,510. The arrangement is similar except that the sides of the heating elements 24 remote from the switching devices are connected in groups, each group being selected by a transistor and Zener diode circuit as described. The other sides of the switching devices from the heaters are connected to lines (as 58) which are coupled to the power supply via diodes and to switching terminals. A positive pulse, larger than the supply voltage, applied to the switching terminal causes the switching device to conduct and remain conducting until the supply is switched off. The possibility is mentioned of using light-activated semi-conductors controlled by rectifiers as switching elements.