Thermal printing method and apparatus

Claim 3

A method according to claim 1, characterized in that the thermal recording is performed row by row, and in which all of said steps are repeated for recording of each row.

Claim 4

A method according to claim 1, characterized in that the durations of activation of the thermal elements (22) are measured in increments of predetermined duration.

Claim 5

A method according to claim 1, characterized in that the step of transferring burn time information comprises a first step of transferring said burn time information in parallel mode from said storage means (64) to parallel-to-serial shift register means (46), and a second step of transferring said burn time information from said parallel-to-serial shift register means (46) in a serial mode to said operating circuitry (28,30,34,36) of said thermal print head means (20).

Claim 6

A method according to claim 1,characterized in that the step of transferring burn time information employs direct memory access.

Claim 7

A method according to claim 1,characterized in that the step of determining burn time information includes computations to produce desired optical density shade variations in the markings produced by the thermal print head means (20) on a record medium.

Claim 8

A method according to claim 6, characterized in that the step of determining burn time information includes the step of multiplying a burn time value by an optical density shade value.

Claim 9

A method according to claim 1, characterized in that the step of determining the length of time that each thermal element (22) must be activated is accomplished in accordance with the equation T = (E/V 2 )R, in which T represents burn time, E represents the desired energy input to a thermal element, V represents the common applied voltage, and R represents the measured resistance.

Claim 10

A method according to claim 1, characterized in that the step of determining the resistance of each individual thermal element (22) includes the following steps: measuring the total voltage drop of a circuit which includes said individual thermal element (22): digitizing such total voltage drop; and subtracting from said digitized total voltage drop a predetermined digital value representing voltage drops across other components (28,60) in said circuit to obtain the voltage drop across said individual thermal element (22), which bears a linear relationship to the resistance of said individual thermal element (22).

Claim 11

A method according to claim 1, characterized in that the step of determining the resistance of each individual thermal element (22) includes the following steps: measuring the total voltage drop of a circuit which includes said individual thermal element (22): digitizing said total voltage drop; measuring the voltage drop across other components (28,60) in said circuit; digitizing said voltage drop across said other components, subtracting said digitized voltage drop for said other components from said digitized total voltage drop to provide the voltage drop across said individual thermal element (22); determining a correction factor (C) to eliminate inaccuracies arising from imprecise current regulation and multiplying the .- voltage drop across the individual thermal element (22) by said correction factor (C) to produce a digitized representation of the resistance of said individual thermal element (22).

Claim 12

A method according to claim 12 characterized in that the activation of individual burn times of the thermal elements (22) can be initiated at different times.

Claim 13

Thermal printing apparatus for carrying out a method according to claim 1, including thermal print head means (20) including a plurality of individual thermal elements (21) and operating circuitry (28,30,34,36) for applying required energy to and controlling the individual operation of said elements (22) characterized by supply means (52,53,54) for providing said common applied voltage (VHEAD) for said thermal elements (22); measuring means (56,60,61,62) including processing means (62) and adapted to measure the resistance of each individual thermal element (22) and provide a digital value representing each such resistance; data input means (68) for selection of individual thermal elements (22) to be energized, said processing means (56,60,61,62) (56,50,61,62) being further adapted to compute individual thermal element burn duration values in accordance with which elements (22) are selected by the data input means (68), the measured digital resistance values of each such element (22), the required energy and the head voltage value (VHEAD); storage means (64) for storing the computed individual element burn duration values for said elements (22); parallel-to-serial conversion means (46) adapted to receive data in parallel form from said storage means (64) under control of said processing means (62) relating to which elements (22) are to be energized and for what duration, and for providing said data in serial form to said operating circuitry (28,30,34,36) for selection and timed operation of each of said thermal elements (22); interface circuitry means (42) for providing operating signals to said thermal print head means (20), and bus means (70) coupling said.-measuring means (56,60,61) said data input means (68), said processing means (62), said storage means (64), said interface circuitry means (42), and said parallel-to-serial conversion means (46) for communications.

Claim 14

Apparatus according to claim 12, charaterized in that said measuring means includes means for determining the total voltage drop of a circuit which includes the individual thermal element (22), digitizing means (61) adapted to digitize such total voltage drop, said processing means (62) being adapted to subtract from said digitized total voltage drop, a predetermined digital value representing voltage drops across other components (28,60) in said circuit to obtain the voltage drop across said individual thermal element (22), which bears a linear relationship to the resistance of said individual thermal element (22).

Claim 15

Apparatus according to claim 12, characterized in that said measuring means includes means for determining the total voltage drop of a circuit which includes said individual thermal element (22), digitizing means (61) for digitizing said total voltage drop, and means for determining the voltage drop across other components (28,60) in said circuit, said digitizing means (61) being adjusted to digitize said voltage drop across said other components (28,60) and said processing means (62) being adapted: to subtract said digitized voltage drop for said other components (28,60) from said digitized total voltage drop to provide the voltage drop across said individual thermal element (22); to determine a correction factor (C) to eliminate inaccuracies arising from imprecise current regulation, and to multiply the voltage drop across the individual thermal element (22) by said correction factor (C) to produce a digitized representation of the resistance of said individual thermal element (22).

Claim 16

Apparatus according to claim 12, characterized in that the transfer of data from said storage means (64) to said parallel-to-serial conversion means (46) is effected under-the control of said processing means (22) by direct memory access.