Controlling the TPH

This following describes how the interface pins are used to print an image.

 Overview

For printing purposes an image in a data file must be decomposed into image lines, each of which is individually sent to the printhead. The printhead produces a dot of heat for each dot in the image line. A thermal printhead is useful only when it is installed in a thermal printer, which presses thermal paper against the printhead while it is firing, advancing the paper one line at a time.

concept of printing data tragsfer to printhead

printing

The printhead only prints singular dots, so complicated images like letter characters, bar codes or pictures must be reduced to lines of dots by computer software or printer electronics. Imagine that the image is cut into narrow strips as shown in the diagram above. Each strip must be so narrow that the contents are made up of single dots. It is easier if you assume the dots are square shapes. The width of the narrow strip matches the spacing of the heater elements on the printhead. For example, for the most common printhead resolution of 8 dots/mm, the line pitch should be 0.125mm, meaning 8 image lines per mm. This is the same as 203 dots per inch or 203 lines per inch.


 Driver IC Equivalent Logic Circuit

Printheads have integrated circuits (ICs) mounted on the ceramic wafer along with the heater elements. The purpose of the IC is to switch the heaters on and off. Data is loaded into the IC for each dot indicating whether it should print or not. Then a strobe signal is asserted by the printer control electronics indicating the length of time that the current must flow.

There are three layers of logic in a driver IC: The shift register array accepts the data, the latch array freezes the data in place, and the NAND gates switches the heater current. The following equivalent logic diagram represents a driver IC that switches 64 dots. Other common IC sizes are 96, 128, 144 and 192 dots. 

schematic of driver IC

 The 64 heaters labeled H1 - H64 are assumed to be connected to a common voltage source at their top and they are switched to ground (GND) as shown by the row of NAND gates. The control signals enter the IC on the left and exit on the right. Other ICs are connected to the left and right of this one to make up an entire printhead. The DATA OUT shown above becomes the DATA IN of the next IC to the right. The direction of data loading is left to right when you see the printhead with the heater line facing up and the connector facing down. The line over some signal names indicates that they are active low. They are written in text as /LATCH and /STROBE.


 Sequence of Printhead Operations
data input sequence

This diagram shows the sequence of operations for printing three lines.The meaning of the data bits is "high" to print and "low" to not print. The printer controller presents a data bit on the DATA IN pin and pulses the CLOCK pin. The printhead copies this data bit into the leftmost shift register on the rising clock pulse. The bits in the other shift registers shift to the right to make room for it. The controller repeats this step for the number of times equal to the number of heaters on the printhead. Then the controller pulses the \LATCH pin low, which causes the printhead to copy all the data bits to the latch registers. Next the controller asserts the \STROBE and BEO pins. Current will flow in all intact heater elements having a high data bit in their latch register, for as long as \STROBE is low and BEO is high. If the controller leaves the current switched on for too long, the heater element will burn out, ruining the printhead. BEO stands for Block Enable Out and is provided as a safety feature on most printheads. During power on and power off, printer controller electronics can be unstable and accidentally assert pins low or high, but probably not both at the same time. As shown in the diagram above, once the data is latched, the controller can begin loading data for the next line, even though the first line is still printing. Strictly speaking, the printhead has no knowledge of paper movement. For history control or gray scale printing, the controller can send several sets of data bits to control supplemental heater pulses, all on the same printed line on the paper.


 Data Loading Details

Below is an illustration of a print line of 6 dots prior to loading.

data transfer to shift resistor

Here is the situation after the first clock pulse.

data transfer to shift resistor by single pulse

After five more clock pulses, the six data bits are completely loaded.

data transfer to shift resistor with five more pulses

Next the controller must pulse the /LATCH pin to cause the data bits to be copied into the latch registers.

data transfer from shift resistor to latch

Next the controller holds the \STROBE pin low for a period of time. Assuming that BEO is also held high, the four heaters with high data bits are switched on and current will flow for as long as \STROBE is held low and the heater is intact.

heating by strobe

Some low cost printheads, such as KPB and KYT series, do not have a BEO connector pin. In these cases the BEO terminal on the driver is tied high within the printhead. For some series, Kyocera can produce a semi-custom flex printed circuit which features an "active high" STOBE pin by utilizing the IC's internal BEO signal instead of its internal \STOBE signal.


 Dot Numbering Scheme

Below is a diagram of a typical printhead. The heater line is facing forward and the connector is on the bottom. Therefore the direction of data transfer is left to right. The direction of paper movement should be from the connector to the heater line. If one reads printed output and define dot #1 to be the leftmost printed dot, then it follows that data bit #1 must be the first data bit loaded. This dot numbering scheme makes the most sense, but it is the opposite of the gate numbering scheme on the driver IC equivalent circuit shown earlier. In the diagram below, data bit #1 would be on the right and printed output would be face down.

direction of data transfer


 Multiple \ Strobe Pins

Printheads often have multiple strobe pins. Each strobe pin would be connected to one or more adjacent driver ICs. By firing the strobe groups individually, one after the other, the maximum current would be dramatically reduced, enabling use of a smaller power supply or battery. Common voltage drop and maximum number of simultaneous dots are issues that can also be addressed by multiple strobes. Print speed is usually reduced because the cycle time must be long enough to fire all strobes. However the shift from one strobe to two strobes will not reduce the print speed because heater elements require a cooling time, during which the other strobe group can be firing. A disadvantage of using multiple strobes is the possibility of a visible print image disruption at the boundaries between strobe groups. If a printhead is equipped with multiple strobe pins, they can still be asserted together, as if the printhead had only one strobe pin.

For example, here below is the a block diagram for the KPA-56-8MPA1 printhead for high-speed bar code applications. Driver ICs are numbered 0 - ~6 and each one switches 64 heater elements. The heaters numbered R1~ - R192, whose dots would appear on the left side of a printed receipt, are controlled by the /STROBE3 pin. A 24 volt printer might choose to fire the three strobe groups sequentially to minimize peak current.

block diagram of 56mm 200dpi TPH


 Multiple Data-in Pins

Printheads sometimes have multiple data-in pins. Each such data-in would load one or more adjacent driver ICs. A line of print data can be loaded faster if there are multiple data-in pins. This is necessary for wide and high-resolution printheads that require multiple data loads for each line. History control and gray scale printing are printer features that typically require multiple data loads for each line.

For example, here is the block diagram for a KYT-106-12MFW4 printhead for video printer applications. Driver ICs are numbered 0 - 12 and each one switches 96 heater elements. The DATA-IN-1 pin loads data into ICs 0 - 3, a total of 384 data bits. These heaters are numbered R865 - R1248 and would appear on the right side of a page of printed output. DATA-IN 2 and DATA-IN 3 also load 384 data bits into 4 driver ICs. DATA-IN 4 loads only IC #12. All ICs are controlled by the same /STROBE pin.

block diagram of 106mm 300dpi TPH
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