Developing highly-integrated thermal printer-based applications

Whenever a money transaction is completed, there is always the need for a printer-based system for printing receipts of transactions, be it a sales terminal, ticket dispenser, weighing scale, gas station, or ATM. Almost all of these systems use thermal printers. The reason thermal printers are more common as compared to other printer technologies is because of their reliability and lower maintenance costs given they are inkless printers. These printers work by selectively heating regions of heat-sensitive paper that is infused with a chemical that changes color when it comes in contact with the heated print head.

Thermal printers, however, are much more than just a printing unit. They have many other components, including a key matrix to enter data, some communication protocols to communicate with the host, an LCD to display data, an ADC for measuring voltage, and so on. This article talks about the basics of designing a thermal printer based system.

Before we jump into the complete system, let us quickly go through what comprises a thermal printer unit. Figure 1 below shows the block diagram of a thermal printer module, though there can be variations based upon the manufacturer and model of module.

Figure 1: Thermal Printer module

Thermal head: The primary block of a thermal printer unit is the thermal head. It consists of a number of heating elements (dots). Figure 2 shows the diagram of the thermal head with its associated signals.

Figure 2: Thermal Head

The number of elements varies from model to model. The distance between one heating element (dot) and another, called pitch, determines the printing quality and font size which can be achieved. Each heating element is controlled by a register bit. The thermal head has two registers called the latch register and shift register.

The size of these registers is equal to the number of dots. In these registers, ‘1’ corresponds to the pixel needing to be printed and ‘0’ otherwise. These values are shifted into shift registers by the host. Each shift takes one clock cycle. Then a latch signal is applied to latch the shifted data into the latch register which in turn controls each dot.

Now the strobe signal comes into the picture. Dots are divided into a number of groups (defined by manufacturer) and each group has one strobe signal associated with it. To heat the elements which have a value ‘1’ in their corresponding latch register cell in that group, the strobe should be asserted.

Care should be taken while selecting the strobe signal’s duration. The longer the duration of the strobe signal, the darker the printing. However, if the strobe is asserted for too long, it may burn the paper and heating element. Also, it will increase the thermal head’s temperature, resulting in slower printing. The reasons for how temperature affects printing speed are explained later in this article.

Temperature Sensor:  For the best printing quality, the temperature of the head should operate within a specified range. Also, to avoid permanent damage of the head, the temperature should not go outside the absolute maximum rating. So, if the temperature goes outside the reliable operating range, printing should be halted, which requires the need for a temperature sensor to be integrated in thermal printer module to measure head temperature.

Note that there are thermal printer modules available which do not have an integrated temperature sensor for temperature measurement in order to keep cost down. While this does affect the reliability of the system, there are ways to reduce the probability of damaging the head due to overheating, discussed later in this article.

Stepper motor: Another integral part of this module is a stepper motor.  The speed of the motor is a function of the operating voltage, and so the maximum speed is defined by the operating voltage of the motor. Commonly drive current for the motor is on the order of 1A so an H-bridge circuit is needed to drive the motors. There are numerous options available in the market for the H-Bridge ICs and one can be selected based upon local availability and pricing.