Using the USB interface of ECG thermal printer system design (1)

ECG is drawing remember heart tissue caused by biological signals as well as modern medical science research and clinical diagnosis using l very broad a precision electronic medical devices.

Due to its diagnosis is reliable, easy in operation, no damage to the patient, in a wide range of applications, and as a result of ECG detects intuitively and ways and means of output, ECG machine micro printers is essential.

Currently, the hospital more than used in traditional mechanical ECG printer, print low efficiency, noise pollution is serious, ECG waveform distortion, etc.

In recent years there has been rapid uSB technology, many desktop printers have applied this technology. But using the USB interface of ECG thermal printer are few. This system uses a USB interface and is attached to the ECG machine, but also and PC connect electrocardiogram machine printing.

1 ECG thermal printer hardware design

1.1 system's overall structure

The hardware system is composed of controller, paper machines, uSB interface, the thermal print head and its detection and protection circuits and other components that make up the block diagram shown in Figure 1.

1.2 ARM controller selection

Because the system requirements for printing at the maximum speed of up to 50 mm/s, so for the processor to the printhead speed data transfer requires very high.

General thermal print head data transfer via serial, SPI port transfer is the best way, but ordinary 8-bit microcontroller in processing speed is defective, it is difficult to meet overall demand, therefore decided to use Atlnel company ARM7 core chip AT91SAM7S64 as controller. The chip has 64 KB of Flash memory chip high speed, 16 KB of on-chip high-speed SRAM (can be the highest clock speed single clock cycle access operations); 1 x USB 2.O full speed (12 Mbps) device ports, chip transceiver, 328 bytes programmable FIFO; master/slave serial peripheral interface (SPI), 8 ~ 16-bit programmable data length, 4 tablets line selection. This design will use USB port with desktop or PC ECG machine communication, SPI port control head with large capacity of data storage and high speed data processing. So in terms of integration, or price/performance ratio, is the ideal choice.

1.3 thermal print head of choice and control

Thermal printing technology is the earliest use of the fax machine, its basic principle is that the printer is receiving data into dot matrix signal control thermal unit heating. the thermal paper on thermal coating developer.

This technology can only use a dedicated thermal paper. Thermal paper is coated with a layer of heating will produce a chemical reaction and discoloration of the coating, similar to the photographic film, but the coating is heating after color developer. Use of the thermal coatings of the characteristics of the thermal printing technology. Thermal print technology lies in heating elements. Thermal printer core with a row of small Semiconductor components. These components are very dense, from 2 · 90dpi ~ 600dpi. These components will be through certain current produces high temperature very quickly, when thermal paper coating experience these elements, in a very short period of time the temperature rises, the coating chemical reaction occurs, showing color.

Thermal printer receives the print data, the print data is converted to bitmap data, and then follow the bitmap data point control printer core of heating elements through current, so the print data into the content of the print on paper.

Because of this design requirements up to and printing 12-lead ECG waveform, so the width of the print head and very high resolution requirements.

After serious performance comparison, use the print head-row-C216, print width of up to 216 mm, i.e. the 1728 point/line. Thermal print head of circuit connection as shown in Figure 2.