tailieunhanh - Parallel Port Complete- P8

Parallel Port Complete- P8: From its origin as a simple printer interface, the personal computer's parallel port has evolved into a place to plug in just about anything you might want to hook to a computer. The parallel port is popular because it's versatile-you can use it for output, input, or bidirectional links-and because it's available-every PC has one. | Interfacing Table 6-5 Results of informal tests of current-sourcing ability of the Data outputs on assorted parallel ports. Card No Load Voltage Source Current at Data output milliamperes 4V 3V 2V Original-type LS374 outputs - 11 25 Monochrome video card single-chip design 18 35 35 Older multifunction card with IDE and floppy controller 5 7 SMC Super I O controller 27 VIN ANY DATA-PORTC OUTPUT 0 . 1 zF z-p i _5 LBI LX 8 2 3 5 GND 7 _4 L B0 OUT 6 3 VREF SHDN 1 MAX756 STEP-UP DC-DC CONVERTER j VOU T 5V OR Figure 6-5 Maxim s Max756 can convert a Data output to a regulated 5V or supply. Maxim s MAX756 step-up converter can convert to 5V with over 80 efficiency. Figure 6-5 shows a supply based on this chip. As an example assume that you want to power a circuit that requires 2 milliamperes at 5V and assume that the parallel port s Data outputs can provide milliamperes at minus a drop for the diodes . This formula calculates how much current each Data pin can provide load supply V output current A converter efficiency source voltage V source current A which translates to 5 output current and this shows that each Data pin can provide just under milliampere at 5V. Three Data outputs could provide the required total of 2 milliamperes with some Parallel Port Complete 127 Ple ase purchase PDF Split-Merge on to remove this watermark. Chapter 6 to spare. In fact there is a good margin of error in the calculations and you could probably get by with two or even one output. If the port has Level 2 outputs each pin can source 4 milliamperes so all you need is one pin. You can do similar calculations for other loads. The 756 has two output options 5V and . The 757 has an adjustable output from to . The selection of the switching capacitor and inductor is critical for the MAX756 and similar devices. The inductor should have low DC resistance and the capacitor should be a