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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 3.5 - 11 25 Monochrome video card single-chip design 4.9 18 35 35 Older multifunction card with IDE and floppy controller 4.9 2.7 5 7 SMC Super I O controller 4.9 0.6 7.5 27 VIN ANY DATA-PORTC OUTPUT 3.3V 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 3.3V Figure 6-5 Maxim s Max756 can convert a Data output to a regulated 5V or 3.3V supply. Maxim s MAX756 step-up converter can convert 2.5V 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 2.6 milliamperes at 2.1V 2.4V minus a 0.3V 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 0.8 2.1 0.0026 and this shows that each Data pin can provide just under 0.9 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 www.verypdf.com 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 3.3V. The 757 has an adjustable output from 2.7V to 5.5V. 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