Tài liệu Parallel Port Complete- P7 doc

Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. [...]... your circuit to use a serial interface directly Port- powered Circuits Most devices that connect to the parallel port will require their own power supply, either battery cells or a supply that converts line voltage to logic voltages But some very low-power circuits can draw all the power they need from the port itself When to Use Port Power The parallel- port connector doesn't have a pin that connects... current-sourcing abilities of a port' s outputs are equal to those of the original port Most ports do in fact meet this test, and many newer ports have the more powerful Level 2 outputs It's a good idea to also include the option to run on an external supply, which may be as simple as a couple of AA cells, in case there is a port that isn't capable of powering your device On the original port, the eight Data... an option for these ports, but it isn't practical for a general-purpose circuit intended for any port I ran some informal tests on a variety of parallel ports, and found widely varying results, as Table 6-5 shows The port with 74LS374 outputs actually sourced much more current than the specification guarantees, about the same as the Level 2 outputs on an SMC Super I/O controller A port on an older monochrome... a wavelength of 60 meters, and 1/20 of that is 3 meters, or about 10 feet, which is the length of a typical parallel cable From this, you might think that a parallel cable isn't a transmission line because the parallel port' s maximum rate of transmitting is much less than 5 Mhz But what's important isn't how often the voltages switch, but rather how quickly they switch This is because the frequencies... especially suited to using parallel- port power is anything that uses a synchronous serial interface, such as the DS1620 digital thermometer described in Chapter 9 These require as few as one signal line and a clock line, leaving plenty of bits for other uses Abilities and Limits One problem with using parallel- port power is that the outputs have no specification that every port adheres to If you're... www.verypdf.com to remove this watermark Chapter 6 could do the same for each of the lines in a parallel link, but as the distance increases, it makes sense to convert to serial and save money on cabling One drawback to the parallel- to-serial converters is that most are one way only, and don't include the parallel port' s Status and Control signals You can use the converters for simple PC-to-peripheral transfers,... that both the port and the device that connects to it have Level 2 interfaces, this type of cable should go 30 feet without problems Parallel- port extenders are also available from many sources One type adds a line booster, or repeater, that regenerates the signals in the middle of the cable, allowing double the cable length Other extenders work over much longer distances by converting the parallel signals... on just about all ports Typical output current for a 74LS374 is 2 6 milliamperes at 3 1 V (2.4V guaranteed) A logic-low output of a '374 can sink much more than this, but a low output doesn't provide the voltage that the external circuits need Level 2 outputs can source 12 milliamperes at 2.SV If you know that your port has Level 2 outputs, you have more options for using parallel- port power What about... parallel port' s inputs require TTL logic levels, so any logic-high outputs that connect to the parallel- port inputs should be at least 2.4V (Status -port inputs may have pullups to +SV, but this isn't guaranteed ) Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark Interfacing The source for port power is usually one or more of the Data pins If you bring a Data output high by writing... levels defined in proportion to the supply voltage (Minimum logic high input = 0.7(supply voltage); maximum logic low input = 0.3(supply voltage).) National's LP324 quad op amp draws under 250pa of supply current and can use a single power supply as low as +3V If you need +SV, there are new, efficient step-up regulator chips that can convert a lower voltage to a regulated +SV The parallel port' s inputs require