tailieunhanh - Parallel Port Complete- P7

Parallel Port Complete- P7: 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 6-1 Level-1 driver and buffer chips for parallel-port circuits. Drivers for the Data Status and Control inputs 74LS244 74HC T 244 octal buffer 74LS240 74HC T 240 octal inverting buffer 7405 7406 open-collector hex inverting buffer 7407 7417 open-collector hex buffer Use open-collector drivers for the Control lines. Schmitt-trigger buffers for the Data or Control outputs 74LS14 74HCT14 hex inverter 74LS374 octal buffered flip-flop 74LS244 octal buffer 74LS240 octal inverting buffer In normal operation the outputs don t provide their maximum rated currents continuously but the ability to source and sink high currents means that the output has low impedance and this in turn implies that the output can switch quickly. As an output switches the voltage must charge or discharge through the cable s capacitance and the lower the output impedance the faster the voltage can change. Ordinary LSTTL logic gates like the 74LS14 hex inverter are guaranteed to sink just 8 milliamperes at so these aren t recommended for driving a parallel cable. Standard TTL such as the 7405 does meet the requirements. The drawback to using standard TTL is that each chip draws 20-40 milliamperes compared to 8-12 milliamperes for an equivalent LSTTL chip or 15-35 milliamperes for an LSTTL octal driver. The HCMOS family has equivalents to most LSTTL chips. However the data sheets for the 74HC24X buffer drivers don t include enough information to guarantee that these chips meet the Level 1 requirements. With a power supply of the outputs are guaranteed to sink 6 milliamperes at . The sink current will be greater with a 5V supply and output but the data sheets don t include figures for these conditions. Overall the outputs of HCMOS driver chips aren t are strong as LSTTL although in most situations they ll work without problems. Receivers These are the characteristics of Level 1 receivers Logic-high inputs maximum at sink current. Logic-low inputs