tailieunhanh - Parallel Port Complete- P10

Parallel Port Complete- P10: 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. | Output Applications Sub LabelTheGrid Dim Row Dim Column 0 For Row 1 To 8 grdXY . Row Row Y Row - 1 Next Row 0 For Column 1 To 8 Column X Column - 1 Next Column 8x8 Crosspoint Switch End Sub Listing 7-2 Controlling an 8 x 8 crosspoint switch Sheet 2 of 2 display the switch matrix. When you click on a cell the associated switch opens or closes. An X indicates a closed switch an empty cell indicates an open switch. You can make and break as many connections as you want by writing appropriate values to the chip. All previous switch settings remain until you change them by writing to the specific switch. The switches can connect in any combination. For example you can connect each X pin to a different Y pin to create eight distinct signal paths. Or you can connect all eight Y pins to a single X pin to route one signal to eight different paths. The X and Y pins may connect to external inputs or outputs in any combination. Figure 7-9 shows the 22106 powered at 5V but the supply voltage may range from 2 to 10V and Vss and Vdd may be negative. The HCT version 74HCT22106 requires a 5V supply. The chip can switch any voltages within the supply range. However the maximum and minimum values for the address and control signals vary with the supply voltage. For example if Vdd is 5V and Vss is -5V the address and control signals can no longer use 5 V CMOS logic levels because the logic levels are in proportion to the supply voltage. The maximum logic low for these signals drops from to -2V Vss Q3 Vdd-Vss and the minimum logic high drops from to 2V Vss Vdd-Vss . At 5V the switches typical on resistance is 64 ohms dropping to 45 ohms at 9 V. The chip can pass frequencies up to 6 Megahertz with supplies. In Figure 7-9 the parallel port s D0-D7 control the switch array. The 74HCT244 buffer has TTL-compatible inputs and CMOS-compatible outputs. If you use a 74LS244 add a 10K pull-up resistor from each