read barcode from image c#.net FIGURE 7.12 The details of the PC parallel port. in Software

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FIGURE 7.12 The details of the PC parallel port.
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port also has an input port at the same address and is used to read the status of the CONTROL port pins. So, in reality the parallel port has five ports. Figure 7.13 illustrates the detail of the DATA port. The figure shows the internal PC bus connected to an output latch. The outputs of this latch are the DATA port outputs. Similarly, these 8 bits of the data port are also connected to the input of a buffer, which connects to the PC bus. Thus a program running on the PC could output any combination of 8 bits on the DATA port through the output DATA port and, when required, read this combination back through the corresponding input port. It is easy to access the DATA port either for sending data out or reading the state of the DATA port pins using a C program running on the PC. However, to be able to send data to or read data from any port on the PC, it is essential to know the port address. These port addresses are not completely fixed. Different PCs may have different addresses. Rather than guess the address values, the user can find out the actual port addresses in a PC by reading the contents of certain RAM locations called BIOS data area (the address off these RAM locations is known and fixed!). For the PC parallel port, the RAM address is 0040:0008 (hex) for LPT1, 0040:000a (hex) for LPT2, and 0040:000c (hex) for LPT3. In a particular PC, if the number found in these locations turns out to be zero, then it means that the corresponding LPT does not exist on the PC. A nonzero number on these locations indicates that the corresponding LPT exists. This number is the base address of the parallel port. The port addresses of the various ports are as follows:
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R=27 Ohms DATA0 DATA1 DATA2
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FIGURE 7.13 The DATA port.
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1. DATA Port Base Address 2. STATUS Port Base Address 1 3. CONTROL Port Base Address 2
I will show some C code to do various things with the parallel port in this section. To read the base address and to calculate the port addresses of the various ports of the parallel port, do the following:
int offset, DATAport, STATUSport, CONTROLport, BASEaddress; offset = 0x08; /*0x08 for LPT1, 0x0a for LPT2, 0x0c for LPT3*/ BASEaddress = peek(0x40, offset); /*read the address from BIOS data area*/ DATAport = BASEaddress; /* address of DATA port*/ STATUSport = BASEaddress + 1; /*address of STATUS port*/ CONTROLport = BASEaddress + 2; /*address of CONTROL port*/ To write some value to the DATA port pins, we do the following: int offset, DATAport, STATUSport, CONTROLport, BASEaddress; unsigned char data_value; data_value = 0x55; /*write a binary sequence 01010101 on the DATA port pins*/ outportb(DATAport, data_value); To read the state of the DATA port pins, we do the following: int offset, DATAport, STATUSport, CONTROLport, BASEaddress; unsigned char data_pins; data_pins = inportb(DATAport); printf(``\nThe DATA pins have the value = %x , data_pins);
PC PARALLEL PORT 169
Figure 7.14 illustrates the detail of the STATUS port. The figure shows the internal PC bus connected to an input buffer. The inputs of this buffer (some are through inverters) are the STATUS port inputs. The STATUS port has 5 inputs only connected to the upper 5 bits of the 8-bit port. The STATUS port is used to read external data into the PC. The following code shows how to read external data from the STATUS port and how to press the date to account for the inversions on certain bits.
int STATUSport; unsigned char status_pins; status_pins=inportb(STATUSport) & 0xf8; printf( \nActual STATUS port = %X , status_pins); printf( \nSTATUS port shifted = %X , status_pins}}3); printf( \nSTATUS port shifted and corrected = %X , 0x10 (status_pins>>3));
Figure 7.15 shows the block diagram for the CONTROL port of the parallel port. The CONTROL port has 4 bits which are placed in the lower 4 bits of a byte. Of the 4 bits, 3 of the bits are inverted before being output of the port pins. These pins are also read back through an inverting buffer. An inverter on one of the lines accounts for the signal inversion on the output lines, and if this buffer is read, it provides the bit information that was originally sent to the CONTROL port.
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