read barcode from image c#.net D0-D7 (I/O): The data bus. The 8-bit data bus is bidirectional and is used for data in Software

Create Code-128 in Software D0-D7 (I/O): The data bus. The 8-bit data bus is bidirectional and is used for data

1. D0-D7 (I/O): The data bus. The 8-bit data bus is bidirectional and is used for data
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transfer from and to the adapter cards that fit into the card slots.
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2. A0-A19 (O): The address bus has 20 bits and indicates the address of the data trans-
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fer between the CPU and other devices or the DMA controller and other devices.
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3. IOW* (O): This signal is generated either by the processor or the DMA controller to
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indicate that data transfer to the addressed destination port is in progress.
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4. IOR* (O): This signal, generated by the processor or the DMA controller, indicates
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that data is read from the addressed port (Figure 7.20).
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5. MEMW*(O): This signal, generated by the CPU or the DMA controller indicates that
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the CPU or the DMA controller wants to write data into the addressed memory location (Figure 7.21).
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ISA BUS 173
ISA Bus
Buffer
Latch
AVR Processor
Data Bus
PORTB
Strobe
Busy
Control Bus
Address Bus Address Decoder
FIGURE 7.17 ISA bus interface for the AVR.
6. MEMR* (O): This signal, generated by the CPU or the DMA controller, indicates that
the CPU or the DMA controller wants to read data from the addressed memory location.
7. RESET DRV (O): This signal provides the reset signal to ports and other devices dur-
ing power up or during a hardware reset. It is an active high signal.
8. IRQ2-IRQ7 (I): These are the interrupt inputs to the Priority Interrupt Controller
(PIC) chip on the motherboard.
9. CLK (O): This is the highest frequency available on the card slot and is three times
the OSC frequency.
10. OSC (O): This is the clock signal to which all the IOW* and other strobe signals are
referenced. It has a frequency between 4.77 MHz on the original PC to 8 MHz on the new PCs. 11. ALE (O): The is the Address Latch Enable signal. During a transfer from/to the CPU, the CPU places the address on the address lines. The original CPU had the lower 8 address lines multiplexed with the 8 data bits. The ALE signal is used as a demultiplexer signal for the address information. On the system bus, the address and the data bits are already demultiplexed and the ALE signal is only used as sync signal to indicate the beginning of a bus cycle.
174 COMMUNICATION LINKS FOR THE AVR PROCESSOR
Data
Strobe
Busy
FIGURE 7.18 ISA bus interface data transfer protocol for the AVR. 12. TC (O): This signal is generated by the system DMA controller to indicate that one
of the channels has completed the programmed transfer cycles.
13. AEN (O): The AEN signal is generated by the DMA controller to indicate that a DMA
cycle is in progress. A DMA cycle could involve a port read and a memory write. However, the port address on the expansion card should not respond to the port read bus cycle if it is not intended. By using the AEN signal, the card circuit can detect if the bus cycle is issued by the CPU or the DMA controller and respond accordingly. A high AEN indicates the bus cycle issued by a DMA controller. I/O CH RDY (I): This signal is used by the card circuit to indicate to the CPU or the DMA controller to insert wait states in the bus cycle. Up to 10 clock cycles can be inserted. I/O CH CK* (I): This signal can be used by the circuit on a plug-in card to indicate an error to the motherboard. A NMI corresponding to INT2 is generated by the motherboard circuit in response to a low I/O CH CK* signal. DRQ1-DRQ3 (I): This is an input signal to the DMA controller on the motherboard. When a port device wants to transfer data to and from the memory, it can use the DMA transfer cycle to do that. The operation of the DMA transfer cycle is controlled by the DMA controller. DRQ1 to DRQ3 are the three inputs to the DMA controller. The system ROM BIOS put DRQ1 at the highest priority and DRQ3 at the lowest at reset. The DMA controller has four channels, but one of them, DRQ0, is used on the motherboard to generate dummy read cycles to refresh dynamic memory. DACK0*-DACK3* (O): These are the four status outputs of the DMA controller that indicate the acceptance of the DRQ request. The DMA transfer cycles begin after the DACK* line is put to 0. Power supply: The motherboard provides 5 V, 12 V, 5 V, and 12 V voltages to the card slots. The ve voltages are guaranteed to be within 5% of their nominal values, and the -ve voltages between 10%.
7.9 Universal Serial Bus
The USB (Universal Serial Bus) is one the most upcoming and recent interfaces available on the PCs. It is being used to connect all sorts of peripheral devices to the PC, including
UNIVERSAL SERIAL BUS 175
Signal Names
Pin Numbers
Signal Names
GND RESET +5V IRQ2 -5V DRQ2 -12V RESRVD +12V GND MEMW* MEMR* IOW* IOR* DACK3* DRQ3 DACK1* DRQ1 DACK0* CLOCK IRQ7 IRQ6 IRQ5 IRQ4 IRQ3 DACK2* T/C ALE +5V OSC GND
B1 B2 B3 B4 B5 B6 B7 B8 B9 B10 B11 B12 B13 B14 B15 B16 B17 B18 B19 B20 B21 B22 B23 B24 B25 B26 B27 B28 B29 B30 B31
A1 A2 A3 A4 A5 A6 A7 A8 A9 A10 A11 A12 A13 A14 A15 A16 A17 A18 A19 A20 A21 A22 A23 A24 A25 A26 A27 A28 A29 A30 A31
IOCHK* D7 D6 D5 D4 D3 D2 D1 D0 I/OCHRDY AEN A19 A18 A17 A16 A15 A14 A13 A12 A11 A10 A9 A8 A7 A6 A5 A4 A3 A2 A1 A0
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