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BUSY nACK PE SELECT nERROR
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FIGURE 7.14 The STATUS port.
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170 COMMUNICATION LINKS FOR THE AVR PROCESSOR
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FIGURE 7.15 The CONTROL port.
The data can be sent on the CONTROL port in the same manner as the DATA port (except that the port address is at an offset of two from the base address) and the CONTROL port data is read back in the same manner as the data is read on the STATUS port, again with the offest of two from the base address. Table 7.3 illustrates the signals, connector pin numbers, and functions of all the parallel port signals. A working example of interfacing an AVR processor to the parallel port is illustrated in a later chapter as a project. There is a simple arrangement to connect an AVR processor and the parallel port (Figure 7.16). This mechanism allows 4 bits to be transferred between the parallel port and the AVR. We have chosen the CONTROL port, as it can be used both for data output and data input. To output data, appropriate data is written to the CONTROL port and when reading data, first the CONTROL port bits are all set to 1 (logic high), and then the read buffer (at the same address as the CONTROL port) on the CONTROL port is read. The read value (after correction for the inverters) is the value output by the AVR. One of the STATUS port lines is used by the parallel port as one of the input handshake lines. The output handshake line is by using the DATA port pin as illustrated.
PC PARALLEL PORT 171
TABLE 7-3 ADAPTER DB-25
THE SIGNALS OF THE CENTRONICS PARALLEL PRINTER
CENTRONICS
REG.
NAME
FUNCTION
Control
nSTROBE
Active low. Indicates valid data is on the data lines. 8 data lines Output only in older SPP A low asserted pulse to indicate that the last character was received. A high signal asserted by the printer to indicate that it is busy and cannot take data. Paper empty Asserted high to indicate that the printer is online Active low. Instructs the printer to automatically insert a line feed for each carriage return. Signal by printer to the computer to indicate an error condition. Active low. Used to reset printer Active low. Used to indicate to the printer that it is selected GROUND
2 to 9 10
2 to 9 10
Data
D1 to D8
DATA-1 to DATA-8 nACK
Status
Status
BUSY
12 13
12 13
Status Status
In In
S5 S4
PE SELECT
Control
AUTO FEED
Status
nERROR
16 17
31 36
Control Control
Out Out
C2 C3*
nINIT nSELECTIN
18, 19, 21, 23 to 25, 27, 29 25 30, 34
172 COMMUNICATION LINKS FOR THE AVR PROCESSOR
Parallel Port
C0-C3
PA0-PA3
FIGURE 7.16 Connecting an AVR processor to the PC parallel port.
7.8 ISA Bus
The ISA (Industry Standard Adapter) bus is inside the PC used by the processor to communicate with its peripherals. However, it is slowly being replaced by the faster PCI bus. Even so, it is a useful bus to understand and can be used in many applications, as it is easy to build and is fairly fast. This bus is accessible only on the PC motherboard through a 62-pin female connector. You can connect to it with the help of a PCB with a matching male-type printed connector. The ISA bus has signal lines for data, address, control and power. Figure 7.17 shows how an AVR processor (even an 8-pin processor can be connected this way) can be connected to the ISA bus. The strobe and busy signals are used for handshaking and synchronizing data transfers between the PC and the AVR processor. The way the data transfer is synchronized is illustrated in Figure 7.18. A working example of data transfer using handshake signals is illustrated in one of the projects later. The ISA bus signals are illustrated in Figure 7.19. The signals and their functions are:
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