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PC ISA adapter circuitry.
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TABLE 21.16
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U1 U2 U3 U4, U5 CR1 Y1 R1 C1 C3 C5 J3 Misc.
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PIC16F877 04/P 75176 RS-422/RS-485 interface 74LA138 8 to 1 demultiplexor 74LS85 value comparators Red LED 4-MHz ceramic resonator with internal capacitors 4.7 k , 1/4 W 0.1 F tantalum 0.1 F any type 4 1 screw terminal
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ISA prototyping PCB, wire-wrap sockets, write-wrap wire, telephone cable
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for the 75176 (especially when it was driving the line low when the ISA card s 75176 was driving it high). The solution to this problem was to add a 7805 to provide power for it and the DS1820. The nal circuit is shown in Fig. 21.39 and was built using point-to-point wiring with the bill of materials shown in Table 21.17.
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Vcc Power In
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Power In
0.1 uF Tantalum Gnd Power In
Vcc 75176 Vcc R
0.1 uF
0.1 uF 4 Terminal Screw Connector
10 K
4 15
_RE 18 17
_MCLR Osc 1 Osc 2
B DE A D Gnd
Power In
4 MHz
1 Top
10 K
Figure 21.39 circuit.
The PIC16HC650 microcontroller based remote thermometer
PIC16HV540 7805 75176 DS1820 4 MHz 0.1 10k Connector Misc.
PIC16HV540 JW 7805 5-V regulator
75176/RS-422/RS-485 interface DS1820 TO-92 electronic thermometer 4 MHz with internal capacitors 0.1 F tantalum capacitor 10 k , 1/4 W resistor Four-terminal screw connector Prototype card, sockets, wire-wrap wire
The PIC16HV540 source code (RemPIC.asm) can be found in code\PCTherm folder. The code can be converted to any other 18-pin PIC microcontroller without modi cation. You also might want to use a PIC16F505 and have it powered from the 7805. The problem with the PIC microcontroller I/O pins being unable to supply power to the different components of the circuit is one that I suspect you will be encountering more and more as more sophisticated chips become available with lower current output I/O pins, and you may nd yourself in a situation where you have to go back to earlier designed parts to avoid the need for adding additional power supplies or drivers. The reason for using the 12-V power from the PC to the remote card was to avoid very long wire voltage drops that would cause problems with the voltage from the PC being high enough to run the PIC microcontroller and the other parts on the card reliably. Using 100 feet of four-conductor telephone cable, I found a 0.35-V drop across the line. For debugging the application before it was installed in the PC, I connected the ISA card to a bench 5-V power supply and the remote card to a 12-V power supply and connected them with two conductors of the four-conductor telephone cable. Note in both Figs. 21.38 and 21.39 that I have put the two 4 1 screw terminals in the same orientation. This was to avoid issues with keeping track of the wiring. The convention I used for the wiring is listed in Table 21.18.
Red Black Yellow Green
12 V Ground A pin (positive RS-485 voltage) of 75176 B pin (negative RS-485 voltage) of 75176
- Drive Line High for 15 msecs - Send P ing Character - Drive Line High for 5 msecs - Stop Driving Line - Wait for Data Reply for 25 msecs - If Data Reply V alid: Return Byte Else Return 0 0FF
Remote PIC Micro
- Wait for Line Valid High for 10 msecs - Wait for P ing Character - Drive Line Active High for 15 msecs - Wait 10 msecs After Input Character - Send Reply Character - Stop Driving Line Active
Figure 21.40 thermometer.
The data- ow diagram for the remote
Note that there is no physical difference between this cable and a real telephone connected in your home, so if you are going to install this cable alongside telephone cable, make sure that you keep track of which cable is which. Note that plugging phones into this cable or a PC could damage either device (which could be very expensive if phone company equipment is damaged). For the data communication, I used 1,200-bps NRZ serial communications with some special 75176 control timings to ensure that the two PIC microcontrollers do not get into contention with one another. The communication s ow for the two PIC microcontrollers is shown in Fig. 21.40, and note that when the communication is working correctly, neither device will be pulling the RS-485 line low. When I introduced RS-422 and RS-485, I did note that multiple devices could drive at the same time, but I found that in this application, owing to the method of providing power to the remote PIC microcontroller, I could not get enough current to reliably pull the data line low. This method of communication works quite well, as you can see in the oscilloscope pictures shown in Figs. 21.41 and 21.42. The top line in both diagrams shows the actual RS-485 positive-voltage signals. Figure 21.41 shows the signals the PIC16F877 master works with, and you can see the 75176 driver being turned off (the lower line) when the temperature data from the remote PIC microcontroller is expected. During this time, the line remains driven, and valid data is received by the PIC16F877. In Fig. 21.42, the incoming ping character (P or ASCII 0x50) can be seen on the 75176 data line, after which it drives the line, waits 15 ms, and drives the current temperature back on the line. When the transmission has been completed, the remote PIC16HV540 stops driving the RS-485 line (at which point the PIC16F877 resumes driving the line high). After the remote PIC microcontroller has responded to the PIC16F877, it polls the DS1820 for the current temperature. This temperature is transmitted back to the PIC16F877 the next time the ping character is received. The PIC16F877 polls the remote device once every second, so there is plenty of time for reading the current temperature from the DS1820.
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