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Figure 17.34 The unused handshaking lines can be used to provide power for your PIC microcontroller application.
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PIC MCU INPUT AND OUTPUT DEVICE INTERFACING
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Positive Signal Negative Terminal
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Figure 17.35 For very long distances, you can transmit data using differential-pair wiring.
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Now, with this method of powering the external device, you do not have use of the handshaking lines, but the savings of not having to provide an external power supply (or battery) will outweigh the disadvantages of having to come up with a software pinging and handshaking protocol. Externally powering a device attached to the RS-232 port is ideal for input devices such as serial mice that do not require a lot of power.
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So far in this book I have discussed single-ended asynchronous serial communications methods such as RS-232 and direct NRZ device interfaces. These interfaces work well in home and of ce environments but can be unreliable in environments where power surges and electrical noise can be signi cant. In these environments, a double-ended or differential-pair connection is optimal to ensure the most accurate communications. A differential-pair serial communications electrical standard consists of a balanced driver with positive and negative outputs that are fed into a comparator that outputs a 1 or a 0 depending on whether or not the positive line is at a higher voltage than the negative line. Figure 17.35 shows the normal symbols used to describe a differential-pair connection. There are several advantages to this data-connection method. The most obvious one is that the differential pair doubles the voltage swing sent to the receiver, which increases its noise immunity. This is shown in Fig. 17.36; when the positive signal goes high, the negative voltage goes low. The change in the two receiver inputs is 10 V rather than the 5 V of a single line. This is assuming that the voltage swing is 5 V for the positive and negative terminals of the receiver. This effective doubling of the signal voltage reduces the impact the electrical interface has on the transmitted signal.
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Positive Signal Negative Signal
Combined Signal
Figure 17.36 Differential data transmission consists of two opposite lower-voltage signals creating a high-quality signal.
ASYNCHRONOUS (NRZ) SERIAL INTERFACES
Another bene t of differential-pair wiring is that if one connection breaks, the circuit will operate (although at reduced noise-reduction ef ciency). This feature makes differential pairs very attractive in cars, aircraft, and spacecraft, where loss of a connection could be catastrophic. To minimize ac transmission-line effects, the two wires should be twisted around each other. Twisted-pair wiring can either be bought commercially or made simply by twisting two wires together; twisted wires have a characteristic impedance of 75 or greater. A common standard for differential-pair communications is RS-422. This standard, which uses many commercially available chips, provides
1 Multiple-receiver operation 2 Maximum data rate of 10 Mbps 3 Maximum cable length of 4000 m (with a 100-kHz signal)
Multiple receiver operation, as shown in Fig. 17.37A, allows signals to be broadcast to multiple devices. The best distance and speed changes with the number of receivers of the differential pair, along with its length. The 4000 m at 100 kHz or 40 m at 10 MHz
Driver + RXn
+ RX 0
8 Vcc 4 3
RX 1
8 Vcc 7 6 7 6 3 4
Gnd 5
Figure 17.37 A. The RS-422 protocol allows multiple receivers on the same differential-pair wiring. B. The 75176 driver chip is used to create a single RS-485 differential-pair communications medium.
Gnd 5
PIC MCU INPUT AND OUTPUT DEVICE INTERFACING
are examples of this balancing between line length and data rate. For long data lengths, a few-hundred-ohm terminating resistor may be required between the positive terminal and the negative terminal at the end of the lines to minimize re ections coming from the receiver and affecting other receivers. RS-422 is not as widely used as you might expect; instead, RS-485 is much more popular. RS-485 is very similar to RS-422, except that it allows multiple drivers on the same network. The common chip is the 75176, which has the ability to drive and receive on the lines, as shown in Fig. 17.37. In the right 75176 of Fig. 17.37. I show the RX and TX and two enables tied together. This results in a two-wire differential I/O device. Normally, the 75176s are left in RX mode (pin 2 reset) unless they are driving a signal onto the bus. When the unused 75176s on the lines are all in receive mode, anyone can take over the lines and transmit data. As with the RS-422, multiple 75176s (up to 32) can be on the RS-485 lines with the capability of driving or receiving. When all the devices are receiving, a high (1) is output from the 75176. This means that the behavior of the 75176 in the RS-485 (because these are multiple drivers) is similar to that of a dotted AND bus; when one driver pulls down the line, all receivers are pulled low. For the RS-485 network to be high, all unused drivers must be off, or all active drivers must be transmitting a 1. This feature of the RS-485 is taken advantage in small system networks such as CAN. The only issue to be on the lookout for when creating RS-485/RS-422 connections is to keep the cable polarities correct (positive to positive and negative to negative). Reversing the connectors will result in lost signals and misread transmission values.
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