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The PC communications model.
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Understanding how different equipment ts in the RS-232 model is critical to connecting two devices successfully by RS-232. With a pretty good understanding of the serial data, we can now look at the actual voltage signals. As I mentioned earlier, when RS-232 was rst developed into a standard, computers and the electronics that drove them were still very primitive and unreliable. Because of this, we ve got a couple of legacies to deal with. The rst is the voltage levels of the data. A mark (1) is actually 12 V, and a space (0) is 12 V. From the gure, you should see that the hardware interface is not simply a TTL or CMOS level buffer. Later in this section I will introduce you to some methods of generating and detecting these interface voltages. Voltages in the switching region ( 3 V) may or may not be read as a 0 or 1 depending on the device. You always should make sure that the voltages going into a PIC microcontroller RS-232 circuit are in the valid regions. Of more concern are the handshaking signals. These six additional lines (which are at the same logic levels as the transmit/receive lines and are shown in Fig. 17.29) are used to interface between devices and control the ow of information between computers. The request to send (RTS) and clear to send (CTS) lines are used to control data ow between the computer (DCE device) and the modem (DTE device). When the PC is ready to send data, it asserts (outputs a mark) on RTS. If the DTE device is capable of receiving data, it will assert the CTS line. If the PC is unable to receive data (i.e., the buffer is full or it is processing what it already has), it will deassert the RTS line to notify the DTE device that it cannot receive any additional information.
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0 > +15 Volts Valid Space Switching Region Ground
Valid Mark 1 > 15 Volts
Figure 17.29 RS-232 data is speci ed at unusually high levels to and from which your circuit will have to translate.
ASYNCHRONOUS (NRZ) SERIAL INTERFACES
The data transmitter ready (DTR) and data set ready (DSR) lines are used to establish communications. When the PC is ready to communicate with the DTE device, it asserts DTR. If the DTE device is available and ready to accept data, it will assert DSR to notify the computer that the link is up and ready for data transmission. If there is a hardware error in the link, then the DTE device will deassert the DSR line to notify the computer of the problem. Modems will deasset the DSR line if the carrier between the receivers is lost. There are two more handshaking lines that are available in the RS-232 standard that you should be aware of, even though chances are that you will never connect anything to them. The rst is the data carrier detect (DCD), which is asserted when the modem has connected with another device (i.e., the other device has picked up the phone). The ring indicator (RI) is used to indicate to a PC whether or not the phone on the other end of the line is ringing or is busy. These lines are used very rarely in PIC microcontroller applications because the AT command set provides a text message for these functions. There is a common ground connection between the DCE and DTE devices. This connection is critical for the RS-232 level converters to determine the actual incoming voltages. The ground pin never should be connected to a chassis or shield ground (to avoid large current ows or be shifted and prevent accurate reading of incoming voltage signals). Incorrect grounding of an application can result in the computer or device with which it is interfacing reseting or having the power supplies blow a fuse or burn out. The latter consequences are unlikely, but I have seen it happen. To avoid these problems, make sure that chassis and signal grounds are separate or connected by a highvalue (hundreds of kiloohm) resistor. Before going too much farther, I should expose you to an ugly truth: The handshaking lines are almost never used in RS-232 (and not just PIC microcontroller RS-232) communications. Normally, three-wire RS-232 connections are implemented as in Fig. 17.30. I normally accomplish this by shorting the DTR/DSR and RTS/CTS lines
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