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Figure 17.31 The Maxim MAX232 is a commonly used chip that generates its own 12 V for RS-232 communications.
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From Purchase RS-232 TX
Figure 17.32 Voltage-stealing RS-232 interface that inverts the RS-232 signals to and from the PIC microcontroller.
handshaking interface). Ground for the incoming signal is connected to the processor ground (which is not the case s ground). Along with the MAX232, Maxim and some other chip vendors have a number of other RS-232 charge-pump-equipped devices that will allow you to handle more RS-232 lines (to include the handshaking lines). Some charge-pump devices that are also available do not require the external capacitors that the MAX232 chip does, which will simplify the layout of your circuit (although these chips do cost quite a bit more). The next method of translating RS-232 and TTL/CMOS voltage levels is to use the transmitter s negative voltage. The circuit in Fig. 17.32 shows how this can be done and will be demonstrated later in this book. This circuit relies on the RS-232 communications only running in half-duplex mode (i.e., only one device can transmit at a given time). When the external device wants to transmit to the PC, it sends the data either as a mark (leaving the voltage returned to the PC as a negative value) or as a space by turning on the transistor and enabling the positive-voltage output to the PC s receivers. If you go back to the RS-232 voltage speci cation drawing, you ll see that 5 V is within the valid voltage range for RS-232 spaces. This method works very well (consuming just about no power) and is obviously a very cheap way to implement a three-wire RS-232 bidirectional interface. As an aside, before going on to the last interface circuit, I should point out a big advantage of the preceding circuit. The advantage results from the fact that the PIC microcontroller s RS-232 transmitter circuit receives its negative voltage from the PC s RS-232 transceiver if the external device (with this circuit) is connected to a PC. This means that the PC s TX and RX are connected together, and it can ping (send a command that is ignored by the external device) via the RS-232 port to see if an external device is connected to it. To do this, you need to specify the ping character as something that the external device can recognize and modify so that the PC s software can recognize that the interface is working. The method that I have employed in the past is to use a microcontroller with bit banging software to change some mark bits when it recognizes that a ping character is being received.
Transmitted Character From PC (0 0F0) Transistor Control At External Device
+12 V Gnd 12 V
+5 V Gnd
+12 V
Data Received by PC (0 090)
Gnd 12 V
+5 V
Figure 17.33 Sending 0xF0 and letting the receiver modify it is one way of detecting whether or not there is a receiver and if it is active.
In Fig. 17.33 I show a ping character of 0xF0 that is modi ed by the external device (by turning on the transistor) to change some bits into spaces. If the PC receives nothing at all, then there is nothing attached to it, and if it receives 0xF0, then the external device is not active. With the availability of many CMOS devices requiring very minimal amounts of current to operate, you might be wondering about different options for powering your circuit. One of the most innovative that I have come across is using the PC s RS-232 ports themselves as powering devices that are attached to it using the circuit shown in Fig. 17.34. When the DTR and RTS lines are outputting a space, a positive voltage (relative to ground) is available. This voltage can be regulated and the output used to power the devices attached to the serial port (up to about 5 mA). For extra current, the TX line also can be added into the circuit as well, with a break being sent from the PC to output a positive voltage. The 5 mA is enough current to power the transistor/resistor type of RS-232 transmitter and a PIC microcontroller running at 4 MHz, along with some additional hardware (such as an LCD). You will not be able to drive an LED with this circuit, and you may nd that some circuits that you normally use for such things as pull-ups and pull-downs will consume too much power, and you ll have to specify different resistance values.
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