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RS232 with single power supply

In most personal computers, the power supply provides +5V and +/- 12V lines. The positive 12 V line is needed for the disk drives. and the +/- 12 V for the RS 232 interface. Over the past few years, ICs have become available (such as the MAX 232) that can drive serial channels (which needs +/- 12 V) from single +5 V line. In the diagram, a Motorola MC 145407 and four electrolytic capacitors provide +/- 10 V (the supply for RS 232  connections may lie between  +/- 5V and +/- 15 V). The circuit also provides three input buffers and three output buffers (the MAX 232 provides two of each). If more buffers are needed. the IC can supply a 145406 via its Vdd and Vss pins to give a total of six input buffer and six output buffers. The 10-volt potentials are generated by an integral 20 kHz oscillator and two voltage doublers. When the supply is loaded. these voltages drop a little but remain well within the RS 232 requirement. The open circuit current drawn by the IC is only 1.5 mA. but this increases, of course, under load conditions. It is advisable to keep the construction as compact as possible and to locate the electrolytic capacitors very close to the relevant pins. The 330 nF decoupling capacitor must be soldered with the shortest possible leads between pins 2 and 19. Preferably . do not use an IC socket, because fairly high peak currents flow when the electrolytic capacitors are being charged.

RS232 with single



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