Compact RS232 isoltaer: The MAX252 from Maxim is a special IC for designing an electrically isolated RS232 connection. Such links are particularly important when several pieces of equipment (which may or may not be isolated from the mains) are to be interconnected and they have different earth potentials.
The chip is available in two modes: A. (expensive) and B (inexpensive). Note, however, that even inexpensive is relative because the B type still costs more than 30.
The maximum isolation voltage that the B-type can handle for one minute is 500 V, whereas the A-type can handle up to 1250 V. Moreover, the A-type can have a constant voltage across its terminals of 130 V. The maximum transmission speed of both types is 9600 baud.
The internal transformer of the chip provides the supply voltage for the right-hand side of the diagram. The input and output levels at the left-hand side are TTL compatible. The inputs have a hysteresis of roughly 0.5 V and pull-up current sources of 0.4 pA for the suppression of noise signals. The inputs and outputs at the right-hand side are RS232 compatible. Outputs T1out and T20„t deliver a voltage of 7.5 V to a load of 3 kΩ.
With a supply voltage of 5 V, the IC draws a current of 90 mA.
Pin 11 of the MAX252 is an active-low out-put-enable input and pin 3 an active-high shut-down input. A high level at pin 3 causes the 130 kHz oscillator of the integrated supply to be switched off, Tlout and T2out to become high-impedance, the 4 μA pull-up inputs to be deactuated, the power acceptance to be reduced to 50 μW.
RS232 compatible levels at the left-hand side can be provided by the MAX233 as shown in the diagram. This IC also arranges the necessary inversion of the various signals.