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Signal-powered switch connects devices

09 Jul 2012  | James Dean

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Figure 1's switch can selectively connect a terminal, personal computer, or any other originating device (DTE) to one of four receivers such as modems and printers. Using a thumbwheel switch, you select which device you want to connect. The circuit handles data rates to 19.2k baud without any signal degradation or crosstalk.

signal-powered selector

Four Maxim DG509A dual 4-channel multiplexers switch the eight critical RS-232C lines. The Maxim multiplexers suit this application because they consume only 20 μA typ, handle ±18V signals, exhibit only 130Ω on-resistance typ, and have input protection.

The unit doesn't require an external power supply because it gets its power from the RS-232C ready lines (DTE pins 4 and 20, DCE pins 5, 6, and 8); the S17661 converter provides the negative supply. The Schottky power diodes in the dual-diode MBR2045CT packages ensure that the switch draws its power from whichever connected device has the highest output voltage at its RS-232C connector. The low forward-voltage drop of these diodes guarantees that the internal-protection diodes of the switch ICs do not conduct. The circuit's V+ supply is merely one Schottky-diode drop below the highest signal potential.

Note:Originally published in the April 14, 1988, issue of EDN

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