This article describes two interface units for deriving a keying control from RS-232/V.24 DTR and RTS control lines.
This design uses an opto-isolator to isolate the output terminals from the input terminals.
The FOD852 isolator is chosen because it is low cost (<$1) and has a Darlington pair in the detector, which gives it much higher gain than the simple single transistor isolators. At 5V on the input, LED current will be about 2mA and the output transistor should be able to pass up to about 70mA at saturation. The output transistor is rated at 300V and 150mA, and has an inverse parallel diode for protection within the package. (A 4N28 single stage opto-isolator would be good to switch up to about 3mA in this circuit, and it does not incorporate the protection diode.)
The input circuit has a 2k2 resistor to limit LED current, and a 1N4148 inverse parallel diode to protect the LED from reverse breakdown.
Fig 2 shows an example built into a DE9 backshell.
Fig 3 shows a dual RTS and CTS interface built on a small piece of Veroboard fixed to a DE9 connector. Some Aralidite has been applied over the DE9 topside pins (after the blue wire was soldered) to secure the DE9 to the Veroboard, it is not obvious in the pic. I used SM diodes and resistors, you could use through hole parts.
If ground isolation is not required, a simpler circuit can be built using a bipolar transistor.
Fig 4 shows a schematic for a simple interface using a bipolar transistor. The input and output circuits of this interface share a common ground.
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