This article describes a simple interface from a V.24 or RS-232 DTR or RTS line to the common transceiver PTT interface.
The interface signals are an open circuit voltage:
When DTS or RTS are asserted, they will have positive voltage on the leads, so they are tecnically speaking inverted outputs, /DTR and /RTS where / signifies the logical negation operator.
Most modern transceivers have a PTT control line that must sink current up to some tens of milliamps to ground to activate the transmitter. The open circuit voltage is typically from +5V to +28V. In the common convention of positive logic where a low voltage represents logical 0, then this signal is actually /PTT.
The requirement is for a circuit that creates /PTT from DTR. In voltage levels then, a voltage greater than +3 must cause the adapter to sink positive current on the /PTT output pin.
The computer system and radio system usually share a common ground, or will not be degraded if the ground systems are tied together by an interface adapter. In most situations, there is not real need to implement galvanic isolation of the DTR - PTT control path.
Fig 1 above is a circuit diagram of a suitable interface. The interface adapter is in the middle of the circuit between the dashed lines. The circuit to the left represents the V.24 interface, and that to the right, a PTT relay in a transceiver.
Fig 2 above is a simulation of the circuit in LTSPICE of a short pulse to demonstrate circuit behaviour.
The green waveform is the driving voltage V2 which has been shaped to have a slow rise time to illustrate the circuit behaviour.
The blue line is the collector voltage, it can be seen to rapidly fall when the input voltage increases above 0.5V, turning hard on to allow current to flow in the relay coil (represented by L1 and R2).
Collector current (magenta line) rises slowly due to the effect of
the series L1 and R2, eventually reaching 12mA. The relay coil current
(red line) follows the collector current up to 12mA. The relay would
probably operate when coil current reaches about 80% of its normal
At about t=12mS, RTS is released, and the input voltage changes to
-7V. When the input voltage falls below about 0.5V, the transistor
ceases conducting, and collector current rapidly falls to zero. The
relay would probably release when the current has decayed to a quarter
or so of its normal operating current (about t=13ms on Fig 2).
When the transistor ceases conducting, the current in the relay coil
creates a back EMF that sustains current in the coil via the quenching
diode D3 (cyan line). The voltage drop caused by the current in the
diode causes the collector voltage to rise above the supply voltage
during the discharge of the coil energy. The coil current decays
exponentially until at about t=15mS, the coil current is near zero.
© Copyright: Owen Duffy 1995, 2021. All rights reserved. Disclaimer.