What is your TNC doing? - Push-To-Talk Circuit

Chapter 5 continued - TNC to Radio Wiring and Adjustments - Chapter Table of Contents

Push-To-Talk Circuit

The Push-To-Talk (PTT) line from the TNC attaches to the radio at a pin often called PTT, STBY (standby), SEND, SS, or PKS. The PTT line is used to key the transmitter. Any signal received by the radio on the Transmit Signal line will not be transmitted unless the PTT line is first asserted. There is no buffer in the radio - if PTT is not asserted, the transmit signal will go nowhere. When the TNC has a frame to transmit, it first asserts PTT and then sends the data on the Transmit Signal line. This is comparable to a person pushing the PTT button on a microphone before speaking.

The PTT circuit is shown in Figure 14. There are several ways to design the circuit in the TNC; but no matter how it is designed, the circuit acts as a switch. When PTT is asserted, the switch is closed and current is allowed to flow through the PTT circuit. This enables the radio to transfer power to the Transmitter Keying circuit. When the switch is open (not asserted), the PTT circuit is open and no power can be transferred to the keying circuit. With an open circuit, the radio will not key and there will be no transmit signal output. For radios that connect the transmit signal and PTT to the same radio pin (such as on the Yaesu FT-5100 DATA IN/OUT phone jack), follow the instructions below for hand-helds.

Hand-Held Radios

Many hand-held radios (most Icom and Yaesu) need the PTT wire connected at the same point where the Transmit Signal wire is connected. The PTT switch in the TNC acts the same as explained above - when PTT is asserted the switch is closed. Current can then flow in the PTT circuit and power the radio's Transmitter Keying circuit. However, since the transmit signal enters the radio on the same wire as PTT, things are more complicated.

The PTT circuit is a dc circuit and the Transmit circuit is an ac circuit. The radio is built to accept both circuits on the same wire and separate them as needed. The TNC on the other hand does not expect the PTT and Transmit circuits to join between the TNC and radio. When wired to a base radio, two separate wires enter the radio. The TNC design must be different if there is one or if there are two wires entering the radio. TNCs are designed to work with two separate wires.

When we join the two wires from the TNC, we must place components in the wires to prevent the circuits from joining in an undesirable manner. The components may be either a resistor and capacitor, or a transformer, as explained below. These components can often be placed in the radio connector shell, such as the DB- 9 on Kantronics units. (It is possible to modify the TNC. However, it may then not work with a base radio.)

Resistor and Capacitor

The PTT circuit with a resistor and capacitor is shown in Figure 15. The transmit signal from the TNC will follow the path of least resistance. If there is no resistor in the PTT line, that line looks like less resistance than the line going to the radio. Therefore, most of the transmit signal will go on the PTT line, back into the TNC and straight to ground. To prevent this, we place a resistor in the PTT line. This increases the resistance in the PTT line and makes the line going to the radio contain the least resistance.

The value of the resistor depends on the amount of input signal needed by the radio to produce a desirable transmitter output. If the resistor is too small, PTT will be asserted and a signal will be transmitted. However, there will be no (or little) audio on the signal since too much of the signal went back into the TNC and to ground. If the value of the resistor is too large, there will not be enough current to assert PTT and no signal will be transmitted.

Capacitors block dc. Therefore, we place a capacitor in the Transmit Signal wire to block any dc from the PTT circuit from entering the transmit section of the TNC. As mentioned under Transmit Signal, this capacitor also blocks the dc from the radio that powers the microphone. The capacitor does not affect the transmit signal because ac can pass through capacitors. A capacitor value from 0.1 microfarad to 1 microfarad is sufficient for most situations. (Some TNCs are designed so the capacitor is not needed.)

Isolation Transformer

The PTT circuit with a transformer is shown in Figure 16. The PTT and Transmit Signal wires meet at ground; therefore, the two circuits can not combine in an undesired manner. The transmit signal is transferred through the transformer to the wire that connects to the radio.

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