PTT (Push to Talk/Transmit) Cable

This page has advice for hams who want to build their own sound card to radio interface.

AGWPE will generate the transmit audio, but you also need a way to open the radio's PTT circuit so that the audio can be transmitted. AGWPE and other sound card programs provide for this by sending a signal to a RS232 pin that can activate a simple hardware circuit to open your radio's PTT circuit. AGWPE uses these RS232 pins:

A. Building A Cable With An Isolated Gate Circuit

Note: Because a sound card has two audio channels, it is possible to interface two (2) radios to a single sound card. This page describes how to make a cable to control the PTT for a single (1) radio. See the Interfacing 2 Radios to 1 Soundcard page for a dual PTT cable.

You may find other variations of the circuit below, most commonly one using a simple transistor (in fact, there's one in the AGWPE 'Help' file). While these circuits will work, they do create a direct ground connection between the computer and radio, and thus create a risk of radio/computer damage in the event of different voltage potentials between the two components. For that reason, I don't recommend them.

Instead, I recommend an isolated 'gate' circuit. Here's a schematic for a single port (one Radio) AGWPE setup which uses a phototransistor (optocoupler) as the gate:

The ideal components for your circuit may vary because of your radio's requirements. For example, you might be able to use a different type of IC, such as a 4N25 or PS2601, but that may require a different value for R1.

If you plan to build your own interface, Bux CommCo can provide advice on components for your radio (Bux CommCo components). You might also want to consider a Bux CommCo's RASCAL kit . It will have all the right components for your radio for a good price.

B. Serial-to-USB Adapters

It's becoming easier to find a fairly inexpensive serial-to-USB adapters. In fact some computers now come with USB ports but no serial port. If this is your case or if you prefer to make use of the USB port to leave your serial and parallel ports free for non-AGWPE purposes, a serial-to-USB adapter may work for you.

Stephen Smith WA8LMF wrote this in the UI-View Yahoo list about USB-to-Serial adapters:

These devices vary wildly in how faithfully they emulate a "real" serial port. This is influenced by both the hardware (chipset that the device is based on) and the software drivers provided. Some are fine for simple three-wire TXD/RXD/COMMON serial hookups (i.e. like connecting a TH-D7) but won't work if the device or software requires handshaking using the RTS/CTS/DTR control lines. Some work for normal serial port data-flow-control usage of the handshake lines, but fail when the handshake lines are used in unusual ways. This includes ham applications such as soundcard interfaces where the handshake lines are set independently of the actual data flow on TXD (for transceiver PTT, RTTY FSK keying, RX squelch detect, CW keying, etc.)

The reliability of these devices is also sometimes influenced by the brand of chipset used to implement the USB port inside your computer, and the version and patch level of Windows you are using. All in all, a very hit-and-miss proposition.

BOTTOM LINE: You may have to try several different brands or models of these devices (and ALWAYS check for the most recent drivers on the vendor's website -- the drivers on the included CD-ROM are almost certainly several revisions out of date!) to find one that works with your particular version of Windows and with your particular application.

There have been consistently favorable comments on multiple ham mailing lists (including this one) about the Edgeport devices. My personal experience is that serial-to-USB cables based on the FDTI chip set and drivers seem to work well. (I've had a USD $15 no-name generic based on the FDTI chipset connecting my D700 to a laptop in the car work flawlessly for over 150,000 KM This includes the last few days in Colorado when the temperature hit -15C... ) The Belkin serial-to-USB devices consistently get complaints.

I had mixed results with two such adapters. The first adapter I bought, a Belkin for about US$30, would not work on either my desktop or laptop. The RTS pin was always left "high" and triggering the radio's PTT line. Belkin may not have been at fault; it may have been a problem with my computer hardware or my version of Windows (XP). Belkin may have corrected the problem since then.

I then bought an IOGear adapter (GUC232A) (apparently made by ATEN International Co.) for about $30 US. It works fine on my desktop using Windows XP.

Some users have reported problems with other USB adapters that change COM port assignments with each computer boot-up. It is possible to "freeze" the COM port assignment using the Windows Device Manager (for the Port device use Properties>Port Settings>Advanced).

So, it's possible such an adapter may work on your computer, but you may have to experiment with different brands. There have also been problems with adapter clones that have a chipset that will not work with the driver of the chip it is cloning.

C. TX Audio-activated Circuit

It's also possible to open the PTT by using a special circuit that opens the PTT when it "hears" tones on the TX audio line, without any COM, LPT or USB port connection .

Stephen Smith's, WA8LMF, Tone Keyer web page describes how to make such a device. If you need your COM or LPT ports for other devices, you should look into this creative solution.

Sound Card Packet with AGWPE