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This web page is quite simply about how I connected several radio transceivers, with varying capabilities, to operate the new Digital Modes. I won't be including a detailed explanation of these modes. You can find detailed explanations in many book and on-line web sites. I have found that once I understood the physical needs, setting up a transceiver for digital modes was probably the simplest thing I ever did in amateur radio.
I made this interface many years ago, before commercial interfaces were readily available. I think the hot operating system of the time was Windows XP, and there wasn't a problem getting the USB to Serial Port converter cables working. My goodness, how things have changed. But, in spite of those changes, this interface still works fine.
The IC-735 is a relatively old transceiver, but it is still very popular. For Digital Modes it has a connector on the rear panel, ACC(1), that provides access to the Audio Input, Audio Output, and the PTT. The only drawback that I have found, with using the rear connector, is that, the Audio Input bypasses the VOX. If the VOX was available, I wouldn't need to include some circuitry that enables the PTT and avoid the problems associated with the USB to Serial Port converter cable. But, after using it for a while, I actually prefer it that way. And, since the levels for PTT are the same for CW Keying, I can get a bonus mode.
Initially, all I knew was that I needed to get the audio from my radio to my computer and the audio from my computer to my radio. So I bought a spool of shielded audio cable and some connectors and then wired the computer audio's to the radio's audio. At the time, my computer was a HP G62 Laptop. The laptop had separate audio in and audio out connectors. Later on I used this same interface on a newer laptop that had a single 4-Position combo audio jack for audio in and out. To resolve the issue, I found a P318-06N-MFF 4-Position to 3-Position audio adapter cable made by Tripp-Lite. The adapter cable provides two connectors allowing me to just plug in the existing audio cables.
Making the computer enable the PTT or CW Key input took a little more work. All of the available Digital Mode software uses a Serial Port to enable the PTT. Some of the software, even include CW Keying via that same port. An exception is FlDigi, which requires more circuitry to key CW directly. See the FlDigi web site for more information. If your computer has a built in Serial Port, part of the work is done. However, most laptops have done away with Serial Ports and only includes USB Ports. So, since my laptop didn't have a built in Serial Port, I bought a USB to Serial Port converter cable. I initially bought one with a DB25 RS-232 connector, but later replaced the USB to Serial Port converter cable with a DB9 RS-232 connector. The schematic reflects the DB9 connections.
That gave me a signal to use for enabling the CW/PTT. But there were two issues. The signal voltage level and the active state. The signals from the converter cable are at RS-232 levels (+/- 12V) and the outputs are Active High. But the CW Key/PTT input requires a Open Collector TTL level signal that is Active Low. But both of these issues are solved with the circuit on the left.
The circuit was built right into the plastic shell used to house the mating RS-232 DB9 connector. When the software initializes the RS-232 port, and when the software is in the receive mode, the RTS (Request To Send) signal on Pin 7 is a negative voltage. The diode and resistor (D1-R1) provides protection for the transistor at this time. When the software enables RTS, it switches to a positive voltage. The positive voltage saturates the transistor (Q1) and causes the Emitter/Collector to conduct. This is sufficient to place the radio into transmit, or operate the keying input.
In the drawing at the upper right, it shows three connectors, as part of the RS-232 to CW/PTT Interface. This is just an easy way to switch between CW and the other digital modes. When I want to run PSK-31, I connect the interface to the PTT input. When I want to run CW, I connect the interface to the CW Keying input.
The drawing below is the interface I used to go from a computer's RS-232 communication port to the ICOM Remote Control Jack (CI-V). I included a section of the IC-735 input circuitry. Its only meant to give you an idea of the circuitry that is being driven, so it is a little incomplete. The IC-735 input circuitry directly drives the IC-735 Microcontroller (U6 - HC63A01V1C80).
The best I can tell is that the interface design is attributed to Nigel Thompson, KG7SG, and appeared in the 7/92 issue of QST. But if you look around the internet, you can find a dozen other interfaces that will do the job. In fact, now you can now find a CI-V interface cable that plugs directly into a USB port and goes directly to the CI-V input port. Like the Icom CT-17 USB FTDI Chipset CI-V Cat Control Programming Cable 3 Feet. Note that this cable will work with my IC-735, because it has a "mono" 3.5mm phone plug at the end. Some Icom radios require a "stereo" 3.5mm phone plug. Check your radio manual to be sure you get the right one for your radio.
The intent of this interface is to convert the Uni-Directional RS-232 Rx/Tx signals to the Bi-Directional ICOM CI-V Rx/Tx signal. The RS-232 signal and control lines switch from a positive to a negative voltage. Whereas the ICOM CI-V is Bi-Directional and switches between ground (< 0.8 Volts) and some positive voltage (> 2.0 Volts). When no data is being received from the computer, anf no data is being sent to the computer, the ICOM CI-V line rests at a positive voltage.
The RS-232 TxD signal line (DB9 - Pin 2) is routed through resistor R8 and transistor Q4 to the Icom CI-V input connector. The diode, D3, protects Q4 from high negative voltages. At the same time, negative excursions of the TxD signal is routed though diode D2 and stored in capacitor C1. This provided the negative voltage required for the RS-232 RxD signal line (DB9 - Pin 3). The positive voltage required by the RS-232 RxD signal line (DB9 - Pin 3), comes from the RS-232 RTS, CTS, and DSR control lines (DB9 - Pin 4, 5, & 6).
Transmitted data, from the Icom CI-V output connector, is routed through Q3 and Q2. Q1 is then used to switch the RS-232 RxD signal line (DB9 - Pin 3) between a positive to negative voltage.
Since my original build, I have come across several other interfaces that would probably work as well as the one I built. Of course, eveyone claims a disclaimer, including myself, that the circuit works fine for them, but doesn't come with any liabilities or guarantees. I am not going to say much about each one, except for a few notes. Where possible, I provided links to the original web page.
The IO connector labeled CIV I/O, in the diagrams below, is a 3.5mm Phone Jack. To connect to the radio's CI-V input, use a shielded cable with a 3.5mm Phone Plug at each end. You could also eliminage one Phone Jack and one Phone Plug and tie the open end of a cable directly to the circuit. The other end of that cable would then have a 3.5mm Phone Plug.
None of the circuits should be picky about the transistors used. You should be able to use a 2N2222, 2N3904, or any other generic switching transistor.
Some of the circuits use a DB9 connector and others use a DB25. Except for the RxD and TxD, the signal lines, including ground, will be on different pins.
This is a super simple RS-232 to CI-V interface from Bill Hawthorne, G3MCS. Its made with three NPN transistors and three resistors. If you don't want the CW Keying interface, you can eliminate one transistor (Q3), one resistor (R3) and the associated output connector for keying. The circuit takes the RS-232 unidirectional RX and TX signals and combines them into a two wire bidirectional open collector bus. The circuit can easily be built directely into a RS-232 DB9 or DB25 shell.
The transistor types are not critical. A BC547B, 2N3904, or 2N2222 would probably work fine. The cable for CI-V communications, from the interface to the radio, should be shielded cable. Shielded audio cable should work fine, but a short piece of flexible coax might work even better.
This circuit is attributed to David Aldridge, G3VGR.
The circuit is powered by DTR (pin4). However, when using other software, it is possible for DTR to be turned off, removing power from the circuit. This is easily resolved by powering the circuit from RTS (pin 7) instead of DTR (pin4).
There are even simpler designs available, such as this opto-isolated model by Gary Dion N4TXI:
In situations like field day, when rig grounding may be questionalble, this interface is a good option
This design draws an average of less than 0.5 mA. The power supply can be taken from either the ACC connector or the microphone jack.
While most of the available Digital Mode software has Rig Control capabilities, Rig Control is not required in order to use Digital Modes. Rig Control describes the ability to control your radio's operating frequency, mode, etc. from a computer. This is a nice feature that is included with almost any of the newer amateur transceivers. But you might have older radios that do not have the capability built in. For example, I have a old Radio Shack HTX-100. It is a nice tranceiver that operates CW and SSB on 10 Meters. It doesn't have any Rig Control capabilities, but that doesn't mean it can't be used for Digital Modes.
As long as your radio is capable of Single Sideband (SSB), specifically Upper Sideband (USB), it can be used for digital modes.
The first thing you need is a transceiver that is capable of Single Sideband (SSB) operation. Specifically, Upper Side Band (USB) is used with most digital modes. The transceiver should also have a way to reduce the output wattage. Digital modes tend to be continuous. Normal voice mode operation, only produces output power when you talk into the microphone. Digital modes, however, use continuous or varying tones that are full on all the time. Some transceivers can not handle a continuous duty cycle so it is necessary to reduce your output power. But digital modes do not need a lot of power. It is common to find users running 1 to 25 watts and still making cross country or cross continent communications.
Most transceiver will have a accessory connector in the rear for digital mode access. It may be described in the manual as AFSK operation for RTTY. But that is what is needed for operating the other digital modes, like PSK. However, do not be detered just because your transceiver does not have an accessory connector for this purpose. While it is less desirable, most digital modes can be operated directly from the microphone connector.
Besides a transceiver capable of SSB operation (USB specifically), all you need is a computer with digital mode software installed. The computer can be a laptop or a desktop. Which ever one you have. Most laptops only have USB ports and will require you to use one or two USB to Serial Port converters. But that is in the more elaborate interface, which is discussed later. Older desktop computers do tend to have built in Serial Ports, but usually only one. So you could by a interface that has multiple serial ports or just use a USB to Serial Port converter when necessary.
The overall idea is that, for reception, your transceiver is tuned to a base frequency (i.e. 14.070 MHz) and is in the USB mode. With normal SSB filters, this will give you about 2.5 KHz of bandwidth. Digital signals have very small bandwidths (PSK-31 = 31 Hz BW) allowing multiple stations in the available receiver bandwidth. This audio is then sent to a computer with digital mode software installed. The software handles the task of decoding the signals within that 2.5 KHz or band width.
To get a feel for digital modes, you can start by just putting your computer near your transceiver and use the computer's built in microphone to receive the signals from your transceiver's speaker. This is usually easier if you are working with a laptop because they have built in microphones. But an external microphone placed near you transceivers speaker, will work for desktop computers.
