How To Add CAT Control To A Ten Tec Omni VI+
The Ten Tec Omni VI+ and CAT Control
In an earlier post, I described acquiring and repairing a Ten Tec Omni VI+ (model 564) transceiver. With the repairs mentioned there, I had a working radio, sought after by DX enthusiasts between 1997 and 2001. The only problem was, I couldn’t get it to connect with my Mac M1 Mini. Now, I can already hear many of you thinking “get a Windows machine,” but strange as it sounds, I prefer the Mac platform.
CAT to enhance DXing
In a different, earlier post, I demonstrated how to connect WSJT-X, MacLogger DX (MLDX) and the Mac. I don’t intend to use the Omni VI+ for FT8, but I would like to use the excellent MLDX “spots” cluster and mapping capability to remotely control the Omni. Seeing a new DX entity pop up on the “Bands” page in MLDX, clicking on a “spot” and having the radio, antenna tuner and amplifier ready in seconds is irresistible. All that is needed is a Computer Aided Transceiver (CAT) connection between the Mac and the Omni, and a custom control cable between the Omni, the Automatic Antenna Coupler (model 253) and the Hercules II (model 420) amp. Piece of cake, right?
We’ll need a cable and some software
Well, it would have been back in the day. Ten Tec provided custom cables for this purpose, but some 20-odd years later, they are pure unobtainium. The radio, antenna coupler and amplifier can be used without this magical cable, but each must be adjusted manually when changing bands. I decided automatic tuning would be a priority, but first I wanted to get CAT control working with the Mac.
Untangling RS-232
In my career, I spent many years integrating “smart” classrooms for a local university using various infrared and RS-232 control methods. From that experience, I can tell you that RS-232, though simple in concept, comes in so many “flavors” that each instance may well need a custom cable. Add the modern twist of Universal Serial Bus (USB) to RS-232, and there are many ways to fail.
I had already conquered the USB-to-RS-232 conversion using an FTDI-based commercial cable. There are many vendors, but this model from “BlueMax49er” was custom configured for the Omni VI+. It arrived in a couple of days, and I connected it between the Mac and the Omni. When I had made a similar connection between the Elecraft K3 and the Mac with the correct settings in MacLogger DX, it worked right away. Not so much this time. Multiple tries with different settings in MLDX were no better. Frustrating.
Troubleshooting with software
Next, I tried connecting a simple terminal program called CoolTerm to the Omni to see if the hardware was working. The Omni communicates through the 25-pin serial port at speeds between 1200 and 19200 baud. It uses the simplest possible serial connection of only three wires: receive data (RxD), transmit data (TxD) and ground. This means that the RTS and DTS signals are not used. After trying all the possible baud rates with no luck, I began to suspect that the port on the radio wasn’t working.
Buried deep within the Ten Tec support site, I found another method for testing the RS-232 port. It involves holding the “FM” button down while turning the radio on. Still nothing. The chip was probably blown.
Maxon, Max off
A little time spent with the schematic revealed that the serial port is connected to a Maxon MAX232 level converter chip which converts TTL serial data into RS-232 levels. It’s important to note that the chip connects directly with the serial connector on the back of the radio. Ten Tec made no provision for isolating the chip from the outside world. MAX232 chips were notorious for Electrostatic Discharge (ESD) sensitivity. When working with them, ESD protections like a static free work surface and grounded wrist strap are essential. Perhaps at one time, a serial cable was connected to the radio when it was powered — damaging the chip.
The chips weren’t expensive or hard to find on eBay. In the Omni, the MAX232 is located on the Logic Board (81606). That board is a little daunting to remove because it has many multi-pin and tiny coaxial connections. I’ll spare you the grizzly details of replacing the chip, but suffice it to say I was successful and now ready to test. And got . . . puzzling results. The radio would now respond to commands manually entered from the terminal program, but the radio’s responses were inconsistent and garbled. After a while, it quit responding all together. It looked like the replacement chip had also failed.
The MAX232 spec
It was time for a deep dive into the chip specifications. They’re not hard to find, and I could see right away there were differences between the recommended application and Ten Tec’s circuit. Best practices even varied from reference site to site. Since the circuit was very simple, I decided to breadboard it and see how they worked. By the way, Ten Tec’s schematic doesn’t match what’s actually on the logic board. One cap isn’t stuffed and instead located on the bottom of the board — between pins 16 and 2. Here’s both the Tec Tec circuit as shown in the schematic, and the recommended version.
The MAX232 is a dual receiver/driver for converting Transistor/Transistor Logic (TTL) to EIA-232-F levels. With just a 5-volt supply, it adds a capacitive voltage generator to supply the plus and minus voltages needed for proper RS-232 operation. Depending on the chip version used, and there are many, the capacitors can be as small as .1 µf or as much as 10 µf. Capacitors must be rated to at least 16 volts. I used 1 µf capacitors rated at 50 volts.
These Chips are sorta touchy . . .
With careful attention to ESD protections, I applied power to the breadboard. I then applied a 1 KHz square wave to one of the RS-232 inputs (pin 13). This produced a TTL level output on pin 12. So far so good. But when connecting that TTL output to the TTL input on pin 11 (as shown in the Tec Tec implementation), current to the chip rose from 3-4 mA to several hundred mA, output ceased and the chip got warm. Toasted.
I tried another. This time the chip failed as soon as power was applied. Another. Same result. It was beginning to look like I had a batch of defective chips. In the end, I tried several of them and almost all I tried were defective — and they didn’t produce the expected plus and minus 10-volt levels on pins 2 and 6. Time to try something else.
Let’s get something real.
Looking through the Mouser online catalog, I discovered there was a newer version of the chip (MAX232ECNE4) with built-in ESD protection. At $1.75, they weren’t much more than the chips I already had. so I purchased a few. Used in my breadboard circuit, they worked fine. Perhaps the eBay chips were counterfeit. I have read about such things online.
Back to the radio. since the ESD-protected version I now had working recommended 1 µF caps, I replaced the original 10 µf caps Ten Tec had used. I also added a Molex DIP socket so the chip could be replaced easily if needed.
In the shack, the Omni now responded to RS-232 commands from CoolTerm and sent reasonable, consistent responses back — although the responses did not match the code examples found in the Omni VI+ manual. More than one version of the radio firmware?
Making it work with MacLogger DX
There is a setting in MLDX for “Omni VIa,” which I thought might work with the Omni VI+. It doesn’t. Yet another fortunate find on the MLDX section of Google groups suggested that the Omni VI and VI+ use a command set similar to the Icom IC-735 with a CIV of 04. I tried it, but still no joy. Then, following a hunch, I tried the IC-751 setting. Bingo! Everything fell into place. This graphic shows the final settings. (click to enlarge)
Which leaves the cables . . .
Once the radio was connecting to the Mac Mini, I needed the custom cables for connecting the Omni VI+, Automatic Antenna Coupler (253), and Hercules II Amp (420). Ten Tec provided these cables at one time, but I’ve never been able to find them. One cable (model 236) had connectors for the Omni, an RS-232 connection and a 15-pin Molex connector.. The other (model 264) provided an extension for connecting the first cable to the amp and coupler via 15-pin Molex connectors.
Building the Remote Cables
The only option was to build them. By happy coincidence, I had several old RS-232 extension cables with DB-25 connectors. They were relics from earlier computer systems, no longer needed. Next, I needed at least two Molex 15-pin male connectors with .093 pins (Molex part #19092151). These parts are obsolete, and can be tricky to find, but I found some on eBay. My old RS-232 cables had 25 conductors, so I cut one end off to create two cables of about 6 and 4 feet. Using the schematics in the Omni VI+, Coupler and Amp manuals, I connected the correct wires to the two Molex connectors, so that the Coupler and Amp remote control ports were connected in parallel.
USB to Omni VI+
That left the RS-232 connection to the Mac Mini. Since the Omni only uses the RxD, TxD and Ground connections, it wasn’t too hard to add those three wires to the DB-25 on the other end of my new custom control cable. I was able to remove The BlueMax49er DB-9 connector and mount the molded end into the DB-25 housing for strain relief. That completed the USB to Omni VI+ serial connection.
Now let’s have some fun!
And that’s it. A challenge to complete, but it’s working great. Now, I can look at the “Bands” or “DX Clusters” tabs in MacLogger DX, select a DX spot and watch everything work. The Omni VI+ changes frequency, the Automatic Antenna Coupler roller inductor and capacitor relays move to the correct settings and the Hercules II is set to the correct band. Magic! Since MLDX is aware of my log contents, it can even recommend which DX spots I should contact. I plan to add antenna switching and rotor control in the future, but for now I’m living the dream!
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