Adding USB to the “Fluidmotion” STEPPIR control box.

If you read some of my other posts you’ll understand that I have no love for the ancient RS-232 protocol , and have found that most items that use a 9 pin RS-232 port can be converted to USB.


Most logic level circuits have signals that use +5 volts for a logic “1” and zero volts as a logic “0”. RS-232 takes these 1’s and 0’s and level shifts them to a much higher voltage level – that has both positive and negative potentials – something in the neighborhood of plus and minus 10 – 12 volts. I guess the reasoning behind doing this is to insure that  determinable voltage levels survived long cable runs. In most Ham applications, the cable length is just a few feet – so signal level shifting of this type is completely unnecessary. USB signals are TTL level (0 – 5 volts) and can survive a journey of many yards without the need for amplification so it seems to me the point is proven.

Any peripheral equipment that uses RS-232 introduces the requirement for a dedicated serial port card or a cumbersome and unreliable USB to serial port adapter. Also required is a cable – wired  null or straight, sometimes gender adapters too – it can quickly become quite a mess – especially if you are dealing with multiple devices.


Wouldn’t it be nice to just do away with the entire mess and make everything run on clean, straightforward USB?


Enter the FTDI DB9-USB-D5-M (male) and the FTDI DB9-USB-D5-F (female) – About $15.50 from Mouser and Digi-Key.


At first glance, this device looks similar to a conventional DB-9 connector,  but instead of 2 rows of pins, it sports a mini-USB connector. It drops into the PC board footprint of a conventional 9 pin right angle DB-9 connector. Getting one to replace a straight DB-9 requires some creativity.

From a software perspective, they appear to your computer to be a plain Jane USB to serial port adapter. From a hardware perspective, the devices TX and RX lines are 5 volt  TTL levels. RS-232 level shifting circuits downstream are not required and in fact must be removed, and bypassed on the modified device.

The FTDI device is powered by the USB bus, so the only connections you generally have to deal with are pins 2, 3 and 5 – TX, RX and ground.

Side note: FTDI make a similar device that is basically a conventional USB to serial port adapter – embedded in the body of a DB-9 connector.  I tried one a while back in a RX-320 . It “worked” – meaning it controlled the radio – but  I abandoned the approach because the DC to DC converter circuits in the device generated SO much RFI – the RX-320 was simply unusable. Keep in mind – ALL conventional USB to serial port adapters have DC to DC converter circuits – they have to in order to make the “high voltage” bipolar RS-232 signals.

Parting company with the USB to serial port adapter may actually work to improve your RX noise issues.


I use a Flex-5000A, and have always wanted to link my Steppir in to my computer in order to keep it synced with the radio. The fly in the ointment was the stinkin’ serial port! I decided it was finally time to do away with RS-232 and install a USB port into the STEPPIR Fluidmotion control box.

The RS-232 level shifter I.C and the controller I.C. reside on a daughter board that mounts to the rear chassis, and attaches to the motor drive board via a 5 pin right angle header. The DB9 connectors on the daughter board are straight – NOT right angled as required by the FTDI device. I realized the only way to make this work was to replace the right angle header with a cable that would allow the daughter board to be relocated. The FTDI device could then be connected by 3 wires, and mounted to the rear chassis.

After unsoldering the right angle header from the motor driver board and daughter board, I removed the MAX202 RS-232 level shifting “transceiver” chip.




I used a 2 inch length of 10 conductor ribbon cable to reconnect the daughter board to the motor drive board. Since there’s only 5 connection points, I connected every other wire and cut short the unused wires. It lined up nicely. A touch of liquid electrical tape fortifies the connection a bit.



A few minutes with a meter and the MAX202 data sheet allowed me to map out where DB9 pins 2, 3 and 5 went. From there the correct connection points to the ATtiny2313 microcontroller were determined.

This is a convenient connection point for the FTDI device pin 2.


Note: all three wires are mechanically secured to the lower DB-9 connector with a wire tie strap.

I couldn’t find a via to connect to, so Pin 3 (orange wire) is soldered to this pad – where the MAX202 pin 10 once was. The solder joint circled in red is where pin 2 (red wire) is soldered.


Pin 5 (ground) can be picked up here. This row of unused holes is just below the lower DB9 connector.


The RS-232 daughter board will lay on top of the motor drive board. Naturally, it requires insulation, so I glued a patch of polyethylene sheet to the motor drive board with a dab of liquid electrical tape. DO NOT USE SILICONE GLUE ON CIRCUIT BOARDS! Some types of silicon are corrosive.


The daughter board lays on top of the polyethylene sheet – held in place by a dab of the non-corrosive,  easily removed liquid electrical tape.


The FTDI device is secured to the rear chassis, and the hole that normally accommodates the upper DB-9 is sealed with a patch of aluminum duct tape.


A garden variety mini to regular USB cable is all that is needed to connect to the computer.

Setup is largely automatic. Windows 7 discovered the device and auto installed the driver. I went into the Device manager and manually configured the com port number and the baud rate. Made the appropriate entries into DDutil, and the Steppir tracks my Flex 5000A as perfectly.


Total Mod time: about 2 hours.






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