Noobie help - Low-Voltage Dual Serial Motor Controller

I have two motors from a toy that runs off of a 7.2V battery, and I want to control it with my computer using serial DB9/RS-232, so I bought the Low-Voltage Dual Serial Motor Controller. I am very new to electronics and robotics, can someone help me understand how to wire it up?

After reading the instructions I am confused because I don’t have a “Robot Controller” and so I think I don’t need to use pins 3,4,5, and instead I should use the RS-232 pins. I don’t understand the logic supply, and the pin on my db9/rs-232 cable I’m using (db9 pin 4 according to the instructions) doesn’t seem to have a wire connected to it in my serial cable. Could it be that I am working with a faulty cable or wrong type? (I will buy a new cable to test, if it helps, I tested my cable/connector by checking continuity and found that my pin 3/SERIN is black, and pin 5/GND is yellow, is this a standard?) Another part of my confusion is because the manual says “The reset line is again optional”, but later the manual says “The motor controller’s reset line should normally be kept high at the logic supply voltage”.

Do I need to add another battery for the controller itself (3-5.5V), or just find the pin/wire from my serial port that provides power in that range?

I have wired everything else up, and even switched on the power for a second, and glad I didn’t see smoke, but the manual cautioned “Before attempting to connect your own electronics to a computer, make sure you know what you are doing!” so I’m scared to actually plug the serial cable into my computer, really can’t afford to fry my computer…

Thanks in advance for any help or advise you can give me!

BTW, once I figure this out, I plan on replacing the pc with a gumstix linux computer, anyone have experience or advise they can share with this setup?

Thanks again,


First off, I should note that the voltage range is up to 7V, so your 7.2V battery (which will be over 8V when charged) will beyond the specs of this product. (We have had people running off of 7.2 V successfully, but you do so at your own risk.)

The motor controller does need its own power source, which a serial port cannot provide. You can connect a 5V regulator to your 7.2V pack to get 5V for the controller.

The reset line is optional and normally stays high on its own. Some cheap cables don’t have all the handshaking lines connected through, and I don’t know of any standards for the internal colors. It should be safe to connect just the ground and transmit lines to the motor controller, but don’t expect anything to happen without connecting the board power supply.

- Jan