Com port communication

Hi ,

One important question ,

I’ve got micro maestro 6 servo output ,and realize a graphic program work fine to can pilot my servo with my computer .

I set in detect baud parameter in the control center to use with com port.

I work with my program without control center ,and work fine too .

But in my Rov application need to work with com port alone for high lenght wiring using .
And when disconnect the usb connector from the micro maestro, to test with com port alone ,nothing appened ?

Don’t understand I read in the documentation ,and emmit 0xAA in first for detecting baud but nothing appened without usb connected ?

If can realize a µcontroler communication on Rx and Tx line (read on doc ) ,it possible have communication with a com port .

I use a virtual com port like FTDI 232 and use Rx and Tx .

Please say me, it can work without USB connection,

Thank for else can help me


It seems like you are trying to control the Maestro through it’s TTL serial connection from the Maestro Control Center; is this correct? Could you please tell me what kind of power you will connect to the Maestro’s VIN pin when it is in your ROV?

If you are using a TTL serial connection, the Maestro can only receive serial commands. You can find more information on using serial commands in the Pololu Maestro Servo Controller User’s Guide in the “Serial Servo Commands” section.


Thanks your help Derrill,

So in first I work both with maestro center an my program software (profilab expert ) .
With my program soft I emit the bytes command ,and see on maestro center if work good to can progress fine, but not realy use maestro center .

But all my command send trought my virtual port com were i connect the tx computer to rx micro mestro and rx computer to tx maestro ,and click on disconnect the device in maestro center.
In Profilab Expert (my graphic programer ) we can realize connection with com port and emmit all byte command .

Power will use ,just be a 12v dc regulated supply on Vin and 5v for servo .

And use these serial command in polulu protocol like :

Start byte : 0xAA (170)
Device number : 0x0C (12)
Command byte : 0x04 (4)
Channel number : 0x00 (my first servo)
first target low byte
last target hight byte

I want precised all work finily ,with this protocol on my virtual com port , and never use or want to use the usb port.
but in uart detect baud rate configuration ,device deconected in maestro center my virtual com port connect on micro maestro tx and rx signal ,I see with my oscilloscope usb communication throught device an computer .

One thing forget say ,when sending byte in uart mode receive same serial echo (like 0xAA send > 0xAA receive etc… )

So this is thing don’t understand ,in serial configuration normally I must not have active usb signal ?

I want control my rov trought underwater ,with a serial signal (RS232 or 485 ) from my computer to my Rov at about 300 ft tether long ,and can’t use a USB communication .

I can send here this little profilab expert program but must modify it cause i use it wit ha PS3 joystik ,and you can test it like a little windows application ,and see it .

Sorry Derrill ,I try explain my problem ,more could I ,If necessary I can send anything to explain it ,but I need to resolve it …

You can have active signals for both USB and TTL serial at the same time, you do not need to disconnect your USB. You mentioned getting an echo of the serial command you sent. Could you give us a schematic of your equipment and how it is connected?

You might try using our Pololu Serial Transmitter utility for Windows to send a basic serial command to your Maestro. This will rule out the possibility that a bug in your software is causing the problem. Please let us know exactly what bytes you send with the utility so we can double-check them.


Hi ,
You say ,
“You can have active signals for both USB and TTL serial at the same time, you do not need to disconnect your USB”

But for my final application ,with my rov I need absolutly to work with usb disconnected ,cause:

I want to control my rov with my computer with a 150ft long tether ,and usb not very appropriate in this configuration , you must use a com port for these configuration .

So I just want use my usb for ,range ,speed ,acceleration ,neutral ,and special configuration .
After ,I want to disconect this usb port and control micro maestro with com port alone ,with my profilab expert program .

But I see can’t control it without usb ,and want to know "why ?! "

This is some picture of these serial test ,one with serial connect alone ,one with serial and usb both connected .
And a rudiment schematic of my mestro connection

in this exemple ,I sent with compact protocol the first Start byte : 0x84 after a sum of four byte : 0x84 -0x02-0x70-0x02

If necessary ,I can do a little videoclip were you can see servo moove with my serial control in profilab expert (with both usb connected ), and what appened when I disconnect usb ,but need to prepare it .

Thank for your help


It appears from your schematic that the problem might be that you do not have the ground connected from the Maestro connected to the ground of the FTDI 232RL Virtual port. When you disconnect the USB cable you lose the ground it provides, killing your serial signal.

Lastly, although you can leave the USB connected, you do not need to in this case. Once you fixed the above ground issue, you will likely be able to send your serial commands.


“It appears from your schematic that the problem might be that you do not have the ground connected from the Maestro connected to the ground of the FTDI 232RL Virtual port. When you disconnect the USB cable you lose the ground it provides, killing your serial signal.”

Sorry forgot it in the schematic, but not in the real .

Hi Derrill ,

After take maesurement with my oscilloscope ,i connect my gnd probe on mettalic subd9 of my devellopement card wich have micro mestro insttaled on .
And all working ! Happy !!!

When disconnect my gnd probe ,nothing work ?!

It’s a curious symptom, to working it is a problem of gnd ,but my gnd power connected to my subd9
I thought if it’s cannot be a problem of virtual com grounding…

Since few time I posted a request like a device com port disfonction

And all in this week had an active communication with your Pololu team
I think I could employed a lot of your precious time ,and can tranfert in my post lot of angry and bad situation ,for a virtual com port modified device .

After continuity search ,I see GND inside my virtual interface connected by a 100ohm resistor …
My virtual com port interface was a virtual com to rs485 inteface, witch i modified for use in USB to com port .

When use it with my labtop computer GND provide from the micro maestro USB connector and work fine .

when disconnect this usb connector no GND no good signal .

When you asked me control my grounding connection on DB9 connector ,is connected but trought a 100 ohms resistor.

Yesterday, with my table oscilloscope ,were for a last time emmiting my control pules trought my virtual port, with nothing work ,I connect my probe ground wich connected to the earth grounding too like my fixed computer ,all working and disconnect it and see nothing work .
Here the final problem resolved …

So I think can deranged and take lot of your time for a stupid electronic bug and could maybe give you all my excuse Derrill,
but I must thank you for you all help in my disfonction device ;
For you and all you team .

Here a shematic explaintion were good and bad working of pc to maestro working communication
Explaintion.pdf (7.74 KB)

Hello. I am glad you were able to solve your problem. Thanks for sharing the details of the problem with us. I have merged your two topics together because they about the same thing. --David