23201a question thinkfailure


sorry my english is not perfect, but i hope for the best entertain.

I order 2 packages of 23201a serial adapter, because in the second step i thougt the first it is damage. But at the moment I think there is a failure in the system.

If i make a loopback on the end the serial cable pin 2 and 3, i get a return from the hypertermina - it’s working.

If i used serial adapter 2301a on a breadboard an put a constant 5,18V GND and VCC look the pics and make a loopback TX and RX I can’t get a return.

Where is the mistake?

Thanks for the help,



I am sorry you are having problems using your 23201a serial adapter. You mention you have two units. Are you observing this problem with both units? Could you verify that you are not using a null modem cable? (Null modem cables have RX and TX internally swapped so it can plug each end into a computer so the two can talk to each other.) If you have access to an oscilloscope, could you try measuring the TX pin of the adapter board and see if there is a signal?

- Jeremy


thanks for the first support. I am working with a serial cable not a nullmodem cable.
I follow your tips and measure something:

how you see:
Voltage on the adapter 5,18V
RX - TX - low level 1V

I measure a high level 11,74 V for the adapter.

What do you think about the pinout
" INV Out Serial port detection pin, high if serial port connected"
Could it be necessary, if we need a high level at this pin?
Yes/ NO
What is the way to make a high level on this pin or voltage number?

Thanks for more information.

The INV pin is an output. It should go HIGH when you connect the serial cable to the adapter; you should not drive this pin HIGH.

Measuring across the TX and RX pins with a voltmeter will probably not give any meaningful results. You will want to use something like an oscilloscope to verify there is a signal being transmitted on the TX pin.

If you do not have an oscilloscope, you might try connecting a microcontroller and having it transmit serial data and see if you can read it with your computer.

- Jeremy

You mentioned you were not using a null modem cable, but could you verify this by measuring continuity of the TX and RX pins on both ends of the cable? If it is a null modem cable, the pins will be swapped (i.e. the RX pin on one end will connect to the TX pin on the other).

- Jeremy

Thanks for the comments.
I measure TX / PX pin on the adapter and you see

~10 V. This is good.

But if I make a loopback at the adapter TX/PX

the adapter can’t get a return.

On the other side if I make a loopback for the microcontroller

I get a return.

What is my mistake?!


In my previous post, I asked if you could measure the continuity of the TX and RX pins on both ends of the serial cable. Did you measure this? From the second picture, it is unclear how the brown wires that go between the DB9 connector and your cable are connected. If you are using a null modem cable, connecting those wires from TX on one connector to RX on the other should make your loopback test work. Could you try the loop back test with these wires connected both directly from one TX to the other TX and with one TX connected to the other RX? If neither of those tests work, you should contact us directly at support@pololu.com with your order information and reference this forum post so we can look into getting you a replacement.

- Jeremy