Electrical properties of AX-12A interface


i have a AX-12A. I found the manual at https://emanual.robotis.com/docs/en/dxl/ax/ax-12a/ an saw the diagram in chapter 5.4.2, but I don’t understand it.

  1. which side is RS485 and which side is TTL to AX-12A? I assume, right is AX-12A?
  2. I assume, the “MOLEX4P” represents the molex connector at the AX-12A. But on my AX-12A, I have only three pins. So, how it is wired actually?
  3. what does “TTL Level Multi Drop Bus” mean electrically? Is that open collector? Or what?
  4. Is “high” logically “true” or “false”?

Maybe someone has answers to my questions.

kind regards

I could be wrong but I don’t think there is an RS485 versions of the as-12a servos? Mine are all TTL (have 3 pins) Which is more along the line of section 5.4.1

Unless you have some of the RS485 like servos with 4 pins, you can ignore that other diagram.

As Kurt mentioned in the above reply, AX series does not support RS-485.
I’ll ping this error to eManual admin.

Yes, no RS485… that’s the reason for my asking.

let me express my question in another way:
I have a serial interface, and I can have RS232 and/or RS485, both with the usual signals, i.e. TXD, RXD, RTS, CTS, DTR, DSR, DCD, RI for RS232 and duplex wiring for RS485.
How can I connect my AX-12A to the serial interface, as simple as possible. My question focuses on the missing of a “direction” signal, at least non of the signals in RS232/RS485 is named in that way. Can I just take the RTS (“Reqest to send”)?


P.S.: there are some ready-to-use RS323 to TTL converter, usually with a MAX232 IC on, it (or RS485 with MAX485). Can I use one of those devices. If yes, how is the wiring?

If you are trying to convert TTL to RS-485, you may need additional board that will do the conversion.
Since the DIRECTION_PORT is directly controlled by the mcu, replacing it with RTS may work depend on your hardware design, but I cannot guarantee such modification.
Please also note that we have RS-485 version DYNAMIXEL from XM430 lineup.

I’am still fighting with the AX-12A.

I use now a MAX232 converter IC, which does convert RS232 signals to TTL and vice versa.
I input signals (instruction packet, i.e. “ping”) at the RS232 side of the MAX232 and connect on the TTL side just RX and TX…
Thats like connecting RX and TX, but with a conversion RS232->TTL and back TTL-RS232.
Thus, I can hear my own echo on RS232 (it works, i tested it).
Now, i connected the AX-12A to the TTL side. I added a resistor of 1200 Ohms, because I was unsure, if the output driver of the AX-12A is able to pull down the MAX232 to ‘0’.
It looks like this:

Ihe serial interface is set to 8N1, no handshake.

Well, I can still see my own echo, but there is no answer from the AX-12A. Just nothing.

I send “ping” packets with ID = 1 (as described in the manual), and ping packets to broadcast address 0xFE, i.e. “0xFF 0xFF 0xFE 0x02 0x01 0xFE”, in order to receive a status packet.

I tried several baud rates, between 115kb and 9,6kb, but i got NOTHING except my own echo.

I took an oscilloscope and looked at the signals at RS232 and at the data input of the AX-12A:

an it seems o.k. (that is a broadcast ping, by the way…)

I changes the time axis to a larger time scale for seeing any activities after sending the ping, but agian nothing, not even a glitch or a peak.

So there is absolutely no reaction from the AX-12A, except a shot flashing of the led when powering on.

Any hints how I can establish a communication to that AX-12A device?


I’m not sure if your oscilloscope is showing the entire single packet signal, but the ping packet checksum is 0xFB, not 0xFE.
Oh, my bad. you are using a broadcasting ID.

By default, AX uses 1Mbps. Have you also changed this when testing under 115,200 & 9,600bps?