Single wire for TX and RX?

i recently bought my first micro servo controller and started interfacing it with my PICMicro. The application i’m using requires to use only use 2 lines as output from the PICMicro. One line for common ground and the other for the signal. Can i use the signal line for both TX abd RX? i’m only able now to use the signal line as TX from MICRO and RX from servo controller side!!


If you want to do bidirectional communication with the micro serial servo controller, you will need two I/O lines (and ground). However, bidirectional communication is not really required, so if you have only the transmit line available, you can just go with that. By the way, ground and a TX line don’t count as “two lines as output”. Most specifically, you need one output and one input line; are you sure you don’t have that available?

- Jan

Yes, i have one signal line plus ground only. i would like to use this signal line to both transmit data from the PICMicro and recieve data from the servo controller like get_position command and readings from sensors / input channel.
Thanks for you support.

Well, it’s not going to happen with one wire (at least not reasonably easily). By the way, it sounds like you’re talking about the Micro Maestro, in which case you should be more clear since we have a different servo controller called the micro serial servo controller.

- Jan

You’re right Jan; it is the micro Maestro.

Hello, sorry to resurrect a semi-old thread, but it pertains to what I’m trying to do. If I wanted to turn the maestro into a half-duplex, one wire, serial mode do I need to add any diodes or resistors (other than a pull-up resistor to power)? I’m trying to communicate with a non-inverted TTL (maestro already does this), open-drain (use a pull-up resistor), one wire device. A lot of parallax devices seem to have this style of serial communication. I’m thinking that I should be able to simply connect the TX and RX pins (should be okay for high-impedance), get rid of echo on the RX from blocking in software or hardware (hence the idea of diodes), and then simply communicate as normal via serial software. What makes it so it’s not reasonably easy?

No problem about adding to the discussion. After thinking about it some more, the circuit itself is probably simpler than what I was thinking, and though I haven’t tested it, you should be able to do it with a diode, a transistor, and a resistor or two. However, the Maestro is still not made for it, and while it’s capable of half-duplex operation in that you can make it only send data in response to a question, there is no characterization of how much later the serial response will come. If you are using something like a Basic Stamp, it’s probably going to be too slow to switch from sending data to receiving it. Why do you want to do this half-duplex approach?

- Jan

I would be using the micro maestro in the dual port mode, such that it simply forwards information to/from the PC. I am currently interested in using it with the ColorPAL from Parallax, but in the future I may also want to use the maestro to communicate with the basic stamp from Parallax. Parallax seems to offer more than a few devices that operate in half-duplex serial modes.

If you’re just using the serial port as an independent port, then of course it’s easy since you can deal with receiving the bytes you send yourself without even doing it in hardware, and you can space the bytes you send and receive as you wish. The original question was about using the servo controller as a servo controller, in which case it sends bytes that you have to prevent it from receiving since it is not made for this half-duplex operation.

- Jan

Is the PIC programmed to have an open drain output on the transmitter? If not it seems as though I will need a diode to the TX line from the PIC. That way both transmitters can only pull the line low, and the PIC isn’t trying to battle the ColorPAL, at least this is the way I understand the matter. In this fashion, I’m thinking that the internal pull-up in the ATTiny13A should pull the line high if no transmitter is operating.

No, the Maestro’s TX output is not open drain, it is push-pull.


I added diode on the TX line, took out TX echos in software, and all seems to be okay now. Thanks for the help all!