Bidirectional isolator for three-wire serial bus?

In summary, Jason was trying to figure out how to transfer data between two microcontrollers using the same SPI bus. He found that chip select lines are used to select which slave is being addressed. If there is more than one slave device on the SPI bus, he will have to use a separate, dedicated line for both directions. If he cannot find an alternative way to use the same wire for both lines, he will have to break the SD line into the two traditional uni-directional SPI data lines.
  • #1
Jdo300
554
5
Hello All,

I have a circuit that uses a microcontroller to communicate via a three-wire serial line (SPI) to a DDS chip to send and receive commands and status information. My question is that if the DDS IC happens to be on a different board and I want to galvanically isolate them from each other, how do I transfer data between the two ICs using the same three-wire serial line? I am thinking some kind of opto-isolators but I have yet to see any that allow bi-directional transfer of information on the same data line. How is this done in other applications where you have half or bidirectional communications taking place on the same data lines?

Thanks,
Jason O
 
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  • #2
By "3-wire" do you mean 3-wire plus ground (MOSI, MISO, SCLK, GND), or do you really mean 3 wires total (BiDir Data, SCLK, GND)?

If you have a Bidir data wire, then yeah, it will be hard to optically isolate it. Can you change to 3-wire +GND? Then the optoisolation is more straightforward, and you mostly have to worry about using fast enough optos (and watch the asymmetry of the output waveforms...).
 
  • #3
Actually, the three wires are the Serial Data (SD), Chip Select (CS), and Serial Clock (SCLK). With the SPI bus, you use the chip select line to flag the IC that you want to send data to, then you clock the data in or out on the rising edge of SCLK.

The CS and SCLK lines are both unidirectional but the SD line must be bidirectional. What do I do in this case?

- Jason O
 
  • #4
Jdo300 said:
Actually, the three wires are the Serial Data (SD), Chip Select (CS), and Serial Clock (SCLK). With the SPI bus, you use the chip select line to flag the IC that you want to send data to, then you clock the data in or out on the rising edge of SCLK.

The CS and SCLK lines are both unidirectional but the SD line must be bidirectional. What do I do in this case?

- Jason O

Yes, you're right. When there is more than one slave device on the SPI bus, chip select lines are used to select which slave is being addressed. Is there any way you can break the SD line into the two traditional uni-directional SPI data lines, MOSI and MISO? Does the slave device that you are using really only support 1-wire data? Is there an alternate device you can use for the slave that supports the full 2-wire SPI interface? Your master supports 2-wire data, right?

If not, I have seen bidirectional opto transfer designs before, but was not very impressed with them. I'll look back in my notes to see if I have a copy of the circuit. I think it was for a bidir I2C bus design though, which would be at lower speeds than SPI, and has the inactive-high shared line characteristic that SPI does not.
 
  • #5
Hi Berkeman,

Yes you're right about having two SPI lines. If I cannot find an alternative way to use the same wire for both lines, then I will have to just add a separate, dedicated line for both directions. I may just go ahead and do this. But I would still be very interested in seeing that bidirectional opto transfer design that you were referring to. Since I am using a BASIC Stamp to do the IO, the transfer rates would be pretty slow (50 kHz max).

Thanks,
Jason O
 
  • #6
Did you finally find a bidirectional isolator circuit scheme?

You can email me if you like at: << e-mail address deleted by berkeman >>

(I don't visit this board very often and you may need to email me to contact me.)
 
Last edited by a moderator:
  • #7
peralta_mike said:
Did you finally find a bidirectional isolator circuit scheme?

You can email me if you like at: << e-mail address deleted by berkeman >>

(I don't visit this board very often and you may need to email me to contact me.)

Just ask him to PM you, and turn on your e-mail notification for PMs in your Control Panel. We discourage posting e-mail addresses in the open forums, as it invites spam.

Oh, and this thread you posted in is over a year old... But welome anyway!
 

1. What is a bidirectional isolator for three-wire serial bus?

A bidirectional isolator for three-wire serial bus is a device that allows for communication between two devices on a shared bus while also providing electrical isolation between them. It can be used to protect sensitive components from voltage spikes or to ensure data integrity in noisy environments.

2. How does a bidirectional isolator work?

A bidirectional isolator uses optocouplers or transformers to electrically separate the input and output sides of the device. This allows for data to be transmitted between two devices on a shared bus without the risk of electrical interference or damage.

3. What are the advantages of using a bidirectional isolator?

One of the main advantages of using a bidirectional isolator is its ability to provide electrical isolation, which can protect sensitive components and ensure reliable communication in noisy environments. It also simplifies the design of circuits by eliminating the need for additional components such as level shifters.

4. Can a bidirectional isolator be used with different types of serial buses?

Yes, a bidirectional isolator can be used with various types of serial buses such as I2C, SPI, and RS-232. However, it is important to ensure that the specifications of the isolator match the requirements of the specific serial bus being used.

5. Are there any limitations to using a bidirectional isolator?

One limitation of using a bidirectional isolator is that it can add some delay to the communication between devices, which can affect the overall performance of the system. Additionally, bidirectional isolators may have a limited data transmission rate, so it is important to select the appropriate isolator for the desired data transfer speed.

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