Bi vs unidirectional channel type? digital isolators

Hello all!

This may be a poor question, but may I ask what bidirectional and unidirectional channel types are for digital isolators?

Our company is looking for alternatives to the LTM2884 and I chose the LTM2894. However, this may be an incorrect assumption now that I see that the LTM2894 is only unidirectional. I just want to verify with the community that I should be looking elsewhere for a replacement.

Thank you in advance.

Entechbob

Hi entechbob,

When it comes to USB isolators, they are always bi-directional channels. Unfortunately, we have an error in the parametric data for the LTM2894 on our website. I actually just noticed this the other day and notified the appropriate people. They are in the process of correcting that on our website.

Please note that, unlike the LTM2884, the LTM2894 does NOT provide an isolated power source for the down-stream side.

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Hello David_1528,

You are everywhere! Thank you for answering this post. However, I am still unclear about the differences between bi and uni-directional.

Also, for clarification, the LTM2894 is bi-directional?

And yes, I read that the LTM2894 does NOT have the isolated power feature after I ordered the samples! haha that is my bad.

Entechbob

Hi Entechbob,

USB is always bi-directional. According to the standard, packets of data must be able to be transmitted both directions on the same pins, which is the definition of “bi-directional”. So, yes, the LTM2894 is bi-directional.

Part of the confusion probably lies in the fact that we have other “isolator” types within the same category on our website. Most of those do have unidirectional lines.

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David_1528,

Ah I see, learn something new everyday it seems! Thank you for your answer and time!

Entechbob

I can’t keep up with David, either, but I will add some drawings and unidirectional part number examples to further demonstrate the difference.

As he explained for part number LTM2894, the datasheet has a quote describing the transfer of data along the bidirectional channel: "The integrated USB transceiver on both sides of the isolation barrier supports full and low speed modes defined in the USB 2.0 Specification. The communication through the isolation barrier for USB is bidirectional and as such the LTM2894 determines data flow direction based on which side a start of packet (SOP) begins first. The direction of data is maintained until an end of packet (EOP) pattern is observed or a timeout occurs due to a lack of activity."

Note that the parameters for part number LTM2894IY#PBF are Inputs - Side 1/Side 2 = 1/1, Bidirectional, and Number of Channels = 1. This is shown in the drawings on pages 01 or 08 of the datasheet where each transceiver (transmit/receive) structure is visible on either side of the isolation barrier.

LTM2894


In contrast, we can look at three other digital isolators of the unidirectional type that Dave mentioned. Compare the individual specs of these part numbers to the drawing, below, and notice the difference in structure compared to the bidirectional type. The third option may appear to be bidirectional, but in this case it’s referring to the direction(s) within a channel.

SI8410BB-D-IS Inputs - Side 1/Side 2 = 1/0, Unidirectional, Number of Channels = 1
SI8420AB-D-IS Inputs - Side 1/Side 2 = 2/0, Unidirectional, Number of Channels = 2
SI8421BB-D-ISR Inputs - Side 1/Side 2 = 1/1, Unidirectional, Number of Channels = 2

This was taken from page 24 of the datasheet covering all three Silicon Labs parts.

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Bill_Gust,

Wow! Now that is an in-depth answer! I can clearly see the difference between unidirectional and bidirectional.

Thank you for clarifying!

Entechbob