Modbus isolate electronic design

[jolopez]

Hello everyone,

I'm here to ask you one question. I have a electronic system with Modbus integrated working perfectly for several months. In this PCB I need 2 grounds (one Modbus dedicated). The fact is the system I'm using to isolate Modbus is quite expensive. I'm using this IC:
Analog ADM2587. Its great because it has auto voltage source to isolate Modbus.

Now I'm looking for another way to make an isolate Modbus design. I have some options in mind.

1. Use IC Texas ISO3080 (Isolated RS-485 transceiver). Problem here is I need to get a new voltage source. I have thinking to use a DC-DC converter, but options I see are expensive (DC-DC). And isolated transceivers is not the cheaper I would like...

2. Use IC Texas SN65176 (Differential Bus Transceiver) and digital isolator. This is the "best" option I think. Problem again, I need another voltage source. I have not never worked with digital isolator but Its not a problem. Its a very cheap solution.

As you can see I'm trying to get the same with less cost, designing as in old times.

So, here is the quesiton. Is there another option I dont have in mind?
and, how can I get another source voltage to use with these transceivers? As I wrote, I was thinking in DC-DC converter. Use a trasformer with 3 secondary I think it's bad choice (if it exist), and to use one more transformer increase the cost.

Thanks in advance :wink:

Best regards,

 

[FvM]

If you are designing a product rather than a DIY circuit, occupied area, part count and assembly costs are an important factor.

Most automation products incoporating an isolated RS-485 bus (e.g. PROFIBUS which is isolated by specification and many MODBUS implementations) end up with a small 5V DC/DC module, 3 digital optocouplers and a standard RS-485 transceiver. Alternatively a multichannel digital isolator IC instead of individual opto couplers.

 

[jolopez]

Hi,

Yes, option about simple transceiver and 3 optocoupler I got in mind, but is not my first choice because I'm trying to runaway from optocouplers. It's the cheapest alternative no doubt.

I understand from your words that use a DC-DC is a good way to start. The main doubt is get another voltage source and DC-DC is not the cheapest way (maybe could be the only way).

Use digital isolators with transceiver is the option I like in this moment (with some way to create the voltage source).

Regards,

 

[edsonmoky]

I use Analog Device, The IC has a power supply and isolated system.

 

[jolopez]

Hi edsonmoky,

You mean ADM2587 I referred before or another IC you know? I have been looking for in Analog Devices others IC many times and all I saw was the same features and price (even higher) than ADM2587.

Reagrds,