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Timothy Puls

Safeguard RS-485 Links Against Electrical Transient Threats

Timothy Puls
vasanjk
vasanjk
7/15/2014 1:45:27 PM
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Re: Termination
chirshadblog

 

These principles are very important and are very much in practice.

 

But there are no thumb rules which can be implemented just like that. Every design is unique and one solution that was best may not be even remotely effective in another design.

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vasanjk
vasanjk
7/15/2014 1:39:30 PM
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Re: Termination
Hi Timothy

I would like to bring in a little bit of specificity into my question.

 

Is it Connector>>>TVS>>Rest of the circuit or

Connector(TVS just on connector pin)>>Rest of the circuit ?

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chirshadblog
chirshadblog
6/24/2014 2:02:04 AM
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Re: Termination
@jayaraman: Do you think the policies are effective ? I feel the policies are being created but not put into practice 

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millikans drop
millikans drop
6/23/2014 2:55:37 PM
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Newbie
Re: SM712 device location
Hi Thomas,

Thank you for your comments. You are definitely a respected expert on RS-485 circuits and issues of EMC in RS-485 networks.  Thank you for your favorable comments regarding the TClamp1202P.

Best regards,

Timothy Puls
tpuls@semtech.com

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millikans drop
millikans drop
6/23/2014 2:47:50 PM
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Re: Termination
Hi Jayaraman,

Thank you for your comments.  The TVS devices should have minimal impact to the termination scheme as they will appear as a high-impedance in the circuit.  The leakage current under normal operation is on the order of 10s of nA.  Thus, the TVS components should have very little effect on the drive current and the termination characteristics.

Regarding the TVS placement, generally it is good practice to place the TVS as closely as possible to the connector.  

Best regards,

Timothy Puls
tpuls@semtech.com

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JAYARAMAN KIRUTHI VASAN
JAYARAMAN KIRUTHI VASAN
6/21/2014 1:36:00 AM
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Newbie
Termination
Hi Timothy,


Nice post. It has become a standard practice to include the TVS diodes in the lines. We always have the circuit as a complete block and cut-paste across all our designs.


Can these diodes have any effect on the overall termination impedance/resistance, say, if I have around 32 RS485 participants in the network?

Also, where to best position these diodes - near the connector or near the A,B pins of the RS485 chip?

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TKUG
TKUG
6/17/2014 9:49:55 AM
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Artist
Re: SM712 device location
I advise RS-485 system designers to distinguish between node design and bus design.

In the past items like failsafe biasing, termination, and TVS belonged to the bus design. This changed slightly when Semtech came out with the SM712 TVS because of its relative low capacitance of 75pF. Previous TVS diodes or arrays had capacitances of 1000pF and were better placed strategically along the data link.

The SM712 power rating of 400W however is rated only for 8/20 us currents pulse (or often falsely quoted 1.2/50us voltage pulse) specified in IEC61000-4-5 for surge protection.

IEC61000-4-5 however clearly states that for symmetrical (differential) unshielded data lines, the cobination wave generator (CWG) is a 10/700us (voltage) or more correctly 5/320us current pulse. This pulse will exhaust the capabilities of the SM712.

However, Semtech recently intrduced the TClamp1202, which is rated for a 10/1000us pulse with 300W and for the 8/20 us pulse with almost 2kW (derived from the device's power rating curve). Further more, this device has a junction capacitance of only 12pF typical. Hence it quickly made it into my RS-485 design tool-box.

Regards,

Thomas Kugelstadt

Senior Applications Engineer, Industrial Interfaces, Texas Instruments Inc.

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etnapowers
etnapowers
6/13/2014 4:54:29 AM
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Master
TVS diodes
I guess that the reliability of the Transition Voltage Suppression diodes is a top priority to guarantee the correct functionality of RS-485 links in an harsh environment.

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DaeJ
DaeJ
6/11/2014 2:44:53 PM
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Master
SM712 device location
This article proposed SM712 device installation in RS-485 cable. I am wondering where this device is located in the cable line. This device would be located either the inside of RS-485 or other side of terminal. Potenial signal distortion could be caused by clamping the voltage.

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