I am trying to add the external fail safe biasing for my RS485 bus. However, I am planning to use a 5 volt source that is seperate from the 5 volts supllied to the transceiver. I am wondering if there are any current (amps) or other ratings I should be concerned about. Fore example, what should the output current be from the voltage regulator suplying the 5 volts to the biasing resistors?
I have attached a quick design guide. The failsafe network in here requires under maximum common-mode loading (375 ohms at each conductor) a total current of 7.23mA. So 10mA for your power supply budget would be the minmum requirement.
Thank you Thomas. This is very helpful in my work.
However, most of the design guides I have read, recommend 22AWG Shielded Twisted Pair. Why in your attachment, do you recommend unshielded twisted pair?
using shielded twisted pair is of course better in noisy environment because the shiled prevents noise signal coupling into the bus cable. The UTP 22AWG to 24AWG is more a minimum requirement I recommend when someone looks for low-cost cabling. I have many Asian customers who use 18AWG lamp wire for low-cost wiring. This type of cable consists of two parallel conductors without a shield and allows for easy noise-coupling into the signal conductors. Often these companies don't apply termination either and then wonder why their networks work fine in the lab environment but fail at the end customer's premisses, such as factory halls. Here electric motors, generators, and other noise sources generate a plenty of noise that couples into the data link and causing data errors which eventually leads to network down-time.
By all means, please keep using shielded twisted pair cable to maintain a robust network in all environments.
Best regards, Thomas
Thank you so much for all of your time and effort. I truly appreciate all of your detailed explanations and timely responses. I have really learned a lot from you.
Just curious about how you determined the value of the Biasing and termination resistors. I have been following the RS485 design guide(SLLA272B). - Please see attachment. However, why does the design guid use 523 Ohm resistors instead of 528 Ohms? Why do you use 135 Ohm termination in stead of 120 Ohm? And 530 instead of 523? I am not sure that it is a significant difference. But I am curious about why the particular values were chosen for each example.
Joseph, you discovered a typical "copy and paste" error in the RS-485 Design Guide (SLLA272B).
The 120 W termination on the right bus end (RT1) is okay, the one on the left (RT2) is not. Here the two 523 W failsafe resistors (RB) are differentially in parallel to the termination resistor (RT2). The whole parallel combination however must yield a 120 W value: RT2 || (2 RB) = 120 W. To determine the value of RT2 simply solve for it : RT2 = 1/ [1/120 - 1/(2 RB)] and you get 136 W (rounded up). Choose your next closest resistor (135 W from the E-192 series) and you're done.
The values in the latest example using 530 W and 135 W assumed a minimum Vcc of 4.5V and not 4.75V as in the design guide. It also depends whether you choose the E-48, E-96, or E-192 series of resistors. I usually use the E-192 series since resistors are dead cheap nowadays. However when writing the design guide I must have used the E-92 series due to a customer request (sounds stupid but it happens).
However the short design equations I sent you earlier were derived from a document on idle-bus failsafe biasing. You can see it when clicking on the FILES section above.
Hope this all helps. Regards, Thomas
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