For Thvd1500, RS485 Bus add pull-down resistance, 3k, A pull up, B pull down.
general node less than 20, drive will be ok?
I was hoping you could clarify the question a little bit. Do you want to check if 3K is a good pull up and pull down resistor?
Also when you say general node less than 20 what unit is this in? Is this impedance, a timing characteristic, or something else?
When it comes to failsafe resistors this is a good document that goes over the calculations for the pull up and pull down resistors. Let's assume this is your circuit.
I highly recommend reading the document however the generic equation for the fail safe resistors is:
Where RFS is your pull up and pull down resistor rating, VS is your supply voltage, 27.8 is the common-mode loading and line-matching constraints of an RS-485 system. To find VAB you can use this equation:
Where VIT, is your receiver input threshold and VNoise is the differential noise.
From here you can solve for RFS, and using RFS you can solve for RT2 using this equation:
RT1 should be equal to the characteristic impedance of the line, for RS-485 this is typically 120 ohms.
I am going to reach out to my team to confirm that 20 nodes will work, however I am pretty sure that the THVD1500 can support up to 256 Bus Nodes.
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