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Over 485-interface slaves are connected as multi drop, physically daisy chain.

Those have built-in termination resistor, and the value is 120 ohm and no select switch for termination on/off.

So it makes termination value as 120hm if I connect 10 slaves.

If I want to make it as properly, just put 60 ohm resistor on each data Line(D+, D-)?

Master A(D+) -----(60 ohm)-----~~~

Master B(D-) -----(60 ohm)-----~~~

My guess is it can affect as like below:

60 ohm + 12 ohm(parallel connection from 10 slaves) + 60 ohm = 132 ohm

Surely, the best way is changing termination resistor, but I want to make it work simply.

Regards, Jiwon

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  • \$\begingroup\$ Your RS485 slaves don't have a switch to enable/disable termination? That's not good ... Are you sure they're RS485 and not RS422? Do they have any way of selecting different addresses so you know which individual one you're talking to? \$\endgroup\$ – brhans May 27 '15 at 13:54
  • \$\begingroup\$ No switch for en/dis termination. And it works based on only 485, I checked with the chip number. They provide software address for accepting signal when it arrived. Physically those are all connected now. My point is above my guess can be workaround. \$\endgroup\$ – Aveugles May 27 '15 at 14:19
  • \$\begingroup\$ The termination should be only at the very ends of the string and any stubs off the pair should be of minimum length possible. \$\endgroup\$ – Spehro Pefhany May 27 '15 at 14:20
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    \$\begingroup\$ I would open them up and physically remove the terminating resistors from all but the last unit. Multiple termination like you describe is just asking for trouble. \$\endgroup\$ – brhans May 27 '15 at 14:25
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    \$\begingroup\$ I think you've misunderstood your slaves. With fixed termination, they can only operate as RS422. However, the voltage levels for RS422 and RS485 are close enough that you can use an RS485 receiver on an RS422 bus. I suspect that the manufacturer uses the same chips for RS422 and RS485 as a way to simplify inventory and purchasing. \$\endgroup\$ – WhatRoughBeast May 27 '15 at 16:19

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