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House battery project is almost done but the required terminating resistor is AWOL. I only have a couple 100s and lots of 470s. I could get close by paralleling 2 100s and put another 100 in series --> 150 Ohms. Or parallel four 470s --> 118 Ohms.

I don't know about the tolerances for Modbus termination. Anyone know if a single 100 Ohm resistor has a chance of working?

Edit: Cable length is about 29 ft (~9 meters) Cat6

House battery project is almost done but the required terminating resistor is AWOL. I only have a couple 100s and lots of 470s. I could get close by paralleling 2 100s and put another 100 in series --> 150 Ohms. Or parallel four 470s --> 118 Ohms. I don't know about the tolerances for Modbus termination. Anyone know if a single 100 Ohm resistor has a chance of working? Edit: Cable length is about 29 ft (~9 meters) Cat6

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[–] • 1 pt

Holy wall of text, batman. Thank you so much!

I'll try with the 100 Ohm (reads 98 on the meter) one.

Time to get one of those resistor sets in through hole. Only have SMD booklet type sets. Suppose they won't stand up to the mechanical stress of converting them to wired ones. But I'll try, if I'm desperate.

Thanks again.

[–] • 1 pt

Time to get one of those resistor sets in through hole.

When I've ordered the cheap Chinese ones they are all blue as if they're metal-film, with an extra band reading a higher precision of value, but when testing they are still 5-10% tolerance (which is fine for most things). Just know they lie about them being 2% metal film resistors.

[–] • 1 pt

Glad to help out. It's not too often I get to put on my old EE hat and think about some classic electronics stuff so this was fun.

Yeah the is probably where I would go if I were in your situation just simply because it's going to be a closer fit and you get to make a Frankenresistor in the process. But contrary to prairie's thoughts on the matter, I'd be wary that the Frankenresistor will become a problem long term and will make troubleshooting more of a pain if it is forgotten about or someone new comes encounters it. But it should be good for now. :)

[–] • 2 pts

'Frankenresistor' :D It's a quality build, measures right at 115. Used Sn60Pb40 solder, no worries.

We have factory shut down until January 9th. After that I'll be able to borrow the correct value. Glad we could have some fun. Let's see how it goes.

[–] • 2 pts

Only real EE/IE/EET guys will get this...

https://pic8.co/sh/7fcjSk.jpg

[–] • 1 pt

Frankenresistor will become a problem long term

Higher inductance? I'd agree, use it until you get a proper 120 ohm to drop in place.

[–] • 1 pt

There will be a slight change in cross-sectional inductance as well as a new parasitic capacitance that forms between the resistors. But since RS-485 is current loop, the inductance and capacitance changes will only be a problem at very high frequencies, which is unlikely to be the case here. Ultimately my concerns with the Frankenresistor are those related to their connection. Though OP used 60/40 Sn/Pb solder, there is the chance that cold joints will change the resistance/impedance of the compound resistor over time. It's probably not a major issue at all, but if it's for some critical industrial control application, I would definitely change it out as soon as possible with a good high quality, tight tolerance metal film resistor just to avoid liability issues that could pop up in a failure incident investigation.