MODBUS maximum wire length
MODBUS maximum wire length
(OP)
I'm going through calculations for various equipment and the one item I haven't found much information on was calculation the maximum distance for MODBUS communication based on wire size. I seen general rule of thumbs and basic equations in regards to data rate and distance, but I would think there would be something out there that would calculate your distance based on wire size for resistance (expected Vdrop of the signal) and the capacitance. For voltage drop conditions, would this be just your 12V signal voltage drop across the wire and capacitance of the wire affects the signal?
Thanks,
guilio
Thanks,
guilio





RE: MODBUS maximum wire length
xnuke
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RE: MODBUS maximum wire length
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RE: MODBUS maximum wire length
https://www.belden.com/products/industrialnetworki...
RE: MODBUS maximum wire length
We currently run 16/4 wire, 80pf/ft between conductors, 4.35ohms per 1000'. We have no issues with to date. I want to reduce the wire size to 20/4, 60pf/ft, 12.35ohms/1000'.
From my standpoint, and I am no communication expert, I'm trying to break this down with what I know of 2 components in the system. Resistance and Capacitance. The resistance will help me know what my voltage drop will be (to determine that we are still within the 12 to -7 volt input range). I'm not sure how the current is calculated. If the driver will output all the current it needs per device attached, which I think it will, then current will be the driver current per device....I think. Thus 12V - Voltage drop = what Voltage the receiver will see.
The other part is capacitance, which when I see this, makes me believe that in order to ensure I see the difference between High and Low of the signal at the device, the higher the capacitance of wire, the slower the rate I need to allow so that the capacitance will discharge.
Thanks,
guilio
RE: MODBUS maximum wire length
2-pair Modbus 485 Cable
xnuke
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RE: MODBUS maximum wire length
RE: MODBUS maximum wire length
Keith Cress
kcress - http://www.flaminsystems.com
RE: MODBUS maximum wire length
For the wire, we generally run the same wire for all the devices. One pull can contain 12 to 20 of the same wires that will land on the 485 device but also provide power, AI, DI, DO, etc...by using the same type of wire, construction doesn't need to worry about separating different spools for different runs, landing the wrong type of wire on the wrong device, etc...I'll definitely look into a 20AWG with lower capacitance. At first glance it seemed that the capacitance was related to the number of strands.
RE: MODBUS maximum wire length
Keith Cress
kcress - http://www.flaminsystems.com
RE: MODBUS maximum wire length
You're trying to make it too easy on the contractors in my opinion. They should be able to pull at least two kinds of cable without screwing it up. I second itsmoked's post to go with larger wire, then - 18 AWG at least since you said you're distributing power.
xnuke
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RE: MODBUS maximum wire length
RE: MODBUS maximum wire length
itsmoked mentioned the insulation thickness and I got some reading last night. From what I am interpreting, of all this, the capacitance of the wire experiences a charge that effect the voltage, similar to a basic RC circuit. The Voltage Source can goes HIGH/LOW when transmitting data, but the wire will experience a delay due to this capacitance charging effect. (IMG14) When I see this, I go back and say if we are experiencing issues on older installation, I could slow the data rate, which will allow for more time for the charging discharging of the wire so that receiver can detect the voltage levels. Is this a correct analogy?
Also, I never understood the drive voltage range on RS485. I don't see why the output high isn't always 12V but rather a 12V to -7V. What effects the drive output voltage and is there a constant current that it is sending?
Last, why is the max distance always 4000'? It seems that if you had a low impedance/capacitance wire and did a low data rate (if the above capacitance analogy is correct), you could go further. Is this just a recommended standards? Hence the RS of RS-485?
RE: MODBUS maximum wire length
Here is a link to a simple primer if you’re interested.
http://www.lammertbies.nl/comm/info/RS-485.html
RE: MODBUS maximum wire length
RE: MODBUS maximum wire length
TTFN
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RE: MODBUS maximum wire length
As for "larger diameter cable" robertjo24 has used badly chosen wording (perhaps a language issue). He is using the term "larger diameter" to mean non-twisted power transmission wires which are indeed not communications wire. My capacitance description to you stands.
And, YES, your charge/discharge description fairly well describes the capacitance issue.
BTW: Consider dishing out out some stars here a lot of posts deserve them.
Keith Cress
kcress - http://www.flaminsystems.com
RE: MODBUS maximum wire length
Irstuff. Thanks for the document. I actually found in the datarate vs cable length that explained a question about can I go further distant then with slower data rate. I reworked out your example with 9600 bps and came up with 9800' but from the document: Section 3 presents the lower frequency range where the line resistance, and not the switching, limits the cable length. Here, the cable resistance approaches the value fo the termination resistor. This voltage divider diminishes the signal by -6 dB. For a 22 AWG cable, 120 Ω, UTP, this occurs at approximately 1200 m. Thanks for the link.
The only thing I need to find out is how the 485 has a driver voltage between 12 and -7 and how it's determined. Not that it matters but curious. Thanks.
RE: MODBUS maximum wire length
I'm not real clear on your question here... The device will have to have a +12V and a -7V source in it to provide those needed values. Now days there are integrated circuits that will provide those voltages without some extraneous extra power supplies involved.
Why those particular voltages is that 485 is a differential signalling method where the information is conveyed as the DIFFERENCE in voltage between the two wires and not the voltage between a ground and one of the wires. That means the signaling can be anywhere in a "region". Why specifically -7V and +12V is, I believe, because that's what the IC designers needed to provide the chosen differential signal robustly while allowing some unequal ground potentials "common mode" between ends of a long cable.
Keith Cress
kcress - http://www.flaminsystems.com
RE: MODBUS maximum wire length
Thanks,
guilio
RE: MODBUS maximum wire length
TTFN
I can do absolutely anything. I'm an expert!
homework forum: //www.engineering.com/AskForum/aff/32.aspx
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers