RS485 termination resistor

[hamdi.tn]

Hi,

Am starting a project where i aim to connect 100 unit with an RS485 bus, and the length of this bus could be around 1KM, and i was reading this document about RS485 from TI
http://www.ti.com/lit/an/slla272b/slla272b.pdf

And looking at Bus loading (Page 4) they said

failsafe biasing contributes up to 20 unit loads of bus loading

i don't get how they calculated that !! any help please.

[Yansi]

Capacitance is the main issue.

I would highly recommend to split the cable run to 100m (maximum) sections and use sort of signal repeaters.

Termination resistors have to be used on both ends (of each cable section) and the stub length have to be minimized.

What baud rate are we talking about?


And by the way, the appnote talks about the bus load units and how the max number of stubs can be calculated.

[NW27]

Using their figures, a standard input presents a 12k input impedance.
12k / 20 = 600. 600 is approximately the value of the Rb + (Rt/2) = (532+60) = 592.

[hamdi.tn]

Capacitance is the main issue.

I would highly recommend to split the cable run to 100m (maximum) sections and use sort of signal repeaters.

Termination resistors have to be used on both ends (of each cable section) and the stub length have to be minimized.

What baud rate are we talking about?


And by the way, the appnote talks about the bus load units and how the max number of stubs can be calculated.

for all those problem i will keep the baud rate to minimum, the amount of data to be transferred is not huge and timing is nor critical either so the lowest practical baud will be used.

And i will design a repeater too, to solve any problem on the field.

yeah i saw how max number of unit is calculated , it's just the detail that those termination resistors are equivalent to 20UL that i don't understand

[hamdi.tn]

Using their figures, a standard input presents a 12k input impedance.
12k / 20 = 600. 600 is approximately the value of the Rb + (Rt/2) = (532+60) = 592.

yeah that's it ! 

[mikeselectricstuff]

Capacitance is the main issue.

Cable resistance is also a significant limiting factor, so for long runs, cable selection is important, i.e. not CAT5!

[nctnico]

Still you'll find it will be surprisingly difficult to find cable which has a low capacitance! At long distances it is necessary to use cable (or coax) with a well defined impedance (usually 100 Ohms) and terminate it properly so it at least looks like a resistive load to the driver. Nevertheless ane (copper) cable will have a significant attenuation (increasing with frequency) due to the resistance of copper and the capacitance. At 1km I think shielded twisted pair is about the best you can do before going to optical fiber.

[ebclr]

The speed of the communication is a crucial point, if you plan to do at very low speeds even nothing will work, but take a note that if you plan to have 100 units on the same line will not be a easy task to do without using repeaters.

No general answer is possible, you need to know the exactly cable characteristic, a practical "way " is to put a trimpot near 500R and with a scope adjust to have a best "square wave" on full speed.

[hamdi.tn]

At 1km I think shielded twisted pair is about the best you can do before going to optical fiber.

That's the plan , to use shielded twisted pair.
also what am planning to do is to use the units boards to link between those twisted pairs, each board will have two RJ45 connector that will link the previous unit to the next unit. am sure the pcb itself will only make it worst.

but take a note that if you plan to have 100 units on the same line will not be a easy task to do without using repeaters.

the most annoying thing for me is that even if this work in the lab , there is 1000 reasons not to work when they install the units. So yeah am not eliminating the need for repeaters.

it's the first time i have to use rs485 bus on such scale, so every datasheet and app note are a bit confusing about the limit of the system and the number of parameters involved. yeah this will not be easy.

[ebclr]

Don't forget a Hugh lightning protection

[hamdi.tn]

Capacitance is the main issue.

Cable resistance is also a significant limiting factor, so for long runs, cable selection is important, i.e. not CAT5!

we will use CAT6

[mikeselectricstuff]

Capacitance is the main issue.

Cable resistance is also a significant limiting factor, so for long runs, cable selection is important, i.e. not CAT5!

we will use CAT6

For 1km you'll need something more like 22AWG. The DC resistance of cat6 may be too high for reliable operation at any baudrate