Hi,

I currently working in the refrigeration industry. On the monitoring system we use, I noticed it says that the maximum number of devices is 100. We also regularly connect more than 32 devices. Which leads me to my question. I thought RS-485 could only have a maximum of 32 connected devices?

The monitoring system we use is called "Televis GO". It's from Eliwell, an Italian manufacturer in the HVAC industry. We have PLCs, parametric controllers, alarm modules (all of which are Eliwell as well), all connected on a RS-485 bus.

What am I missing?

 

Back when I started dabbling in this in the 1990's, the spec was a maximum of 32 "loads' on a 485 bus, as defined by the chip manufacturers and the EIA-485 standard. Wikipedia says that EIA-485 got revised and renamed to TIS-485 in 1998 and the maximum number was not defined by the standard but was stated as a "minimum of 32 loads".

Lynn Linse in post #2 in this thread from the early 2000's summed up what one might expect for approaching the maximum number of 32 loads: https://control.com/forums/threads/...ions-on-number-of-multi-dropped-devices.6023/

He mentions a 'fractional' load spec, meaning more loads per bus. I haven't looked up such high sky numbers because my experience has typically been with nodes of mixed devices from various manufacturers over fair distances where common mode ground loops have been the limitation to bus size because of the cost and need for multiple 485 repeater/isolators.

 

The load limitation of RS-485 is defined as 32 "Unit Loads". For older RS-485 devices, it may be true that 1 device presents 1 unit load on the bus. However, nowadays, there are many devices out there that present a fraction of a unit load, such as 1/4, 1/8, or even 1/10, onto the RS-485 bus. For example, with 1/8 unit load devices, you can have up to 256 devices on a single RS-485 bus.

 

Ah, thanks you guys so much. That makes lots more sense now. I've been programming PLCs for a little while now, and I'm now starting to become quite interested in the comms protocols.

There's quite a lot to learn and it does feel a bit overwhelming. Modbus RTU (RS-485, 232), Modbus TCP, CAN, Profibus, Profinet. These are all things we use but I've hardly given a second thought to, being more focus on becoming a competent programmer. RS-485 is the one we use most frequently, and therefore I decided to dive into that a bit more first.

I've since realized that this is quite an important topic to be well versed in, especially in my field of work. It is quite interesting albeit intimidating.

 

Hi, I have been reading up a little bit and i have been curious about the following:

1) I have noticed that we do not use any termination resistors on the rs485 line I initially described. Would there be any reason for this?

2) Could the fraction unit load become infinitely smaller? i.e. would it be possible to have a unit load about 1/20 or 1/30 etc

 

1) The short answer is termination resistors may not be used because termination may not be needed on this RS-485 bus.

Termination is used to prevent reflections (i.e. the signal reaching the end of the line and "bouncing" back, slightly attenuated and in opposite polarity) on a transmission line. An RS-485 bus is considered a transmission line and will have transmission line effects if the rise/fall time of the signal is less than twice the propagation time of the signal down the line. The main determining factors of whether termination should be used are the baud rate and total length of the cabling. As a general rule, reflections will dampen out within 3 round trips, so if the reflections will dampen out well within a single bit time (1 / baud rate) for the baud rate and cable length used, termination is not necessary.

Here is a general guideline:

Baud Rate	Maximum Length Without Termination
9600 and below	Termination not required
19,200	2,800ft (850m)
38,400	1,400ft (425m)
57,600	940ft (285m)
76,800	700ft (210m)
115,200	470ft (140m)

The recommendations in this table assume bits are sampled in the middle of each bit time, 3 round trips are required for reflections to dampen out, and the RS-485 cable has a worst-case propagation velocity of 66% the speed of light (typical values are between 66% and 75% and should be stated in the cable's datasheet). At baud rates of 9600 and below, the maximum length before termination is required is greater than 4,000ft, which is the maximum allowable cable length for RS-485 networks.

In general, you can use the following equation:
L < 0.66C / 12B
Where L is the cable length, C is the speed of light, and B is the Baud Rate.

These articles have good discussions on termination:
https://blog.opto22.com/optoblog/rs-485-to-terminate-bias-or-both
https://e2e.ti.com/blogs_/b/analogw...nation-is-necessary-and-how-to-do-it-properly

2) There is a limit to a device's fractional unit load, so it could not become infinitely smaller. It comes down to how much signal strength is required by the device for proper detection - the less load imposed on the bus, the lower the signal strength in the device's detection circuitry. As the technology advances, it becomes more and more possible to detect smaller and smaller signals, allowing the unit load to become less. I'm not certain what the lowest available is these days, but I believe the lowest I've seen is 1/10 unit load.

 

120 ohms.
485 termination and how to do it right