RS485 Cables: Why you need 3 wires for 2 (two) wire
RS485 needs 3 conductors and a shield. Many people say it's a two-wire network but it is not. Two conductors are used to carry the RS485 Differential voltage signal. The Shield is connected to earth/ground at one end only and provides shielding against induced noise.
So why the 3rd conductor?
The driver sends data by modulating the differential voltage. The receiver must sense and decode the differential. There are limits to the voltages the transmitters and receivers can work with. These limits are specified by the code. They are -7Volts to +12Volts. What happens if you have two devices and a ground potential exists between the two devices of 24 volts? You can see that one of the devices will be operating outside the specified voltage range. While you might expect that all the electrical equipment in an installation is ultimately connected to the same ground in practice this is rare especially in cold climates where building architecture and frozen ground can conspire against you. That is why you need the 3rd conductor - to connect the ground (of each RS485 driver) to the same reference. Now we don't care about ground potentials.
Ever wonder why you blew a 485 device when you connected your laptop or computer?
It's this problem - there is a ground potential. That's why its good practice to connect your laptop's 485 ground conductor before you connect the differential conductors.
Can you get away with 2 conductors?
Yes. That's why lab or factory tests suddenly stop working when installed at a site. In your lab or on your desk you can be sure the devices are all commonly grounded. Now if you measure the difference between the ground of an RS485 driver on one device and another device you will find zero volts.
Can you get away without the shield?
The value of the shield is controversial. If you are using a twisted pair and you don't ruin the twists by
unwinding them by more than an inch or two at each end then the shield probably is of little value. But,
most cables come with a shield. If it's not easy to terminate the shield or you can't be bothered then at
least have your design drawings tell the installer to coil and tape the shield drain wire so that you can
use it if you want to.
Where can you purchase 3 wire 485 cables?
I don't know. Purchase two twisted pairs with an overall shield/drain. Use one pair for the differential and join the conductors of the other pair to make your ground wire.
What size conductors?
The bigger the better. Most installations are done with 24awg but remember the higher the baud rate the greater the signal frequency and the greater all kinds of captives and inductive effects. If you can, get bigger conductors.
What kind of cable?
Choose one designed to present an almost constant nominal impedance because it will make the addition of terminating resistors easy - just read the cable jacket and get a resistor of the same impedance. Most cables listed for 485 use are designed to present an almost constant nominal impedance. Almost constant means a cable whose impedance is reasonably independent of length.
What about biasing?
If an RS485 driver is idle then what? The line is disconnected from the driver in its idle state which means that the + and - are floating. What happens if for an instant there appears a differential voltage of more than 0.2 Volts? Well, a differential greater than 0.2 is considered a signal and hence constitutes data noise. Solve this problem by pulling the lines to no voltages when they are idle. To do this connect them to the ground or some other voltage using pull-up/pull-down resistors. Nice vendors include this. Nicer vendors provide a choice of biasing resistors selected by switches/jumpers. The reason you might not always use the same value is that the cumulative effect of many device's biasing resistors may make it impossible to signal at all. How do you calculate the value of a biasing resistor? How would you know where to connect it? Good luck getting answers to these questions. An easier question to answer is this: How do you know if you need to bias the idle state? If you have a scope you can see what the idle state floating voltages are? Don't forget you can only measure this when the device is idle.
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This is just some small feedback with regard to the Ground connection for RS485 that we discussed two weeks ago on the ECC:
1. Yes, the Ground wire is required, and thus RS485 is not a 2-wire network. Very few companies indicate this in their documentation and diagrams. Even reputable semiconductor manufacturers will only mention this connection in passing, and then one specifically has to look for the issue.
2. But, the problem is that a ground loop can be created when the power to all RS485 terminals is not isolated from each other. This is a problem since in most installations for commercial purposes, unskilled people are used. They generally don't fully understand the principle of EARTH and RETURN connections.
3. Thus, normal installations will receive their power from one central power supply or have separate SMPS, but if these are not specified properly, then the EARTH and NEUTRAL will be connected by either a short or some small resistance to each other on the low cost versions!!!
4. The whole grounding issues thus becomes the problem of the equipment supplier and in particular the installation / hardware design engineer.
5. Most isolated RS485 Ground connections refer to the main Ground with a 100E resistor. This is fine if the common mode current (low frequency syndrome) is minute in the cabling. This phenomenae can be minimized by using a Shielded Twisted Pair where the shield is normally terminated only at ONE side of the equipment. In applications, such as ARC furnaces this will NOT work since 100's to 1000's amps are generated, and the ground wire will be conducting.
6. The track on the PCB thus becomes a fuse when the rating is exceeded e.g. 8 mil track maximum current carrying ability under optimal conditions is 500mA.
7. Proper isolated RS485 networks remove the low cost resistor implementation, and use a proper transformer to generate a isolated RS485 ground. This is also the best and most expensive circuit to implement. This will minimize common mode current pickup and voltages spikes on the line, but each installation has its own unique requirements (the arc furnace is just one of worst case examples that I know of¦)
Regards,
Arno Netter.
You wrote this in one of the blogs about rs485:
2. But, the problem is that a ground loop can be created when the power to all RS485 terminals is not isolated from each other. This is a problem since in most installations for commercial purposes, unskilled people are used. They generally don't fully understand the principle of EARTH and RETURN connections.
3. Thus, normal installations will receive their power from one central power supply or have separate SMPS, but if these are not specified properly, then the EARTH and NEUTRAL will be connected by either a short or some small resistance to each other on the low cost versions!!!
4. The whole grounding issues thus becomes the problem of the equipment supplier and in particular the installation / hardware design engineer. I'm currently in Kuwait working on a project which involves rs485 connections. I face problems with the rs485 connection grounding. Right now, I'm using a Robust DC 485-fiber convertor to convert my data from my substations to transmit across the FO to my local control room. At the local control room, I'm using same convertor to convert the signals back to 485 signals.
My problem is there is no common reference which ideally should have. Now I'm having line breaks in communications are time and data hanging during transmissions. Out of the 8 units I have, only 1 is working.
Can you please help me with it?
Dear Chong,
I am currently in Kuwait working on a project which involves rs485 connections. I face problems with the rs485 connection grounding. Right now, I am using a Robust DC 485-fiber convertor to convert my data from my substations to transmit across the FO to my local control room. At the local control room, I am using same convertor to convert the signals back to 485 signals.
Are you using the recommended isolated power supply as per Robust DC specification or perhaps another type. I stress that this must be a ISOLATED power supply that supplies power to the transmitter and receiver of your RS485 convertor. When you measure the GND on the input side of the power supply with reference to the power output ground/negative, this must be open circuit or at least in the mega Ohm region. In effect, this is also called in industry an isolation transformer that is not always identical with a normal transformer, although some
My problem is there is no common reference which ideally should have.
That should be the problem of the isolated power supply as these units are referenced on their input side to each other over distance. The communication should not suffer unless there are other problems associated with your network. RS485 is a differential pair, thus in your specific application with fibre their is no need for a common reference, but that depends on the driver/receiver pair in the actual convertor. How are these driver/receiver pairs (the actual semiconductors inside the box) referenced to the power of the actual unit. Are you introducing additional external resistors to make a common? If so, why because from a quick inspection of Robust DC datasheet this is not required?
Now I am having line breaks in communications are time and data "hanging" during transmissions. Out of the 8 units I have, only 1 is working.
I suspect that your problem is more related to a bad fibre optic cable. Do you use single mode or multi mode cable in your installation? Are you exchanging these units with each other to confirm a bad unit? Or are all the units on the fibre optic cable at the same time? Has the fibre been properly terminated? Is there is damage to the cladding? Are you within the length specification of the fibre? Are there any loops in the fibre cable that will cause higher optical dispersion in both single mode, but especially multi mode fibre? Are using exactly the same fibre optic cable for all the units?
Can you please help me with it?
For help, more information is generally required. Also never underestimated the physical environment in which you are installing and operating equipment. Kuwait for an analysis would imply a desert which implies relatively little humidity thus higher electro-statics, thus your equipment needs to be more robust to withstand continuous static discharge. You probably have a shot driver/receivers on either the fibre interface or the RS485 interface especially when you say that data is intermittent.
But this is all maybes since I do not know more about the installation, the environment, etc.
thank you.
RS485 is a 2 wire network if done right. Profibus is a typical example of a true 2 wire RS485 system. The secret to trouble free RS485 is to use isolated power supplies to run the RS485 transeiver. Low cost, low current, high speed non optical isolators are now available from Analog Devices (the ADUM1300 series) and simple to implement power supply isolators are also available from Maxim (MAX253 & MAX845). Combining these parts with any standard RS485 transeiver builds you a fully isolated interface that truly needs only 2 wires. You must provide biasing resistors with this setup to center the RS485 signals within the isolated interface common mode range. A sample of an isolated interface can be found in the booklet The rapid way to Profibus or I can send you as sample circuit if you are interested.
Hello Douglas,
I'm sorry but basically you are wrong.
Voltages are not defined by themselves, they are only defined as the difference in the force of the electrical field. To measure a voltage you always need two points to put your probes on.
In RS485 one point is the signal wire A or B, and the second is the local ground of the receiver circuit. When you have an isolated device, the voltage of the isolated ground compared to the local ground of your device is undefined. This is especially true for the voltage difference between the signals and the isolated ground, because the signals are generated in respect to the local (isolated?) ground of the *transmitter* circuit. When these two grounds are completely isolated, there is no guarantee the receiver understands the data. It often happens the difference is too big. One proof of that is the real-world experience, transmissions become successful when you just add the ground line.
Biasing resistors do not really help, because for the purpose you describe you have to attach them to *every* device connected to the bus. This requires a recalculation of their values every time you add or remove a device from the bus.
(fact: calculation of resistors is not done by average customers.)
And in fact in this way you use the data lines to clear all voltage differences between the isolated grounds of the devices. At least you connect them via the resistors and the Data- line, which is pull-downed.
Even without bias resistors there may be some kind of parasitic connection between the data lines and the receivers ground, which also clears the differences between isolated components. That may also be the reason why in many applications the communication is fine with only two wires.
I agree, when using bias resistors as you describe it, there is only a reasonable current when a new isolated device is attached to the network. Later the isolated ground planes share the same electric potential, and there are only very small currents, if any.
By the way, even maxim-ic says the ground wire is required.
Hello Douglas,
Your point about Profibus is a good one but I think it re-inforces the the fact that rs485 requires 3 conductors.
The reason I say this is that Profibus is a standard that encapuslates RS485 and imposes other electrical requirments on it (like the isolation). Thus you get a bus with devices with a uniform electrical interface.
This is not the case with other RS485 applications. For example BACnet MSTP merely references the RS485 standard and does not impose other physical layer requirements.
I have to agree with Wilfried, viasing resistor calcs should not be required for field installations.
Hello Wilfried.
I think that the receiver side isolator output is measured by its ground. therefore receiver device can read data correctly.but the third wire is required as shield to reduce the noise.
Hi All,
I agree with Douglas, a third wire is not always necessary.
As long as the difference in ground potential between all the devices, including noise, is within the common mode range for TIA485 specification then it should work without a ground.
In this situation is side A more positive than B or vice versa, this is what is meant by differential signals.
This common mode range of TIA485 was mentioned early in the article, -7 volts to 12 volts.
If this is exceeded in the system than one or more isolation devices will be needed.
Though personally I would always use a ground.
Biasing resistors, not to be confused with terminating resistors, only need to be small enough so that noise on the signal line does not get through the receiver, at least 200mV.
This has become confusing over time as the original specification required the receiver chips to have a defined input resistance which means that there is always going to be current flowing into the input pins. This limited a bus to 16 devices owing to the drive available from the driver chips.
Since then there has been quarter power, eighth power and even sixteenth power devices, so resistors for biasing with early devices are now too low in value to work properly in large systems, the driver will become overloaded. This means that later systems will have higher value resistors, and can lead to providing insufficient bias for older chips allowing noise through the receiver and affecting the integrity of the communications.
Trevor
I have been struggling with the 2- versus 3-wire, grounded versus ungrounded, shielded versus unshielded issue with RS485 wiring for 20 years. Steven says pretty much the same as I have been saying for over 15 of those 20 (I was a slow learner).
Comments:
1) Shielding is always good. This can be proven theoretically and in practice. Double shielding is even better. Single shields must be grounded at one end, double (insulated from each other) are grounded one at each end. This prevents currents from flowing through the shield and inducing currents into the signal lines via the transfer impedance between the two.
2) Bigger may be better but there is a very serious law of diminishing returns as the gauge of the wire gets larger. High frequencies are carried on the surface of a conductor and the more surface area the lower the impedance " this is good. 18ga is overkill. 20ga is OK for very long runs, 22ga is good enough for pretty much every application.
3) There are RS485-specific cables out there. They have a shield & drain wire (ground at one end), a ground wire and a twisted pair for the signal. Belden 3106A is a good example.
I wish I could get more people to understand that carrying a third wire between RS485 devices was necessary. I wish everyone who implemented an RS485 device had put the third wire on the connector so you could connect to it.
Apropos PROFIBUS: Profibus may be RS485, but the reason it works is because the purple cable and connector is EXACTLY DEFINED and full network performance is only achievable using EXACTLY THAT CABLE (and connector). If you grab any old twisted pair cable use it to hook up your Profibus network, you deserve the problems you have.