RS-485 and CAN standards?

[Chad1]

Please, PLEASE!!! can someone with knowledge of these standards explain what exactly they are? The wiki pages create more questions than answers. I'm a test engineer that's responsible for testing chips that have those standards (e.g. 3.3V Supply RS-485), and I can't get a good explanation from anyone about what exactly they are. Thanks!

[AndyC_772]

I know a bit about them - they're differential, serial communications standards used primarily in industrial and automotive applications - but you knew that much already.

What is it you need to know? Have you looked at the data sheets for the particular chips you're testing? What tests are you carrying out?

Happy to help out, but don't want to waffle on for hours without actually answering your question 

[Chad1]

Well the tests are independent of standard, for example continuity, leakage, etc. I just wanted to know what the "standards" ARE exactly. They're completely meaningless to me...what defines a chip as an RS-485 device? Is it simply a set of specs that are consistently met that makes something RS-485? I'm not even really sure what to ask because I'm so lost on the subject. Any info I can get from you is appreciated, no worries about rambling

[AndyC_772]

RS-485 is a communications standard, so the idea is that any product which conforms to that standard can, in theory, communicate with other products that also conform to the same standard. Same with CAN (or Ethernet, or RS232, or USB, or whatever).

If you're testing for continuity and leakage, then much of what's contained within those standards won't concern you. All you're looking for by the sound of it is a way to check the integrity of the wiring used to connect equipment together, not necessarily the details of how the equipment actually communicates. Am I right?

An RS-485 chip will be one which specifically implements some or all of the circuitry needed to talk with other equipment according to the RS-485 standard. Take a look, for example, at these:

http://www.exar.com/connectivity/transceiver/rs485

These chips contain line drivers and receivers, which physically connect to the wires and drive electrical signals onto them. The actual data rate on the wire, and the specific pattern of 1's and 0's used, is determined by the microprocessor or other controller that they're used with.

Physically, there will usually be a pair of wires used to transmit data, and one pair used to receive. A logic '1' is normally represented by one of the wires in each pair being at +5V and the other at GND, and a logic '0' being the other way round. The transmit pair of one device is connected to the receive pair of the other. If the product you're testing has a 3.3V interface then the logic levels on the wire might be +3.3V and GND instead, which is probably OK in many cases.

I believe it's also possible to have multiple receivers driven from a single transmitter.

With CAN, there's only a single pair, which is terminated with a resistor at both ends. Normally the two wires sit at about the same voltage (around 2V or thereabouts, though the spec allows this to vary quite a lot). This is called the "recessive" state. When a device wants to transmit, it drives the CAN+ wire to a higher voltage and the CAN- wire to a lower voltage; this is called the "dominant" state. Data is transmitted by rapidly switching from one state to the other, and every device on the bus will receive the same information. Each message has includes a header which states which device the message is intended for, and every device on the bus must therefore have a unique address.

Does that help at all? Hope so!

[AlfBaz]

I was under the impression that RS485 describes the Physical (PHY) way in which data is transmitted electrically. In the case of 485 it is a differential voltage on a pair of wires. It can be half duplex, where the driver is connected to 1 pair of wires and the receiver to the other. In full duplex both the dirver and receiver are connected to the same pair of wires. It goes on to describe the maximum number of units that can be attached to a 485 "multidrop" system, maximum transmition length, slew rates of signals etc, etc

CAN on the other hand describes the protocol or language used to communicate. For example you can implement CAN on copper wire using RS485 or it can be done with fiber optic. Further to this protocols such as CAN, Modbus, Profibus, etc can be embeded within other protocols. For example you can take modbus protocol and embed it as a data packet into ethernet. When it gets to the other end ethernet stack will strip away its various layers leaving the mosbus data to be passed on to the modbus handling layer

Edit: You can find out heaps about 485, which is multidrop, 482 (nearly the same as 485 but not multi drop) and 232 from maxim/ti sites as they have lots of driver ic's for 485.
CAN IC's can have drivers, such as 485 built into them or they can just have logic lines that you connect to a PHY of your choice. Its a coms protocol developed by BOSCH for the automotive industry with rigorous handshake and data intigrity systems

[tecman]

RS485 is a physical layer standard.  It is the multidrop variant of RS422.  By multidrop a transmitter will go to high impedance when it is not active, allowing other nodes to take control of the line.  As such it is a master-slave communications.  Arbitration and messaging is all software protocol based and not part of the RS485 specification.  The RS485 spec is based on 5 volt based signaling, but has been extended by industry to 3.3 volt levels.

CAN is a multi-node bus standard that includes basic physical layer specs and also the basic arbitration and message formatting.  CAN can be implemented on a number of physical formats, the most common of them being single wire and differential.  Optical links are an adaption to the spec and must be converted to the signaling specified by the CAN spec.  Since CAN arbitration is done in hardware, most CAN drivers and interface chips perform the arbitration before signaling the CAN's software layer.

paul

[vk6zgo]

I don't know anything much about CAN,but I had a bit of a play with RS485 when we thought we had a data comms failure with some transmitter controls.
It turned out to be bodged OHL connectors!

We found a couple of useful websites which helped us "Big Dummies":

http://www.radio-electronics.com/info/telecommunications_networks/rs485/rs-485-tutorial-basics.php
     
http://www.bb-elec.com/technical_library.asp