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A Guide to online information about:

RS-485 Multidrop Networking

by Benjamin Day

Introduction

RS-485, due to its low-cost, simplicity, and robustness, has become widely used in many industrial networking applications. Many microcontrollers provide a serial port that can become a node on an RS-485 network with the addition of an inexpensive RS-485 transceiver. RS-485 wiring uses simple and inexpensive twisted pair wiring. And RS-485's use of differential transmission makes it quite robust even in noisy industrial environments.

Unfortunately, RS-485 is only a physical layer interface. And even then, there is no specification for connectors. You are on your own with respect to the higher layer protocols.

The need for additional protocols is clear when you consider that RS-485 allows for multiple nodes to connect to the same wire, much the same as party-line phone service did in decades past. When you wanted to use a phone on a party-line, you had to pick up the handset and listen first to make sure nobody else on the party-line was talking already. If the line was clear, you were free to dial out. If more than one party tried to use the phone at the same time, the result was confusion. The same is true for RS-485. Every node on the network can generally listen at the same time. However, only one node can talk at any given time. If more than one node attempts to talk at once, the message will be garbled. So, a higher level protocol is necessary to ensure that every node will get a chance to talk and that its message will get through ungarbled.

Some systems deal with this problem partly through wiring. Some systems will require that one device be a master and all others slaves. Then the transmitter of the master is wired to the receiver of all the slaves and vice versa. Then there will be no problems as long as the slaves accept the rule that slaves only speak when spoken to.

However, the master/slave relationship does not always fit well. Often, a peer relationship better describes the interactions between network nodes. In order to realize such a relationship in an RS-485 network, it will be necessary to have some higher-level protocols.

OSI Network Model

To understand networking in general, you should be aware of the OSI Network Model. A network can be described as having several layers of functionality. The OSI Network Model defines seven layers of functionality which from highest level to lowest level are: application, presentation, session, transport, network, data link, and physical.

If you are thinking about creating your own networking protocol, you may want to study the OSI model and data communications in general, before you get too far along. A good place to start would be with the textbook Data and Computer Communications by William Stallings.

How many of the OSI layers will be needed in your application will depend on the capabilities you wish to achieve in your network. But, assuming a peer-to-peer network, you'll minimally need to have a data link layer. Without it, you'll have a party-line nightmare. The data link layer will ensure that messages are not garbled and will permit everyone an opportunity to talk.

Additional protocols may also be necessary if you wish to route traffic from your network to other networks. Or, if you have multiple applications on a single node that may need to talk at the same time over the network.

Of course the easiest solution would be to use an existing protocol.

Cimetrics offers BACnet, a sophisticated network protocol originally designed for communications of environmental systems. It works over a variety of physical layers including RS-485 and Ethernet. Cimetrics also offers a simple 9-bit serial protocol well suited to standalone RS-485 networks.

High Tech Horizon has developed a free and open protocol called SNAP or Scalable Node Address Protocol that can be used with half- and full-duplex, master/slave, and peer-to-peer systems.

Chip Vendors

A variety of chip vendors provide RS-485–compliant transceivers. A few of these vendors also offer UARTs in case the microcontrol-ler you are using does not have a suitable one. This should give you a few places to look for a transceiver or UART to meet your needs.

Analog Devices offers several RS-485 transceivers with a variety of features including ESD protection. Some of their parts can also be cross-referenced to compatible devices from other suppliers, in case you need second sourcing.

Burr-Brown's Isolated RS-485 Differential Bus Transceiver integrates the functionality of an optocoupler and an RS-485 transceiver in a single chip. The chip uses Burr-Brown's capacitively coupled isolation technology in place of an optocoupler.

Exar offers a UART with automatic RS-485 half-duplex control. The UART also sports 128-byte receive and transmit FIFOs which are a real plus for high transfer rates.

Maxim offers a 3.3V RS-485 transceiver that conforms to the industry standard pinout.

National has a large selection of RS-485 transceivers. National also offers an RS-485 bus repeater. National's FailSafe Transceiver offers the added benefit that its receiver enters a known state during floating line, idle line, and line fault conditions. National offers several useful appnotes.

Other Resources

The resources listed here should help you get into the meat of
RS-485.

B&B Electronics provides a helpful technical library, which includes a General appnote on RS-485, An appnote on the Basics of RS-485 and an appnote on Cable Selection.

National Semiconductor has a helpful appnote that addresses the problem of dealing with an unknown bus state when all drivers are in an off state. National also has an appnote that should be helpful in addressing network termination needs.

If you would like to buy a copy of the RS-485 standard, you can find at the Global Engineering Documents web site when you search for document number TIA/EIA-485.

The following magazine articles are also quite informative and helpful.

"Controlling the Transmit Enable Line on RS-485 Transceivers", Nigel Jones, Embedded Systems Programming, August 1999.

UARTs make possible low-cost networks of embedded systems, Jim Butler, EDN Magazine, March 1995.

"Ten ways to bulletproof RS-485 interfaces", John Goldie, EDN Magazine, August 1996.

Programming and Interfacing the 8051 Microcontroller/Book and Disk by Sencer Yeralan, Ashutosh Ahluwalia (I have included a reference to this book, even though you may not be interested in the 8051, there is a section on implementing an RS-485 multidrop network, including information on a simple network protocol.)

If you would like to add any information on this topic or request a
specific topic to be covered, contact Benjamin Day. Ben Day works for Schweitzer Engineering Laboratories in Pullman, Washington making electric power safer, more reliable, and more economical.

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