Troubleshoot RS-485 networks

In spite of the proliferation of more modern alternatives, RS-485 technology remains the mainstay of myriad communication networks. Here's an eight-step recipe for checking common failures and making troublesome RS-485 networks work. 1.RS-485 uses an unbalanced differential pair, which means that every device on the network must be connected to ground through a signal return to minimize noise...

03/01/2005


In spite of the proliferation of more modern alternatives, RS-485 technology remains the mainstay of myriad communication networks. Here's an eight-step recipe for checking common failures and making troublesome RS-485 networks work.

1. RS-485 uses an unbalanced differential pair, which means that every device on the network must be connected to ground through a signal return to minimize noise on the data lines. Data conductors should consist of a twisted pair of wires plus a shield if the environment is noisy.

2. Termination causes more problems than it solves in the vast majority of RS-485 networks. To check which nodes are terminated, power down each one and disconnect it from the network. Use an ohmmeter to measure between the A and B or + and - lines of the receiver. Terminated nodes will usually read less than 200 Ohms. Unterminated nodes will read greater than 4,000 Ohms.

3. It's not always clear which line is the A line and which is the B. Different manufacturers have adopted different labeling conventions, even though the B line should always be the one that is more positive in the idle condition. Thus, the A line is equivalent to - and the B line is equivalent to +. Check the idle network with a voltmeter. If the B line isn't more positive than A, there's a connection problem.

4. A tristate condition in an RS-485 network occurs when no devices are transmitting, and all are listening. This can cause all the drivers to go into a high impedance state, leaving floating wires feeding into all of the RS-485 receivers. A node designer typically cures this precarious state by installing pull-down and pull-up resistors on the A and B lines of the receiver to simulate an idle condition.

Networks & Communication

An oscilloscope trace can determine if the 1s and 0s of each byte have voltages well outside of the undefined state and if the minimum turnaround delay (equal to the time occupied by one byte of data) is being satisfied before the device reaches the tristate condition.

To check the biasing, measure the voltage from B to A with the network powered up and idle. At least 300 mV is required to keep well away from the undefined state shown in the graphic. If no termination resistors are installed, bias requirements are very forgiving.

5. A two-wire-plus-ground RS-485 network sends data up and down the wires. Since no two transmitters can successfully talk at once, there's a slice of time immediately after the last bit has been sent when the network appears to be idle, but in fact the node has not yet put its driver into the tristate condition. A collision with unpredictable results will occur if another device tries to talk during this interval. To check for collisions, use a digital oscilloscope to capture a few bytes of ones and zeros. Identify the time it takes for a node to reach the tristate condition at the end of a transmission, as shown in the figure. Make sure the RS-485 software is not trying to respond to a request faster than one byte time (a little more than 1 millisecond at 76.8 kilobits/second).

6. Every reliable mid-to-long distance networking technology has some form of isolation built in, except for RS-485. It's up to the system designer to ensure that the network does not include any ground loops. Isolating every node will increase the network's reliability by orders of magnitude.

7. Isolation is the first line of defense against power surges, but adding a multistage surge suppressor takes the edge off larger surges, keeping them in the range that the network's isolation can tolerate. It is best to install a surge suppressor at one location on the network with a high-quality ground. Tie it to the earth ground at the same point as the rest of the network equipment or the facility's electrical system.

8. Once the RS-485 network is up and running, write down every detail of its configuration. Include termination information, biasing, wire types, and information on spare parts. Buy some spares now, if they're affordable, and keep them in the cabinet.

For more information about RS-485 technology, see www.bb-elec.com .