4 Tips to Successfully Run an MD485 Network with Extreme Cable Lengths

您是否拥有使用的设备网络，连接使用MD485界面, so spread out that extremely long cables are required? Are you worried that the distances, and resultant cable lengths, may be causing communication problems? In this brief article, we’ll look at four tips that can help ensure your MD485 network operates correctly.

MD485网络使用RS-485通信硬件，总最大长度引用为“最多”1,200米（4,000英尺），网络上有最多256个设备。正如您可能怀疑的那样，“最多”的单词确实表明您可能无法在实践中达到所引用的限制 - 或者至少没有仔细设计。

So whilst setting up a small network with short cables is a simple task, as the network size grows, you can run into problems with data corruption that result in lots of retries and delays. Sometimes extending the network can “push it over the edge,” and it will stop working altogether.

To help you set up a reliable network, here are some tips to follow.

#1 – Use a high-quality cable and a simple linear layout

Ensure that you are using a twisted pair, high-quality cable. Ideally, the cable should be shielded, with the shield connected to ground at one end. Campbell Scientific offers a suitable cable (CABLE2TP), but you can also use existing structured wiring such as CAT5/6 cable. Whichever type of cable you use, make sure that the A/B wires used for the data connection are from the same pair of conductors in the cable. Also, it is best to design a network with a linear daisy chain of devices rather than a star arrangement.

#2 – Check the resistive/reference ground connections

Make sure that all of the resistive/reference ground terminals between your MD485 interfaces are connected together using an extra conductor in the cable. Prior to doing this, you can check that the voltage difference between the grounds of the different MD485 interfaces are not too large. If the voltage differences are too large, this can prevent communication and could even be hazardous.

To check the voltage difference, disconnect the ground-referencing wire temporarily and then use a voltmeter to measure the voltage between the end of the wire (connected to the rest of the network) and the MD485 reference ground. If the potential is more than a few volts, review the grounding of the different systems. If the potentials cannot be brought closer, consider adding a third-party RS-485 isolating repeater.

#3 – Reduce the baud rate

如果您必须使用较低级电缆和电缆运行很长，您可以尝试将RS-485电缆本身的波特率降低到38.4 kb / s或更慢。如果这在从另一端传输数据时，如果远程站的活动LED在远程站处不亮起，则可能必须提高RS-485信号。您可以通过将网络分为一半并添加某种类型的中继器来完成此操作。中继器可能是通过其RS-232端口背对背连接的两个MD485接口，或者您可以使用两个MD485接口（带有不同的SDC地址），该接口连接到一个数据记录器设置为充当路由器。另一种选择是使用第三方RS-485中继器。

#4 – Install termination resistors

If the activity LEDs flash as expected but your data is getting corrupted, try installing a 120 ohm termination resistor across the A/B lines at each of the two extreme ends of the network. This modification reduces the reflections of signals in the cable and also helps eliminate some types of noise. (These resistors are rarely required for most MD485 networks, and fitting them can increase power use slightly, but their use is considered to be good practice by RS-485 network designers.)

Conclusion

使用极端电缆长度使MD485网络运行良好可能是棘手的。我希望这些提示有助于使工作更容易。

Tip:You can apply the principles outlined in this article’s four tips when you make RS-485 connections to aCR6 datalogger，通过其控制端口，或者当您使用一个连接传感器到数据记录器时SDM-SIO1模块.

If you have any questions or comments, please feel free to post them below.