SymPol™: An alternative to RS-485 for easier and faster equipment installation

Last year, I experienced one of the great “joys” of home ownership: shelling out for a new AC system. While the cost certainly made me blink and swallow a few times, the length of time it took to install it was almost worse. After experiencing that, I can’t even begin to imagine the length of time it takes for a commercial equipment upgrade, replacement or new installation. Many of these applications like HVAC equipment, building automation and security electronics use RS-485 (aka TIA-485 or EIA-485) interface to implement “smart” functions such as power management and diagnostics. It’s pretty standard practice to use RS-485, but there is something brand new in the world of RS-485 that can make installations easier and faster: SymPol™

First, a quick background. RS-485 has been around for a long time. It is configurable into “two-wire” made up of a twisted pair “A” and “B” for half-duplex or “four-wire” for full-duplex systems. It can communicate with up to 32 nodes with a network length of up to 1200 meters. It can be used in point-to-point, multi-drop or multi-point networks. Pretty straight forward. Out in the field during installations though, it’s a different story.

An experienced technician may not have much trouble, but an installer could have a major problem on their hands when everything is hooked up and miscommunication happens. A common mistake is not connecting wire A and wire B correctly. It can take down the entire network until the installer can figure out where the wires were incorrectly connected and correct them. Not something you want happening with your AC installation in the heat of summer. Or causing line down at a factory because of two little wires – that’s money going down the drain by the minute.

Well, if the equipment the installer was using was made with the SN65HVD96 which has the SymPol™ technology, they would have had no issues. SymPol™ which stands for Symmetric Polarity, makes the device, and in turn the equipment, immune to crossed wire bus. SymPol™ uses differential signaling which provides high noise immunity. The internal circuit auto-detects the connection and operates error-free regardless of which way wire A and B are connected.

Check out the test setup below in Figure 1. Normally, an RS-485 transceiver pair set up with cross-wire fault would cause errors. With SymPol™ though, in Figure 2, you see the result – no errors.


Figure 1: Test setup with cross-wire fault 


Figure 2: SymPol™ detects the connection and uses symmetric polarity differential signaling to output correct data

The icing on the cake is that SymPol ™ doesn’t need any software nor hardware changes in existing applications using RS-485. The pin-out is identical to SN75176 standard pin-out, so you can upgrade to the SN65HVD96 very easily. As long as all the devices in the network are SymPol™ transceivers, the system will be fully functional – no matter how you connect the wires.

  • Zatil,

    Nice discussion, and good point about all the devices on the network should be SymPol.  

    On Youtube I saved a couple of videos that illustrate this same idea for a security electronics network:


    This series of four short videos contain a discussion of converting the network for a security electronics application from RS-485 transceivers to SymPol (TM) transceivers. The benefits are that SymPol transceivers are immune to cross-wire fauls on the bus signal lines, and are not damaged by direct shorts to 12Vdc supply wires.

    Part 1a - when wired correctly, the RS-485 signaling controls the PTZ camera.

    Part 1a:

    Part 1b - with signal wires reversed, the RS-485 communication does not work.

    Part 1b:

    The second part of this demonstration shows how, after replacing the RS-485 transceiver chips with drop-in SymPol transceiver chips, the network is not affected by crossed-wire faults on the signal wires.

    Part 2a – when wired “normally” the SymPol signaling controls the PTZ camera.

    Part 2a:

    Part 2b – when wired “cross-wired” the SymPol signaling controls the PTZ camera with no errors.

    Part 2b: