Multi-conductor cables used for Wiegand are usually not twisted, and impedance is not controlled in the cable. This means that reflections can occur, and signal integrity might be compromised when long cables are used. Tests were performed to verify how signal integrity is affected when using a 500 foot cable, the longest cable recommended by HID and Mercury for Wiegand readers.
Overview
Signal integrity was measured with an oscilloscope between a Synergis™ Cloud Link and an HID reader, and between a Mercury EP1502 controller and an HID reader. This was tested and compared in two setups, the first without resistor terminations on either side of the connection, and the second with 120 ohm resistors terminations added on either side of the connection.
Test 1: No resistor terminations
The communication was tested at 115,200 baud without resistor terminations on either side of the connection.
Results
The following image shows the communication between a Synergis™ Cloud Link and an HID reader:
The following image shows the communication between a Mercury EP1502 controller and an HID reader:
Test 2: Added 120 ohm resistor terminations
The communication was tested at 115,200 baud with each 120 ohm resistors terminations added on each side of the connection.
Results
The following image shows the communication between a Synergis™ Cloud Link and an HID reader:
The following image shows the communication between a Mercury EP1502 controller and an HID reader:
Conclusion and recommendations
Even though the tests were performed with HID and Mercury devices, the recommendations in this guide are applicable to any scenario where repurposing existing Wiegand cables for RS-485 readers is required.
- The original installation was done using the recommended multi-conductor cable for Wiegand. Low quality cables can lead to unexpected results when switching from Wiegand to RS-485.
- Cables longer than 10 m require end-of-line (EOL) terminations on both ends of the RS-485 daisy chain: controller and reader. This is especially important when working with high-speed links, for example, at 115,200 baud. If communication errors occur, you can lower the link speed, add EOL terminations to cables, or both.