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Access Control Comms – RS-485 or Weigand?

Q: Which is best for communications between access controllers and door readers – Weigand or RS-485?

A: This is an interesting question. There’s a lot more behind Weigand and RS-485 than you’d think. Some of your choices need to be application specific but it depends on the product you are using, too. 

For a start Weigand readers are unsupervised – this means if they are breached, your customer will not know. If they are smashed or completely removed from the wall and carried away, the system will not report this. The maximum Weigand run is about 150m – this might be fine in some applications but in larger sites it can be limiting. You also need to install Weigand in a star config around a controller, making for complex wiring schedules. 

Along with this, depending on the bit number of the Weigand reader, are only 0-255 site codes and 0-65,535 card codes available. This means the same site codes and card codes are programmed into many cards that are not authorised to have access to a site. Having said this, many manufacturers came up with their own variations of Weigand comms protocol that massively increased security levels and code variations – this area is not as straightforward as it sounds. 

RS-485 has real advantages of the 50-year-old Weigand technology but it’s not a simple swap. Comparatively, RS-485 can support runs of 1200m and it offers linear bus topologies – daisy chains – with only 2 wires at bit rates of 2Mbps at 50m and 100Kbps at 1200m. However, it’s not recommended for star configs because of signal reflections and impedance. You also need to consider the installation of termination and bias resistors in cable runs to ensue signal integrity in your bus. 

But RS-485 only specifies the physical layer – the generator and the receiver. It does not govern the vital comms layer. The comms layer that’s becoming more prevalent in the electronic security industry is SIA’s Open Supervised Device Protocol (OSDP) for connecting readers to controllers. OSDP uses 128-bit encryption, supports multidrop installations and supervises connections to report reader issues. Something else to bear in mind is that using OSDP means card reader, door strike, alarm contact, and Request to Exit functions are supported using 2 wires, not the 10+ that used to be required per door. 

So, which is best? It depends on the application to some extent. For some installers, the known simplicity and cabling flexibility of Weigand teamed up with proprietary (more secure, more codes) readers may be ideal for more compact installations. On large and complex sites with higher levels of situational awareness required and installation designs that expose controller-to-reader cabling, RS-485 and OSDP comms are likely to become increasingly dominant.  ♦

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