RS485

Normally the controllers in the swarm communicate via wifi. With the ftSwarmRS there is the additional possibility to communicate wired using RS485. This has the advantage at conventions & fairs not to come into clone conflict with existing wifis. The feature was developed for the ftSwarm project at the BUGA23 in Mannheim.

In the configuration menu it is possible to switch between RS485 and wifi transmission.

RS485 Basics

RS485 is a serial interface which is widely used in measurement and control technologies. Its advantage is high transmission rates with relatively simple cables up to 100m length. The RS485 specification describes only the hardware - additionally there are different protocols using RS485 hardware. The ftSwarm uses the same internal protocol as the WLAN variant.

Classical interfaces - like I²C or SPI - define the bits via the applied voltage. However, this is very susceptible to interference that e.g. with the I²C bus only low transmission speeds or with SPI only short cable paths are possible. The RS485 interface therefore codes the bits via the current’s flow direction. This is less susceptible to interference; if twisted-pair cables are used, even induced interference from motors is cancelled out.

Since only the current direction is relevant, only two cables (RS284A and RS485B) are required for the RS485 interface. “GND” or “-“ do not exist. For this to work, only one controller can transmit at a time, all other controllers only receive data and switch off their transmition module as long as they do so.

Cabling

The RS485 interface must always be wired as a bus. This means that all controllers are connected in a row:

Always two adjacent controllers are connected 1:1 (shown in the picture via red and green cables). To simplify wiring, RS485 connections are doubled on each ftSwarmRS, so that no terminal has to be assigned twice. Attention: The green shown cable is not a fischertechnik “green” connection and must not be connected to other “green” connections.

Normal fischertechnik stranded wire can be used for short distances. If the total cable length is >10m, twisted cables must be used.

The RS485 jumper (shown in blue) must be set on the first and last controller in the series. This activates the terminating resistor, which additionally improves transmition quality.