What is it?

This shield makes it possible for two or more Raspberry Pi's to communicate over long distances via Serial over RS-485. RS-485 is the industry standard for communication over long distances and is virtually immune to interference. It is commonly used in industrial environments. You could use this shield together with a few Raspberry Pi's and other hardware to make things talk to each other over a distance, without running power to every node or connecting a lot of wires. Imagine an art installation with a sensor at the door which triggers movements and sounds all over the room. We used it to prototype a football-field sized game controller.

The shield has two RJ45-sockets ("ethernet connectors") that allow you to connect up to 100m (300ft) of UTP or FTP cable. Because it has two in parallel, you can easily daisy-chain shields together to form a bus-communication system. The shield allows for full-duplex communication by using the Maxim MAX489 RS-485 chip.

Additionally, the HAT has a wide-input, efficient switching voltage regulator on board, which allows you to power the HAT and Raspberry Pi remotely in a way similar to Power over Ethernet. UTP pin 4 & 5 are used as +, and pin 7 & 8 are -. The voltage regulator accepts anything up to 36V, allowing to power the bus with a high voltage such as 24VDC, reducing voltage drop (energy losses due to resistance in the long cable). As standard PoE injectors can supply up to 50VDC, use caution when using these. If the injector uses the right pinout (probably) and supplies 36V or less, you should be fine. The voltage regulator supplies 5V, 1000mA to the Raspberry Pi, which should be plenty for most applications. Check the power requirements of your application before ordering though. The HAT has an unpopulated socket for a second regulator, so the power can be doubled to 5V 2000mA (10W!)

When two Raspberry Pi's are connected together, both can "speak" at the same time to each other. When more Raspberry Pi's are connected together, it is recommended to make one the "master", which can send commands to all slaves simultaneously and let the rest of the Raspberry Pi's act as "slaves" which only speak when spoken to. The RS-485 protocol and / or shield does not handle this; you'll have to arrange for this in software. The cable between the master and first slave must also be "cross wired" because by default all shields are wired as slaves.

Terminating resistors, RJ45 pins and useful chip pins are broken out so it's easy to use this HAT with other development boards. Supplied as a kit. Surface mount components preassembled.

Refer to the MAX489 datasheet for more information on how to drive the chip and a very useful diagram on how RS-485 buses are wired and operate. Example code will be provided soon.

Why did you make it?

This HAT is a redesign of an Arduino Shield designed and built to scratch our own itch: We were developing a system that needed 80 bus-powered, daisy-chained products and were prototyping the system with Arduino's. Later we reused the shield we made for testing in another installation and realised a lot more people must have applications for a product like this. The HAT and the Arduino shield are compatible, so you can have a system with a Raspberry Pi as a master and lots of Arduino slaves. Or feed sensor data from a remote, bus powered, Arduino into a Raspberry Pi

What makes it special?

If you're designing or building an art installation, interactive installation, game or anything that requires a lot of independent "nodes" communicating together to form a bigger system, this shield is ideal to quickly prototype the system. Especially if the nodes are located further apart from each other. It can also be used in the final application, if your application has space and budget for full-sized Raspberry Pi's with HATs. Because the nodes can be bus-powered, this saves you a lot of wiring and hassle. Please note that this HAT only allows for serial communication, not ethernet other IP communication.

Contact me for discounts on larger orders. After all, this HAT is ideal if you have a lot of them...