PiBridge – How communication between Revolution Pi modules works

The PiBridge is the central connection element, a backplane connector, between the base module of the RevPi Core and RevPi Connect series and the RevPi expansion modules. It enables efficient and fast communication between the modules and ensures automated detection and configuration during system startup.

With the PiBridge, up to 10 expansion modules can be connected to a RevPi Core or RevPi Connect base module. In addition to an RS485 interface for I/O modules, the PiBridge features two Ethernet channels specifically designed for connecting RevPi gateways. This allows the Revolution Pi to be flexibly expanded and seamlessly integrated into industrial networks.

• Bus coupling of RevPi expansion modules through a 20-pin system connector
• 4 digital signal lines for controlling the initial module and position detection
• 2 signal lines for a fast RS485 data channel
• 8 signal lines for independent bidirectional 100 Mbps Ethernet data channels on both the right and left sides of the RevPi base modules

The Ethernet channels of the PiBridge are currently used exclusively for the RevPi gateways to transfer up to 512 bytes of data per cycle in both input and output directions between RevPi base module and RevPi gateway. Our I/O modules are not connected to this communication channel but simply forward the channel to the next module via the PiBridge connector. Since the base module has independent Ethernet channels on both sides, one gateway can be connected per side. Exceptions include the RevPi Connect 4, RevPi Connect SE, and RevPi Core SE, where connecting RevPi gateways is not possible. For the RevPi Connect S, a gateway can only be connected on the left side.

Future expansion modules that need to use the Ethernet channels for large amounts of data can tap into a channel via an integrated Ethernet switch, thus enabling more than one module with Ethernet connection per side. Since the RevPi gateways do not have integrated switch functions and their terminating resistor on the RS485 channel is always enabled, they must always be placed at the outermost slot of the Revolution Pi system.

Status and configuration data are transmitted on the RS485 channel during the startup phase of the system. Since it is not yet known which modules are mounted at the end of the RS485 line at this time, the bus termination is open and the transmission rate is reduced. 

After completion of the startup phase, the last devices on the bus are known and receive the command to switch on the terminating resistor (exceptions are the RevPi gateways, which always have this bus termination switched on). After that, all bus participants switch the data transmission rate to the maximum value of 115.2 kbit/s and cyclically exchange the input and output data. This data exchange uses a proprietary protocol defined specifically for this purpose, optimized for small amounts of data of a few bytes per telegram.

For the experts: We have improved the ratio of payload to total length by minimizing protocol overhead.

This blog post was originally published in 2016. Technical specifications may have changed since then.