Although many consider the RS485 relay output module as an archaic protocol, it is still important to the industry. The RS485 protocol allows up to 32 devices to communicate through the same data line over a cable length of up to 4000 feet with a maximum data rate of 10 Mbps. Not many other protocols can equal those numbers.
The single board computer, the Raspberry Pi (RBPi) is increasingly finding its way into more and more industrial applications. However, the limiting factor for most compatible relay modules is the number of contacts available, which are either too few, or limited by the GPIO pins used.
The RS485 relay interface overcomes this limiting factor. Modules such as the Pi-SPi-RS485 and VP-EC-8K0 support the Modbus protocol. That offers the industrial user up to 253 modules at eight relays per module, theoretically making it possible to use 2,024 relays from one interface. Practically, there are two limitations.
According to the hardware protocol, the RS485 relay can support up to 32 unit loads, before a repeater/amplifier becomes necessary for the next batch of loads. Popular modules use the Texas Instruments RS485 drivers such as the SN65HVD72DR half-duplex IC, which according to the TI data sheet, allow only up to 200 unit loads.
In addition, the hardware protocol of the RS485 relay output module specifies the maximum distance between the extreme ends of the RS485 transmission line cannot exceed 4000 feet. For greater distances, a repeater/amplifier becomes necessary.
Therefore, for any industrial application requiring serious outputs such as few hundreds of easily configurable relays, each with 10 A SPDT contacts with MOV protection, where the distances are within 4000 feet between all modules, the RS485 modules for the RBPi are a perfect fit. Some modules are field ready as they have an optional DIN rail enclosure.
RS485
RS485 is an industrial standard for transmitting serial data via a hard-wired cable—EIA/TIA-485 defines the system. RS485 offers the ability of multi-drop cabling with data speeds of up to 10 Mbps over 50 feet, and slower communication speeds of 100 kbps for up to 4000 feet. Industrial applications such as data acquisition widely use the RS485 protocol.
Simple networks often use RS485 links, connected in 2- or 4-wire mode. A typical application may have several addressable devices linked to a single controller, PC, or SBC such as the RBPi. This typically uses a single line for communication.
Using simple interface converters, linking systems using the RS485 and RS232 protocols is possible. This may include optical isolation between the two circuits. It is also possible to incorporate surge suppression for any electrical spikes that the communication line may pick up.
RS485 makes it easy to construct a multi-point data network for communication. According to the protocol, you can have 32 nodes capable of both transmitting and receiving. Furthermore, you can easily extend this capability further by using automatic repeaters and using high-impedance drivers/receivers. That means hundreds of nodes can exist on a network, extending the common mode range for both drivers and receivers with tri-state and power off modes for power saving.
