Glen Akins Builds Another ProductivityOpen PLC Add-On, an Isolated RS-422/RS-485 Shield

Glen Akins is back with another add-on for Automation Direct's P1AM-100 ProductivityOpen-family programmable logic controller (PLC), this time taking advantage of the design's extensible nature with an isolated RS-422/RS-485 shield.

Billed as an "open source" PLC family, the Microchip SAM D21-powered ProductivityOpen P1AM-100 features Arduino MKR1000-style header sockets on the left-hand side. These headers can be used with compatible shields, Automation Direct's own add-ons, or a prototyping shield — the housing for the latter forming the basis of a multi-function open source board programmable from the Arduino IDE, released by Akins last month.

Now, Akins is back with another add-on: "The RS-422/RS-485 shield is an open source shield designed to add RS-422 and RS-485 communication capabilities to the ProductivityOpen family of open-source programmable logic controllers (PLCs) from Automation Direct," he explains. "It's loosely based on the Arduino MKR RS-485 shield but updated to use an ADM2582E 3.3V isolated RS-485 transceiver from Analog Devices. The completed shield fits inside the P1AM-PROTO prototyping enclosure.

"In my example application," Akins continues, "the state of the eight inputs on a P1-08ND3 input module connected to a first P1AM-100 PLC are relayed over RS-422/RS-485 to the eight outputs on a P1-08TRS output module connected to a second P1AM-100 PLC. Every 20ms, the inputs are sampled and sent as a single byte over the serial communications link to the remote P1AM-100. The remote P1AM-100 receives the byte and updates its relay outputs."

As before, Akins has released the board design files and program source code on the project's GitHub repository under the permissive MIT license; a full write-up of the project can also be found on his website.