The presence of a new board and the type of board are automatically sensed by the meter’s firmware or by Instrument Setup (IS) software. First set jumpers for your type of communications. This MJ20-PRG module clicks in the PLC and provides one RJ11 socket for serial communications with a PC. The Jazz PLC series from Unitronics are not standard equipped with a RJ11 connector for serial communications. The actual functionality is determined by DIP-switches on the mainboard of the PLC. The central four pins remain their original RS232 functionality. The RJ11 connector can be used for both RS232 and RS485. Our plans to upgrade our solar PV system this year are slowly progressing with installing a new Solis 1.5kW inverter and an EM112 Series Energy Meter, which have RS485 output and communicate using Modbus protocol. The RS485 system used for Modbus communication provides a main cable (Bus or backbone), to which all the devices have to be connected with branches (also known as stubs) that are as short as possible. Longer branches could cause signal reflections and generate disturbances and consequent errors in the reception of data.

Two wire telephone cables will not function because in that case the ground signal levels on both sides will be floating. The RJ11 socket in the Unitronics PLCs provides six signal pins. Therefore the pin assignment is a little bit different from the default pin assignment on the RJ11 socket. It shouldn’t however be a surprise that the pin assignment on the RJ11 socket on Unitronics PLCs is different from the two systems mentioned above. The diagram below shows a basic RS485 cable for use with the RJ11 socket on Unitronics PLCs. After several frustrating days of trying different combinations of RS485 hats, different wiring connections, with and without termination resistors and ordering a USB RS485 adapter which would also fail to connect, we tried to install the Data logging WiFi adapter. If the total length of the main cable is less than 50 m termination resistances can be avoided at the ends of the main cable. The maximum number of devices that can be connected to a main cable is 32, including the Master. The main difference is, that pin 1 and 6 on the RJ11 socket of the MJ20-PRG module are used as power supply. The difference between RS422 and RS485 is the fact that RS485 has the possibility to built a multi-point application.

Therefore for RS485 communications no crossover cable is used. The symmetrical pin layout of the RJ11 sockets on the Unitronics PLCs allows an easy way of interconnecting the PLCs with both RS232 and RS485 communications. RS485 is in its design a different interface from RS232. RS485 uses two wires and sometimes a ground connection, so we used a twin-screen cable to go between the inverter to the EM112 Series Energy Meter and then onto the Raspberry Pi with a DB9 connector connected to the RS485 Pi board. See page 7 of the Laurel digital panel meter manual, page 7 of the Laurel counter manual, page 8 of the Laurel scale meter manual, or page 8 of the Custom ASCII protocol manual. To set up communications for your meter or counter, see the Serial Communications section of its user manual. Again you can check with a meter. We can also use this to upload to our long-term storage data logger online. RS485 is simpler because it uses only two wire (or a single pair) to carry out the data in bidirectional manner, using RS232, we need 3 wires at minimum to carry the data bidirectionally. RS485 wiring bus should in theory be terminated with a 120R resistor at both ends.

We found that there are 10K ohm resistors fitted on the A & B data lines with the A channel having a 10K pull-up resistor to 3.3 volts and the B channel having a 10K pull-down resistor to ground. With this boards, a CBL03 lines with RJ11 connectors can branch out to support an RS485 line consisting of three discrete wires. We then added the 10K pull-up and pull-down resistors to the RS485 Pi board and connected it directly to the Solis inverter. To minimise shadows over the top row of panels, we decided to mount 2 on the top row and 3 below, which will give us a theoretical maximum output of 1.6kW, which is suitable for the Solis Solar 1.5kW Mini 5G Inverter. The data COM port on the base of the Solis inverter uses a small round 4-pin plug with a screw thread to hold it in place when connected.