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The RJ11 socket is interesting, because the manufacturer choose to have a symmetrical pin-layout, just as with the MMJ connector on DECconnect systems and the RJ45 jack in the Yost standard. Should I use RJ11 or RJ45 connectors? The board with dual RJ11 jacks is the more popular since is can use commercial straight-through, non-reversing, 6-wire data cables with RJ11 connectors. RJ11 jacks support half-duplex or full-duplex communications. Both boards support the Modbus protocol and Laurel’s Custom ASCII protocol, as well as half-duplex or full-duplex RS485 communications. Below you will see a cabling example for one master and two slaves in a full-duplex and a half-duplex situation. Two wire telephone cables will not function because in that case the ground signal levels on both sides will be floating. Transaction history from POS will be recorded without any protocols. A symmetrical pin-layout allows two DTEs, data terminal equipment, to be connected directly without use of a modem or other DCE, data communication equipment. Install a normal SIM card for the internal MODEM. The normal way to implement this is with a two wire system with differential signal levels. As a rule of thumb, the speed in bit/s multiplied by the length in metres should not exceed 108. Thus a 50-meter cable should not signal faster than 2 Mbit/s.

The RJ11 socket in the Unitronics PLCs provides six signal pins. This MJ20-PRG module clicks in the PLC and provides one RJ11 socket for serial communications with a PC. This cable provides a USB type A connector for the PC end and an RJ11 connector for the meter end. The RS485 data transmission standard requires that each network node be linked via a single transmission line terminated at each end in its characteristic impedance. The Jazz PLC series from Unitronics are not standard equipped with a RJ11 connector for serial communications. PCs are normally not equipped with an RJ11 socket for serial RS232 communications. 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. Normally pin 7 (RTS) and pin 4 (DTR) are used on the DB9 connector. For RS232 a crossover cable is used where pin 1 is connected with pin 6 at the other connector, pin 2 with pin 5, etc. This interconnects the transmit and receive signals of both PLCs, and also the handshaking signals DTR and DSR. Pin 1 and 6 however are reassigned to the A and B lines when the port is used in RS485 mode.

When in RS485 mode, only in 1 and 6 are reassigned. When in full RS232 mode, the six available pins are assigned to the most used RS232 signals. In RS232 mode all pins are used for RS232 signals. The older RS232 standard uses single-ended signals, is more sensitive to noise, and is only rated for distances up to 50 ft (15 meters). RS485 is the preferred serial communications standard for long distance industrial communications. In fact, there is no such thing as an RS485 cable since RS485 is a protocol for serial communication (like between inverter and Zero export device) and Ethernet is a protocol for carrying internet data (between router and inverter) whereas CAT6 is a cable standard. Modbus ASCII makes use of ASCII characters for protocol communication. 3106A is what I typically use to wire balanced audio. If handshaking is not necessary, an ordinary 4 wire telephone cable could be used, provided that the pins are cross-connected as shown in the picture below. Otherwise a full 6 wire cable must be used. Care must be taken when using A/B naming. The only way to add RS232 capabilities to the Jazz PLCs is by using the MJ20-PRG programming port add-on module.

If we want to connect a PC with an Unitronics PLC, for example for programming or debugging reasons, we need a connection cable which is on one side fitted with a female DB9 connector, and on the other side with an RJ11 jack. These pins have to be connected to handshaking outputs at the PC side on the RS232 port. The central four pins remain their original RS232 functionality. Otherwise RS232 would be my next best option. Rs232 is only an option for pc to controller. The RS232-RS485 adapter circuit diagram is show below, RS232 DB9 is used as the RS232 port, while only a terminal block is used as RS485 connector. We connected the A and B wires to the pins on the adapter's circuit board. Laureate RS485 boards are available in versions with dual RJ11 jacks (ordering option 2, board P/N L485) and with dual RJ485 jacks (ordering option 4, board P/N LMOD).