The wide supply voltage range also means that you can allow for a significant voltage drop along the bus (e.g., when using a bus cable of considerable length). RS485 cable length can be as long as 1.2 km, while RS232 only about 25 meters. 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. 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. Therefore for RS485 communications no crossover cable is used. Normally, this connection is made at one end of the main cable. In order to avoid signal reflections, a 120 Ohm termination resistance must be fitted on each end of the main cable. 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 Modbus RTU, sending and receiving are never simultaneous, so half duplex is as fast as full duplex. Otherwise a full 6 wire cable must be used. Depends what you mean by a serial cable - Fibre, Ethernet, RS232, RS485, RS422, USB, HDMI, SMPTE292 are all examples of serial communications media. In RS232 mode all pins are used for RS232 signals. These pins have to be connected to handshaking outputs at the PC side on the RS232 port. Pin 1 and 6 however are reassigned to the A and B lines when the port is used in RS485 mode. Both protocols (or communication languages) are built into the instrument’s firmware and are user selectable. Read rates vary with counters, since they are normally controlled by the user entry Gate Time. The main difference is, that pin 1 and 6 on the RJ11 socket of the MJ20-PRG module are used as power supply. This MJ20-PRG module clicks in the PLC and provides one RJ11 socket for serial communications with a PC. The RJ11 connector can be used for both RS232 and RS485. The circuit uses optical isolation (opto-coupler) to isolate your RS232 port from the RS485 cables. Each device has a communication port with two terminals, which are indicated for the sake of convenience as A and B. In these two terminals the communication cable is connected so that all the devices that take part in the communication are connected in parallel.
To connect an RS485 device or RS485 network to any PC or controller, the PC or the controller should provide RS485 interface also. A Modbus RS485 connects a Master device to one or more Slave devices. 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. This distance does not include the branches (which must nevertheless be short). The branches must be no longer than 1200 m! The main cable must be no longer than 700 m! When a single RS485 meter or counter is connected to a PC via a cable CBL06, set the PC to 9600 baud. Then use front panel programming to set the baud rate, protocol, digital address, half or full duplex, and other available parameters. 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.
It has an EM112 Series Energy Meter fitted in the same box to monitor energy production and consumption of the inverter. 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. RS485 communication cable, then connect back of the DVR panel. Setup is the same when multiple meters are interconnected via cables CBL03, except that all meters now need to be set to a different digital address via their front panel. When in full RS232 mode, the six available pins are assigned to the most used RS232 signals. 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. The RJ11 socket in the Unitronics PLCs provides six signal pins. The solution is provided by the Laureate DIN rail mounted breakout board, which provides two RJ11 jacks with six screw clamp terminals all wired in parallel.
The entire Guide To Understanding Rs485 Cable
by Leonardo Lehman (2024-07-16)
The wide supply voltage range also means that you can allow for a significant voltage drop along the bus (e.g., when using a bus cable of considerable length). RS485 cable length can be as long as 1.2 km, while RS232 only about 25 meters. 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. 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. Therefore for RS485 communications no crossover cable is used. Normally, this connection is made at one end of the main cable. In order to avoid signal reflections, a 120 Ohm termination resistance must be fitted on each end of the main cable. 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 Modbus RTU, sending and receiving are never simultaneous, so half duplex is as fast as full duplex. Otherwise a full 6 wire cable must be used. Depends what you mean by a serial cable - Fibre, Ethernet, RS232, RS485, RS422, USB, HDMI, SMPTE292 are all examples of serial communications media. In RS232 mode all pins are used for RS232 signals. These pins have to be connected to handshaking outputs at the PC side on the RS232 port. Pin 1 and 6 however are reassigned to the A and B lines when the port is used in RS485 mode. Both protocols (or communication languages) are built into the instrument’s firmware and are user selectable. Read rates vary with counters, since they are normally controlled by the user entry Gate Time. The main difference is, that pin 1 and 6 on the RJ11 socket of the MJ20-PRG module are used as power supply. This MJ20-PRG module clicks in the PLC and provides one RJ11 socket for serial communications with a PC. The RJ11 connector can be used for both RS232 and RS485. The circuit uses optical isolation (opto-coupler) to isolate your RS232 port from the RS485 cables. Each device has a communication port with two terminals, which are indicated for the sake of convenience as A and B. In these two terminals the communication cable is connected so that all the devices that take part in the communication are connected in parallel.
To connect an RS485 device or RS485 network to any PC or controller, the PC or the controller should provide RS485 interface also. A Modbus RS485 connects a Master device to one or more Slave devices. 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. This distance does not include the branches (which must nevertheless be short). The branches must be no longer than 1200 m! The main cable must be no longer than 700 m! When a single RS485 meter or counter is connected to a PC via a cable CBL06, set the PC to 9600 baud. Then use front panel programming to set the baud rate, protocol, digital address, half or full duplex, and other available parameters. 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.
It has an EM112 Series Energy Meter fitted in the same box to monitor energy production and consumption of the inverter. 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. RS485 communication cable, then connect back of the DVR panel. Setup is the same when multiple meters are interconnected via cables CBL03, except that all meters now need to be set to a different digital address via their front panel. When in full RS232 mode, the six available pins are assigned to the most used RS232 signals. 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. The RJ11 socket in the Unitronics PLCs provides six signal pins. The solution is provided by the Laureate DIN rail mounted breakout board, which provides two RJ11 jacks with six screw clamp terminals all wired in parallel.