3.4 Bus

This subsection describes the bus types of 2N^® MeterCom PRO including their main use, control and board location.

RS-485

2N^® MeterCom PRO is equipped with an RS-485 bus for connection of additional meters and use of one access point to the Internet. Up to 31 additional devices can be connected to the bus, which can share a master e-power meter equipped with a GSM / UMTS / ETH level. This approach saves communication module mounting costs. RS-485 in 2N^® MeterCom PRO applies two-wire bus connection with reference ground. See the figure below for the screwless connector configuration. 

RS-485 Bus Connection

 

RS-485 features two-wire unit interconnection. The wires are marked A and B, sometimes '-' and '+'. Wire A (-) should show a lower voltage value than wire B (+) at relax. The maximum bus length is 1200 m and the maximum count of nodes (i.e. devices sending and receiving data via a line) is 32, or even higher if repeaters are used. The maximum bit rate is inversely proportional to the cable length. To allow for communication via long distances, the cables must be terminated with terminating resistors on both sides. The purpose of these 'terminators' is to prevent an rf  signal from being reflected back from the end and improve the line resistance against interference. The optimum terminator value is 120 Ω (so-called image impedance) and the resultant line impedance is 60 Ω (120 Ω || 120 Ω). Thus, every 2N^® MeterCom PRO board is equipped with such a terminating resistor, which is provided with a switch and can be activated if necessary. Set the terminating resistor switch to ON for the first and last bus members. See the figure below. 

 

Terminating Resistor Switch

If multiple e-power meters are connected to one bus, make sure that communication is addressed to the appropriate device. If you do not select the opponent at the beginning, all the devices will answer you at the same time. This is undesirable as the received data will be mixed up and nonsensical. Therefore, make sure that communication takes place between just two devices on the bus. Enter the device address (serial number) to select a device. 

Pulse Output S0

There are L+G E350 e-power meter pulse outputs S0 on the 2N^® MeterCom PRO board. These two terminals are just accessible pins of the meter internal connector separated with protective circuits. Refer to Subs. 2.3 for details. The connector is galvanically isolated from the e-power meter. See the figure below for polarity.

Pulse Output S0 Polarity