Physical layer

Figure 1 - Various examples of physical connection to electricity meters

The tested electricity meters had the following types of communication interfaces available:

• Optical - an optical head is attached, which holds with a magnet. On a computer, the interface appears as a serial port. In the picture, the meter C is thus connected, and the optical head is connected to the computer via USB and represented as COM13.
• RS485 serial interface - output on the terminal board of the meter and/or on the plug-in communication module - depending on the type of device. There are several ways to connect to the meter's serial interface.
  • USB/RS485 converter (e.g. Papouch SB485) (meters D, E) - the device driver created a COM14 serial port on the computer.
  • MOXA (pictured is MOXA onCell with GPRS module) (meters F, G) - MOXA serves as a translator between UDP/IP and serial communication.
  • If one meter has a plug-in module with both IP and serial interface (meter B), it can pass communication between them and mediate communication with other meters connected via RS485 (meters H, I).
• IP interface – in the form of an Ethernet port or a GPRS modem. In this case, the "physical" transport layer means TCP/IP connection or UDP/IP communication.

Parameters for serial communication

The serial communication configuration consists of the following parameters.

• Designation of the serial port (e.g. COM13).
• Transfer speed. It is necessary to know the setting of the meter, but the devices tested had by default:
  • all on the optical interface: 300 baud,
  • EMH on RS485: 4800 baud,
  • Landis+Gyr, Addax and Iskra MT-880 on RS485: 9600 baud.
• Number of Start/Stop bits: 1.
• Number of data bits and how parity is calculated
  • for HDLC + DLMS/COSEM: 8 bits without parity
  • for IEC 65056-21: 7 bits and even parity.

Parameters for IP communication

Depending on the circumstances, the meter may be assigned a static or dynamic IP address. The dynamic IP address mode is supported by ADDAX NP73 and is used if we want to save on the price of SIM cards. If it is necessary to connect a small number of meters via GPRS - small enough that it is not interesting to create a special APN (gateway between the GPRS network and another computer network with a private IP space), but large enough that we want to save on SIM cards with assigned fixed public IP addresses. In all other cases, a static IP address of the meter is preferable.

• Static IP address of the meter - the TCP/IP protocol is used, therefore it is necessary to configure the IP address and TCP port on which the meter listens for incoming connections.
• Dynamic meter IP address - UDP/IP protocol is used and SGCom is assumed to have a public static IP address. We configure the IP address and UDP port through which SGCom will communicate in the meter. SGCom must know the unique identifiers of all meters that will communicate through this port.

Communication through a serial server

A serial server (e.g. MOXA OnCell or MOXA NPort) serves as a translator between IP communication and communication over a serial line. It is possible to create a virtual COM port in the system using drivers from the manufacturer of the serial server and communicate through it. In practice, however, it turned out that this configuration is not very stable, it tends to freeze. For that reason, communication via the so-called "udpMOXA" protocol is used. The following parameters are used for configuration:

• IP address of the serial server of the device,
• The UDP port of the serial server, which is bound to the physical serial port,
• The UDP port on the local computer to which the serial server sends UDP datagrams.