Laurel Electronics Modbus Protocol For Series 2 Laureate Digital Panel Meters, Counters, Timers & DIN-Rail Transmitters - Manual (Page 3)

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2. INTRODUCTION, MODBUS PROTOCOL
The Modbus Protocol is an industry-standard communications protocol that is selectable with all
our serial communications signal options: Ethernet, USB, RS485 and RS232. It is implemented by
the microcomputer on the main board and is compliant with Modbus RTU or ASCII transmission
modes (software selectable), as specified in Modbus over Serial Line Specification V1.0 (2002).
Digital panel meters, counters and timers require a plug-in option board for Modbus communi-
cations. This board can be any of the following:
• RS232 board
• RS485 board with dual RJ11 jacks.
• RS485 Modbus board with dual RJ45 jacks
• USB board
• USB-to-RS485 converter board
• Ethernet board
• Ethernet-to-RS485 converter board
Our RS485 and Modbus RS485 boards are both Modbus compliant, but the RS485 board uses
RJ11 jacks while the Modbus board uses RJ45 jacks as recommended in the Modbus Specifi-
cation. With either board, the two jacks are wired in parallel to allow daisy chaining of meters with
no need for a hub.
Our USB-to-RS485 and Ethernet-to-RS485 converter boards allow the host meter to function as a
normal meter, be connected to a host computer or Ethernet local area network (LAN), and also act
as the device server for an RS485 network with up to 31 other meters equipped with an RS485
board. These meters can then be daisy-chained using readily available, straight-through 6-wire
data cables (not 4-wire telephone cables or crossover cables). Use repeaters to increase the
number of addressable meters.
Our DIN-rail transmitters come with a user-selectable Ethernet or RS232/RS485 I/O port in addi-
tion to a scalable 4-20 mA output, which is standard.
Our DIN-rail Ethernet-to-RS485 device server provides an RJ45 jack for connection to the
Ethernet, an RJ11 jack to support an RS485 network of meters, plus screw terminals to support
an RS485 network of DIN-rail transmitters via a set of 3 or 5 parallel wires (half- or full-duplex).
The Modbus TCP protocol is seamlessly converted by our Ethernet Nodes to Modbus RTU or
Modbus ASCII for communication with meters and transmitters on an RS485 bus. Please see our
Ethernet Manual for more information.
The Custom ASCII Protocol is a software-selectable alternative to the Modbus Protocol. It also
allows device addressing of up to 31 devices. It is less complex than the Modbus protocol, but is
limited to use with our devices. Please see our Custom ASCII Protocol Communications Manual.