Modbus Protocol - GE 845 Instruction Manual

DEVICE

Modbus Protocol

4–30
feature triggers the transfer of communications from port 4 to port 5. If LLA is enabled
on a port with a non-fiber SFP, the target message "LLA not supported by Prt (4 or 5)" is
displayed on the keypad and an event is logged.
PRP (Parallel Redundancy Protocol): ports 4 and 5 use the same MAC address and
combine information at the link layer. It is intended to only be used if the two ports are
connected to separate parallel LAN's. In this mode of operation both ports cannot be
connected to the same LAN. The receiving devices (SR relays) process the first frame
received and discard the duplicate through a link redundancy entity (LRE) or similar
service that operates below layer 2. Aside from LRE, PRP uses conventional Ethernet
hardware but both ports must know they are in PRP. Ports of PRP devices operating with
the same Internet Protocol (IP) addresses for traffic that uses IP Management protocols
such as Address Resolution Protocol (ARP) must operate correctly.
Duplicate Discard mode (only mode supported by the 8 Series). This is the normal
setting for PRP operation and once set it allows the sender LRE to append a six-octet
field that contains a sequence number, the Redundancy Control Trailer (RCT) to both
frames it sends. The receiver LRE uses the sequence number of the RCT and the source
MAC address to detect duplicates. It forwards only the first frame of a pair to its upper
layers.
All Ethernet ports and serial communication ports support the Modbus protocol. The only
exception is if the serial port has been configured for DNP or IEC 60870-5-103 operation
(see descriptions below). This allows the EnerVista 8 Series Setup software (which is a
Modbus master application) to communicate to the 845.
The 845 implements a subset of the Modicon Modbus RTU serial communication standard.
The Modbus protocol is hardware-independent. That is, the physical layer can be any of a
variety of standard hardware configurations. This includes USB, RS485, fiber optics, etc.
Modbus is a single master / multiple slave type of protocol suitable for a multi-drop
configuration.
The 845 is always a Modbus slave with a valid slave address range 1 to 254.
DATA FRAME FORMAT AND DATA RATE
One data frame of an asynchronous transmission to or from an 845 typically consists of 1
start bit, 8 data bits, and 1 stop bit. This produces a 10-bit data frame. This is important for
transmission through modems at high bit rates. Modbus protocol can be implemented at
any standard communication speed. The 845 supports operation at 9600, 19200, 38400,
57600, and 115200 bps baud rate. The USB interface supports ModBus TCP/IP.
FUNCTION CODE SUPPORTED
The following functions are supported by the 845 :
• FUNCTION CODE 03H - Read Setpoints
• FUNCTION CODE 04H - Read Actual Values
• FUNCTION CODE 05H - Execute Operation
• FUNCTION CODE 06H - Store Single Setpoint
• FUNCTION CODE 07H - Read Device Status
• FUNCTION CODE 08H - Loopback Test
• FUNCTION CODE 10H - Store Multiple Setpoints
• FUNCTION CODE 42H - Group Settings Read
• FUNCTION CODE 43H - Group Settings Write
When a ModBus master such as the EnerVista 8 Series Setup software communicates to
the 845 over Ethernet, the 845 slave address, TCP port number and the 845 IP address for
the associated port must be configured and are also configured within the Master for this
device. The default ModBus TCP port number is 502.
845 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL
CHAPTER 4: SETPOINTS