(Modbus Rtu) Electrical Interface; Modbus Protocol - GE MM300 Manual
Motor management system/low voltage motor protection and control
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Also See for MM300:
- Instruction manual (198 pages) ,
- Quick start manual (77 pages) ,
- Installation recommendations (21 pages)
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GE
Grid Solutions
MM300 Motor Management System
Chapter 2: RS485 interface (Modbus
RTU)
Modbus Protocol
Electrical Interface
MM300 MOTOR MANAGEMENT SYSTEM – COMMUNICATIONS GUIDE
RS485 interface (Modbus RTU)
The RS485 interface is a serial two-wire port intended for use as a Modbus RTU slave. The
RS485 port has the following characteristics.
•
Address: 1 to 254
•
Baud rate: 9600 to 115200 bps
•
Supported Modbus function codes: 3, 4, 5, 6, 7, 8, 16
The MM300 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 RS232, RS422, RS485,
fibre optics, etc. Modbus is a single master / multiple slave type of protocol suitable for a
multi-drop configuration as provided by RS485 hardware. The MM300 Modbus
implementation employs two-wire RS485 hardware. Using RS485, up to 32 MM300s can be
daisy-chained together on a single communication channel.
The MM300 is always a Modbus slave. It can not be programmed as a Modbus master.
Computers or PLCs are commonly programmed as masters.
Both monitoring and control are possible using read and write register commands. Other
commands are supported to provide additional functions.
The hardware or electrical interface in the MM300 is two-wire RS485. In a two-wire link,
data is transmitted and received over the same two wires. Although RS485 two wire
communication is bi-directional, the data is never transmitted and received at the same
time. This means that the data flow is half duplex.
RS485 lines should be connected in a daisy chain configuration with terminating networks
installed at each end of the link (i.e. at the master end and at the slave farthest from the
master). The terminating network should consist of a 120 Ohm resistor in series with a 1 nF
ceramic capacitor when used with Belden 9841 RS485 wire. Shielded wire should always
be used to minimize noise. The shield should be connected to all of the MM300s as well as
the master, then grounded at one location only. This keeps the ground potential at the
same level for all of the devices on the serial link.
2–1
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Questions and answers
we have a problem with device MM300. it says that the device is invalid through the software and does not read the device. the communication is through RS 485. is it possible to give us an advice or send us some reccomendations or instructions?
For a GE MM300 device that is not recognized by software and communicates via RS485, follow these troubleshooting steps:
1. Ensure correct RS485 wiring:
- Use two-wire RS485 in a daisy chain configuration.
- Install a 120 Ohm resistor in series with a 1 nF ceramic capacitor at each end of the link.
- Use shielded wire and ground the shield at only one location.
2. Check device addressing and termination:
- Confirm proper device address and ensure no address conflicts.
- Verify that termination networks are correctly installed at both ends.
3. Confirm half-duplex communication:
- Ensure that the system supports half-duplex communication and not sending and receiving data simultaneously.
4. If using USB for setup (e.g., USB-to-RS485 converter) and the device is not recognized:
- Repeat the connection procedure 2–3 times.
- If still not recognized, uninstall the USB driver from Device Manager under modems.
- Disconnect the USB cable, wait at least 10 seconds, then reconnect.
- When prompted by the Hardware Wizard, select "No, not this time," then "Install the software automatically," and press "Continue Anyway."
- Finish the installation and check if the USB device appears in the setup software.
These steps help ensure proper hardware connection and correct software recognition of the MM300 device.
This answer is automatically generated