EM 6400 User Manual v03.02 – d12
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8.COMMUNICATION INSTALLATION
8.1.RS-485
Each EM 6400 series instruments have a unique address up to three digits long. The range of the
instrument address is 1 to 247. This allows the user to communicate with up to 247 instruments
on one COM port of the PC. The EM 6400 series works on the Modbus RTU protocol. Under
Modbus, each port supports up to 247 instruments. However, each RS485 segment allows only 32
nodes (instruments). Therefore, Data Repeaters are necessary to amplify the signal between
segments.
Available standard baud rates are 1200, 2400, 4800, 9600 and 19200. To select the proper baud
rate, apply the following rules:
• For best results under normal conditions, leave the meter factory-set at 9600 baud.
• For a smaller number of instruments over a long distance, use a low baud rate 4800 / 2400 /
1200.
• In noisy EMI, RFI conditions, try 4800 or 2400 baud.
• With short cable runs (< 300 meters or 975 feet), 19200 baud speeds up data transfers.
The connection diagrams show one Termination Resistor (RT) across the cable-pair at each end
of the cable. This is a 1W resistor, 5% or better. Its value should approximately equal the
Impedance of the Data Cable, as specified by the cable manufacturer. If you’re not sure, a value
of 120Ω might just work fine.
The termination resistors help absorb the data signal reflections off the ends of the cable, reducing
data packet defects and the corresponding data packet re-transmissions. In cable runs longer than
say 200 meters (650 feet), and with baud rates higher than 9600 baud, these benefits become
more apparent.
Figure 7.1: 2 Wire Half Duplex Communication Connection
EM 6400 Instruments (rear view)
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