Communications Installation; Wiring Guidelines; Port 1: Rs-485 - Siemens SITRANS LR 300 Instruction Manual

Communications Installation

Wiring Guidelines

• RS-485 maximum length is 1,200 meters (4,000 feet).
• Use good quality communication grade (shielded twisted pairs) cable for port 1, as
recommended for RS-485.
• Connect the shielding of instrument cables to ground at one end only.
• Run the communication cable separately from the power and control cables. (Do not
tie-wrap your RS-485 cable to the power cable or have them in the same conduit.)
• Follow proper grounding guidelines for all devices on the bus.
Notes:
• Improper wiring and incorrect cable choices are the two most common causes of
communication problems.
• Increased safety, EEx de version:
• General safety or hazardous EEx d version:

Port 1: RS-485

Note:
COM is not earth
ground: it is floating.
Notes:
• The RS-485 is isolated with respect to earth ground, but its inputs (A,B, COM) are
electrically clamped to earth ground to protect them against electrical transients.
• Before making any connections, measure the common on the cable with respect to
the earth ground that the instrument is connected to, and make sure COM is within
± 3 V dc with respect to earth ground.
• If you are connecting the converter to a portable notebook computer, the COM of the
notebook is normally isolated. To prevent damage to the RS-485 port, one end of this
COM connection must be grounded.
WARNING: Avoid static discharge to terminals.
7ML19985CL02
Port Powered RS-232 to
RS-485 convertor
SITRANS LR 300 – INSTRUCTION MANUAL
use solid 4, flexible stranded 2.5
use rigid 0.2 to 4
flexible stranded 0.2 to 2.5
AWG 24 to 12
Computer
Note: The illustration
shows a desktop
computer system, and
RS-232
assumes the computor
side of the COM
connection is grounded.
Shield, installed
on Intrinsically
Safe mA Output
version only
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