Siemens SITRANS LR 300, Инструкция по эксплуатации, Страница: 37 / 140

7ML19985CL02 SITRANS LR 300 – INSTRUCTION MANUAL Page 35
mmm
mm
Int
erc
onnect
ion
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.
Port 1: RS-485
Notes:
• Improper wiring and incorrect cable choices are the two most common causes of
communication problems.
• Increased safety, EEx de version: use solid 4, flexible stranded 2.5
• General safety or hazardous EEx d version: use rigid 0.2 to 4
flexible stranded 0.2 to 2.5
AWG 24 to 12
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.
Computer
Shield, installed
on Intrinsically
Safe mA Output
version only
Port Powered RS-232 to
RS-485 convertor
RS-232
Note:
COM is not earth
ground: it is floating.
Note: The illustration
shows a desktop
computer system, and
assumes the computor
side of the COM
connection is grounded.
SITRANS LR 300.book Page 35 Monday, June 10, 2002 7:18 PM