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from the Automation List department...
BACnet MSTP distance vs. Wire Gauge
Communications systems and equipment. topic
Posted by Mustafa Mandviwala on 17 May, 2007 - 1:12 am
Hi,

BACnet specs says MSTP run can go up to 1200m @ 18 AWG. If we increase the gauge to 22 or 24 AWG, how much distance can it go assuming fully loaded MSTP Lan?

Appreciate your feedback.

Regards,

Mustafa


Posted by Zvi on 23 June, 2007 - 1:03 am
Mustafa,

18 AWG is a larger cross-section than 22 AWG. I don't think that using heavier wire will enable longer distances. If cable is isolated from inteference, all connections are solid and shield is continuous and properly grounded at a single point, you can stretch the length limits. Otherwire, use repeaters to booste the signal.

Regards,

Zvi


Posted by Peter Chipkin on 20 March, 2008 - 1:02 am
The cable gauge only changes the cables impedance. You should already be selecting a cable whose nominal impedence is relativly independent of length such as Belden 3106a.

Practically speaking you can go up to 4000 feet at baud rates up to 76800 baud. Above that you need to do a little math and reduce the length. For example, at 115k baud your cable should not be much longer than 2500 feet.

However, the higher the baud rate the more sensitive the cable is to the quality of installation issues like how much twisted pair is unwound at each termination start to become very very important.

Our advise: For longer networks with lots of devices, choose 38k400 baud over 76k800 baud and optimize using COV, separate networks and by setting the Max Master to a lower number.

Read more at: http://www.chipkin.com/articles/cable-lengths-and-baud-rates-fro-rs485 -or-bacnet-mstp

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