Comments on: BACnet MSTP Installation, RS485 and Cables http://www.chipkin.com/articles/bacnet-mstp-installation-rs485-and-cables Resources and News for industrial automation Mon, 16 Jan 2012 10:29:36 +0000 hourly 1 http://wordpress.org/?v=3.3.1 By: Peter http://www.chipkin.com/articles/bacnet-mstp-installation-rs485-and-cables/comment-page-1#comment-792 Peter Thu, 08 Jan 2009 22:20:50 +0000 http://www.chipkin.com/articles/?p=174#comment-792 The Standard imposes the limit on the number of unit loads presented by the device and does not limit the number of devices. The limit was imposed for practical reasons. When a device drives the line it sees the load presented by the other devices and the terminating (and biasing) resistors. The people who designed the standard made an arbitrary decision with practical implications - How big a load can the line driver see without the signal being attenuated too much. That decision set the limit at 32 unit loads (15kOhm each). Some vendors may make chips which can drive more but they cant push the standard too far or it may present an electrical risk to the other devices. I don't have practical experience of installing more than 32 devices on a trunk. I think you may be able to add a few more devices and still have a fully operational network depending on the load presented by the terminating resistors. As is mentioned in the article, a repeater is the best way around this limitation or look for devices which present less than one unit load (UL) since the standard defines the number of Unit Loads and not the number of devices. The Standard imposes the limit on the number of unit loads presented by the device and does not limit the number of devices. The limit was imposed for practical reasons.

When a device drives the line it sees the load presented by the other devices and the terminating (and biasing) resistors. The people who designed the standard made an arbitrary decision with practical implications – How big a load can the line driver see without the signal being attenuated too much. That decision set the limit at 32 unit loads (15kOhm each).

Some vendors may make chips which can drive more but they cant push the standard too far or it may present an electrical risk to the other devices. I don’t have practical experience of installing more than 32 devices on a trunk. I think you may be able to add a few more devices and still have a fully operational network depending on the load presented by the terminating resistors.

As is mentioned in the article, a repeater is the best way around this limitation or look for devices which present less than one unit load (UL) since the standard defines the number of Unit Loads and not the number of devices.

]]> By: Chris Smith http://www.chipkin.com/articles/bacnet-mstp-installation-rs485-and-cables/comment-page-1#comment-791 Chris Smith Thu, 08 Jan 2009 12:44:10 +0000 http://www.chipkin.com/articles/?p=174#comment-791 This is an excellent read! I believe that most of the cable mistakes that occur today is due to a lack of education with the guys installing them. If someone had to read just this article before touching a RS485 bus they would in for a world of hurt! :) Could you explain the maximum amount of devices allowed on the bus though? I always wondered if 32 was the actual max or if it was due to the standard. Also, could you explain the limitations of the RS485 protocol and the 32 node 'limit' This is an excellent read! I believe that most of the cable mistakes that occur today is due to a lack of education with the guys installing them. If someone had to read just this article before touching a RS485 bus they would in for a world of hurt! :)
Could you explain the maximum amount of devices allowed on the bus though? I always wondered if 32 was the actual max or if it was due to the standard. Also, could you explain the limitations of the RS485 protocol and the 32 node ‘limit’

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