Concept
Worksheet for Configuration of RS-485 Bus with Generic RS-485 Devices
To make appropriate recommendations, it is necessary to first collect some information about the intended configuration. The worksheet contains questions, which identify some specific attributes that affect the proposed configuration. Review the installation guide and manuals for the products to be connected to the RS-485 port on the automation server. Use the product manuals along with your project details to answer the questions in the worksheet.
After answering the questions on this worksheet, work through the configuration selection flowcharts based on your answers. The flowcharts use your worksheet answers to provide you with a recommended configuration. For more information, see Configuration Selection for Generic RS-485 Network Devices .
Does the product use an RS-485 transceiver with integrated idle-state failsafe receiver?
Integrated failsafe refers to a receiver that will provide the logic 1 level output when presented with any voltage of 0 V or above from an idle and terminated bus. This does not refer to transceivers with what is called open wire failsafe. Those transceivers represent an earlier class of devices that simply provided a small current pull-up and pull-down in the chip. That integrated bias will not withstand DC termination and the resulting near 0 V level would again produce indeterminate output. You must assume a basic transceiver and answer No to this question (Q1) if the failsafe function cannot be confirmed as a feature of the transceiver being used. This will be the case with many third-party RS-485 devices.
This is a very beneficial feature and worthy of pursuing confirmed answers. This feature avoids the requirement for strong bias and the additional restrictions on node count and distance.
If you have a mixture of devices connected to the automation server and any of these do not have a failsafe receiver, answer No here. You must treat the entire bus as a basic RS-485 and operate the bus with the basic rules and restrictions.
What data transmission speed will be used for the RS-485 bus?
This will normally be one of the following selections: 9600, 19200, 38400, 57600, or 76800 bps.
Data rates of 19200 bps and lower offer better immunity to transmission line effects. These data rates allow flexibility with the bias and termination options to avoid some of the guideline restrictions they inject.
What is the objective on maximum length of the RS-485 bus?
Does the device provide an isolated RS-485 interface?
Most product installation guides will make reference to the feature of isolated RS-485 interface if the product contains such a feature. It may be called galvanic isolation.
What is the published unit load rating for the RS-485 device to be installed?
Many times this unit load rating is not provided and instead the guide will simply identify the maximum number of devices that can be attached to a single copper segment (without repeaters). Divide 32 by the listed maximum node count to determine the implied unit load rating.
Many product guides do not include the unit load impact from local bias resistors supplied within the product. While the local bias resistors are typically considered weak (high resistance) bias sources with little impact, they actually can have a significant contribution when used in conjunction with fractional unit load transceivers (such as 1/4UL and 1/8UL). The following question (Q6) uses a resistance measurement to determine the actual unit load presented by a sample device. This will frequently show a higher unit load value than the published number. It is recommended that you should use the higher UL of the two UL values (published or measured).
With no electrical connections to the product, what is the measured resistance between the RS-485 low-side (-) signal and the communications common?
On a standard non-isolated product, the communications common will typically be the recommended ground connection on the product.
On an isolated RS-485 product, the common will typically be a third terminal on the RS-485 interface, for example, REF, COMMON, SHLD, RETURN.
What is the measured/calculated unit load of the device?
Calculate the measured unit load by dividing 12,000 by the measured resistance in ohm (from Q6a).
Example: On a sample b3865V controller, the resistance measured between COM- (terminal 17) and the Ground connection (terminal 11) is 35,760 ohm (12,000 / 35,760 = 0.336 UL).
Enter that calculated result as answer for Q6b.
For the proposed configurations, it is recommended that you use the larger of the two unit load values (Q5 or Q6b).
RS-485 Communications
Generic RS-485 Network Devices
Configuration Selection for Generic RS-485 Network Devices