Concept
Generic RS-485 Network Device Configuration 6
This configuration is recommended if the following conditions apply to the RS-485 network and the devices to be connected to the automation server's RS-485 port:
Not all RS-485 network devices have failsafe receivers.
All RS-485 network devices have isolated RS-485 interfaces.
The maximum data rate required is greater than 19,200 bps.
The maximum network cable length required depends on the size of the cable:
150 m (500 ft) or less with 24 AWG (0.20 mm²)
240 m (800 ft) or less with 22 AWG (0.33 mm²)
The third condition indicates that network termination is needed. The first condition indicates the termination will cause a need for strong bias.
A strong external resistor bias (low resistance pull-up and pull-down) is required to create a minimum +200 mV level (plus noise margin) across the data+ and data- conductors for the full length of the bus when the network is idle (in between all packets). The low resistance bias is needed to overcome the voltage drop created by the wire resistance and current pulled by the termination resistors. This is referenced as failsafe bias and is required because the transceiver is of the earlier type with no integrated failsafe receiver function.
With the first and third conditions defined, there are two bias configuration options to choose from. The choice relates to simplicity of the arrangement and the maximum cable length needed (the fourth condition). This configuration uses single end-point biasing. Dual end-point biasing is required to support cable lengths up to 1200 m (4000 ft). For more information, see Generic RS-485 Network Device Configuration 8 .
This configuration uses the bias supply output from the automation server to generate bias on the bus from the single location where the automation server is located (typically at the head end of the bus). AS-P servers and AS-B servers provide a 5 V bias voltage supply, which supports 120 ohm termination resistors at the two ends of the bus.
Connect a 510 ohm bias resistor from the TX/RX+ terminal to the Bias+ terminal on the automation server (see the figure below). Connect another 510 ohm bias resistor from the TX/RX- terminal to the RET terminal on the automation server.
Connect a 120 ohm termination resistor across the + and - data lines at the head end of the bus (typically at the automation server). Connect another termination resistor across the + and - data lines on the last node at the far end of the bus.
Connect the shield drain wire to earth ground terminal rail in the panel with the automation server. This is the only ground connection of the shield for the complete cable segment. Connect the RET terminal on the automation server to the ground rail in the panel using a 12 AWG (3.31 mm²) to 18 AWG (0.82 mm²) wire.
The shield drain wires from the cable segments are connected to the RS-485 reference/common terminal at each RS-485 network device.
The example diagram below shows the alternate RS-485 terminal block connections for the different automation server models.
The example diagram below shows the RS-485 Com B connections on the AS-P and AS-B servers. The guidelines are the same for Com A. When failsafe bias resistors are required on the Com A network, the pull-up voltage is obtained from the Bias+ terminal.
The addition of termination resistors to a network requiring failsafe bias resistors creates a continuous current flow through the network in the idle condition. The result is a voltage divider network where the termination resistors create a voltage drop in the bias resistors and network wire. The bus biasing from the single point presents cable length limitations based on wire size as seen in the following table. This table indicates the wire lengths where the starting (head end) bias voltage created by the 510 ohm resistors has dropped to the minimum bias voltage level due to the wire resistance.
|
Maximum Length |
Wire Size |
|
150 m (500 ft) |
24 AWG (0.20 mm²) |
|
240 m (800 ft) |
22 AWG (0.33 mm²) |
The recommended node count for the configuration shown in the figure above is determined using the following process.
The unit load rating of the individual network devices along with the remaining available unit load budget will determine the recommended maximum number of nodes you should install on this copper segment. The total unit load budget specified in the RS-485 standard for all components connected to a bus segment (communicating devices and bias resistors) is 32UL. If all devices on the bus segment have isolated RS-485 interfaces (excluding the automation server), it is recommended that we can boost the basic budget limit of 32UL up to 48UL. This extra 16UL (50%) capacity allowance is related to fact that the isolated interfaces provide an avoidance of Common Mode Voltage. For more information, see Expanded Unit Load with Network of Isolated Devices Only (MNB and Generic RS-485 Devices) .
We must determine the UL node budget available after subtracting the load imposed by the bias resistors and the automation server. The single end-point bias configuration presents a unit load of 23.5UL (12,000 / 510 = 23.5). The automation server adds 0.5UL.
For a bus with all isolated interfaces, the remaining node budget is: 48 - 23.5 - 0.5 = 24UL
We must divide our node budget by the device load value to determine node count. For the network device load, select the higher unit load value between answers Q5 and Q6b. For more information, see Worksheet for Configuration of RS-485 Bus with Generic RS-485 Devices . Divide the node budget value by the device UL value (Q5 or Q6b, whichever is greater). The result is the maximum recommended node count in regards to bus loading.
Example: If the answer to Q5 was 0.25 and the answer to Q6b was 0.32, use the value 0.32.
For a network with all isolated devices, the maximum node count is: 24 / 0.32 = 75 nodes
The recommended limits on RS-485 bus node counts discussed here pertain to hardware bias and unit load considerations only. The recommended maximum node count may be further limited based on product and system version.
Generic RS-485 Network Devices
Configuration Selection for Generic RS-485 Network Devices
Generic RS-485 Network Device Configuration 8
Expanded Unit Load with Network of Isolated Devices Only (MNB and Generic RS-485 Devices)
Worksheet for Configuration of RS-485 Bus with Generic RS-485 Devices