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04.13.2018

Use of RS485 Hub Minimizes Downtime

RS485 networks are designed to provide multi-drop serial network connection of 32 nodes at distances up to 4000 feet. Properly implemented, RS485 networks should be trouble-free, however in over 30 years in the industry we have seen common problems in RS485 networking implementations that include:

  • RS485 networks wired as star networks.

The RS-485 network specification requires nodes to be wired in a “daisy-chain” topology where each node is connected to the previous node along a single transmission line with impedance matched termination resistors at both ends of the line. Often in industrial implementations, it makes more sense to have each node wire back to a central control panel in a “Star” type topology. Star wiring can wreak havoc on RS-485 connections as each leg of the network cause reflections on the line and put termination resistors on each leg would overload the bus. The 485HUB eliminates this problem by splitting the RS-485 network into multiple individually isolated sub-networks.

  • Ground loops and common mode voltage offsets.

In installations that cover large distances or have nodes that are powered from independent power supplies, it is common for the ground potentials at each node to vary considerably. This results in common mode voltage offsets in the signal that can easily push the transmitted signal out of the receivers input range. Each port on the 485HUB is individually electrically isolated allowing the receivers to float up to the transmitter’s ground level eliminating the common mode offset as well as any noise that may be induced by the ground loop created between the nodes. This isolation also prevents large ground currents from being conducted on the signal ground connections which can damage equipment and wiring.

  • Signal reflections and noise due to improper transmission line termination.

Both far ends of an RS-485 network should be terminated with 120ohm resistors, however since only the end devices should have the termination installed most devices do not include the resistors and assume that the technician installing the device will have some resistors with him. Each Port on the 485HUB has internal termination resistors enabled by adding a wire jumper on the port’s terminal block. Internal Bias resistors can be enabled by the configuration DIP switches under the unit for use with older RS-485 drivers that require biased lines to prevent noise during a bus idle state.

  • Failure or disconnection of one node or cable segment takes down the whole network.

In a “daisy chain” network a failure of a single node can bring down the whole network, likewise a short or break in the line whether accidental or during maintenance will cause the whole network to fail. With each leg of the network isolated on its own line, these failures on a sub-network will not impede the rest of the network. In addition, the 485HUB’s integrated USB connection allows a technician to tap into the RS-485 network for maintenance of the downstream devices without having to change or disconnecting any existing wiring.

  • Bit timing problems introduced by low cost RS-232 to RS-485 converters

Converting RS-232 to RS-485 requires the synthesis of a direction signal to switch the RS-485 driver from receive to transmit. This is often done in low cost RS-485 converters by using a simple RC timer circuit. While this may get the job done for slow transactions at low baud rates it’s timing is too loose to operate effectively at speeds greater than 9600 baud. Some converters do not even have a configurable baud rate setting meaning the same time constants are used at all baud rates resulting in poor performance at any speed. The 485HUB uses internal UART’s and a microprocessor to control the timing for all ports and is capable of high-speed transactions and the fastest possible bus turnaround.

Troubleshooting problematic RS-485 installations can be quite difficult as many of the above issues can only be diagnosed using advanced tools such as oscilloscopes and protocol analyzers. These are not standard equipment for the electricians and field technicians who typically will be implementing RS-485 networks, meaning the time spent troubleshooting via trial and error can become excessive and often a specialist may have to be brought in to repair problematic installations. Costly field time, as well as production and project schedule delays, can easily be avoided by designing networks with the SCADALink 485HUB.