This morning, one of my colleague asked about isolated RS-485.
What is RS-485?
It is a network communication standard for two or more equipments with distances up to 4000ft.
The RS485 standard only specifies electrical characteristics of the driver and the receiver, it does not specify or recommend any protocol. Because matters of protocol are left to the user, it is often difficult (if not impossible) to connect RS485 devices from different manufacturers on the same network.
The RS485 standard allows the user to configure inexpensive local networks and multidrop communications links using twisted pair wire.
A typical RS485 network can operate properly in the presence of reasonable ground differential voltages, withstand driver contentious situations, provide reliable communications in electrically noisy environments (good common mode rejection using twisted pair cable, shielding provides additional protection), and support thirty-two or more (many IC manufacturers have 1/2, 1/4, 1/8 unit load devices) drivers and receivers on the line.
Why do we need isolated RS-485?
Once an RS485 network exceeds about 32 nodes on a network, serious consideration should be given to using galvanic isolation.
Even though some IC manufacturers offer light loading devices, that can accommodate 256 or even 400 nodes on one RS485 network, you may NOT want to build such a network for a few reasons.
1) Large networks accumulate distributed electrical noise which can make communications unreliable.
In general it is very important not to run communications wires in the same trough or conduit or in parallel with AC power cables.
Maintain as much distance as possible and cross any power cable at a right angle.
While shielding is not specified for RS485 systems, it can help in many instances.
By "isolating" sections of a large network, the accumulated noise on one isolated leg is not so likely to cause a data error that will propagate to another leg of the network.
Galvanic isolation will break a large problem into several small, but manageable ones. Galvanic isolation can also help eliminate "ground loops."
2) Severe damage can occur to your entire system, if a high voltage source is connected (accidentally or otherwise) to your communications lines. Your entire network could be damaged.
With galvanic isolation the damage is generally limited to only one leg of the network, except in extreme cases of very high voltage (induced by lightening for example).
While it goes against conventional wisdom, and can potentially cause a problem with circulating currents by grounding a shielded cable at both ends, this method is very effective at keeping induced lightening noise away from the communications lines.
In the alternative, ground one end of the shield and connect the other end to ground through a bi-directional transient protector (from a few volts to a few hundred volts depending on the situation).
What is RS-485?
It is a network communication standard for two or more equipments with distances up to 4000ft.
The RS485 standard only specifies electrical characteristics of the driver and the receiver, it does not specify or recommend any protocol. Because matters of protocol are left to the user, it is often difficult (if not impossible) to connect RS485 devices from different manufacturers on the same network.
The RS485 standard allows the user to configure inexpensive local networks and multidrop communications links using twisted pair wire.
A typical RS485 network can operate properly in the presence of reasonable ground differential voltages, withstand driver contentious situations, provide reliable communications in electrically noisy environments (good common mode rejection using twisted pair cable, shielding provides additional protection), and support thirty-two or more (many IC manufacturers have 1/2, 1/4, 1/8 unit load devices) drivers and receivers on the line.
Why do we need isolated RS-485?
Once an RS485 network exceeds about 32 nodes on a network, serious consideration should be given to using galvanic isolation.
Even though some IC manufacturers offer light loading devices, that can accommodate 256 or even 400 nodes on one RS485 network, you may NOT want to build such a network for a few reasons.
1) Large networks accumulate distributed electrical noise which can make communications unreliable.
In general it is very important not to run communications wires in the same trough or conduit or in parallel with AC power cables.
Maintain as much distance as possible and cross any power cable at a right angle.
While shielding is not specified for RS485 systems, it can help in many instances.
By "isolating" sections of a large network, the accumulated noise on one isolated leg is not so likely to cause a data error that will propagate to another leg of the network.
Galvanic isolation will break a large problem into several small, but manageable ones. Galvanic isolation can also help eliminate "ground loops."
2) Severe damage can occur to your entire system, if a high voltage source is connected (accidentally or otherwise) to your communications lines. Your entire network could be damaged.
With galvanic isolation the damage is generally limited to only one leg of the network, except in extreme cases of very high voltage (induced by lightening for example).
While it goes against conventional wisdom, and can potentially cause a problem with circulating currents by grounding a shielded cable at both ends, this method is very effective at keeping induced lightening noise away from the communications lines.
In the alternative, ground one end of the shield and connect the other end to ground through a bi-directional transient protector (from a few volts to a few hundred volts depending on the situation).
0 comments:
Post a Comment