What is RS485, EIA-485
EIA-485 (formerly RS-485 or RS485) is a specification for the physical layer of a network that uses the difference in voltages between two wires (Three wire) to conveys data.
One polarity of voltage indicates a logic 1, the reverse polarity indicates logic 0. The difference of potential must be at least 0.2 volts for valid operation, but any applied voltages between +12 V and -7 volts will allow correct operation of the receiver.
EIA-485 does not specify or recommend any data protocol is only a specification for the transmission of the ones and zeros. It is up to the receiver to convert the ones and zeros to a specific data protocol such as BACnet MSTP, Metasys N2 by JCI, ect
Since EIA-485 uses a voltage differential it can be used across large distances. (4000 feet or 1200 meters) and it resists electromagnetic interference from motors and welding equipment. When using a network across large distances the transmitter will use the max potential differences (+12 V and -7 volts). The resistance of the twisted pair cable will drop the voltage difference as it travels down the wire. When the receiver reads the voltage on the other end there could be as little as 0.2 volts difference between the two wires. 0.2 is the minimum recommended voltage difference that receivers use to detect a change in polarity.
The recommended arrangement of the wires is as a connected series of point-to-point () nodes, a line or bus, Ideally, the two ends of the cable will have a termination resistor connected across the two wires. Without termination resistors, reflections of fast driver edges can cause multiple data edges that can cause data corruption. Termination resistors also reduce electrical noise sensitivity due to the lower impedance, and bias resistors are required. The value of each termination resistor should be equal to the cable impedance (typically, 120 ohms for twisted pairs).
How do you put more than 32 devices on a single RS485 trunk?
The simple answer is use a repeater but in practice one isn’t always necessary.
The RS485 standard is based on 32 devices. Since the standard was developed most RS485 chips present less than the full unit load originally specified. Today you get half and quarter load devices. Thus to see how many devices you can install you simply get the data sheets and add the loads. Look for “UL” on the data sheet. It stands for Unit Load.
Four (4) Jumpers are required to connect a FS2510 to a 2-wire RS485 System
In early versions of the FS2510 the serial LED does not flicker even when
there is normal RS485 activity.
Devices with firmware version 5.12k are known to require a firmware upgrade.
Contact Tech support for help.
We have module solutions which allow integration directly into Bacnet products as well as the stand alone solutions shown bottom right.
Range of Conversion Solutions
Depending on project requirements select from units with varying resources. and multi port devices. OEM and embedded solutions too.
|FS20 Series||1x RS232/485||1x Ethernet||1x Lonworks|
|FS25 Series||2x RS232/485||1x Ethernet|
|FS30 Series||2x RS232 and 2x RS485||2x Ethernet||1x Lonworks||FieldBus Support:eg Profibus, DeviceNet etcHot Standby Support|
|FS40 Series||10x RS232 and 2x RS485||2x Ethernet||1x Lonworks||FieldBus Support: eg DH+, MB+, Profibus,
DeviceNet etcHot Standby Support
|SlotServer||1x RS232/485||1x Ethernet||1x Lonworks||For Rockwell ControlLogix 5600|
Lonworks and Fieldbus are options not proviced on base models.
CAS BACnet Explorer now supports BACnet MSTP with the help of a RS485 adapter.
A USB to RS485 adapter provides a virtual comm port that CAS BACnet Explorer can use to talk to your BACnet MSTP devices. These RS485 adapters are relatively cheap approximately ~70 USD and can be bought at your local electronics store or we can provide you with one that we have thoroughly tested in our lab environment.
Now you can test and debug your BACnet MSTP without having to worry about the inconsistencies of BACnet MSTP routers.
Supported RS485 adapter
CAS BACnet Explorer should work with any RS485 adapter that detects as a virtual comm port on your PC.
|USB-COMi-SI-M||USBGEAR||In lab (14 Dec 2006)|
For a full list of supported adapter please read the CAS BACnet manual.
If you know of any RS485 adapter that works with our CAS BACnet Explorer let us know.