The RS-485 Design Guide-TIA
RS485 & RS422 Basics
Configuring the FS-B20 serial port to work as RS-485

RS485/422 Application Note

FieldServer Configuration Manual


Frequently Asked Questions (FAQ)

RS485 Cables – Why you need 3 wires for 2 (two) wire RS485?

  • RS485 needs 3 conductors and a shield. Many people say its a two wire network but it is not. Two conductors are used to carry the RS485 Differential voltage signal. The Shield is connected to earth/ground at one end only and provides shielding against induced noise. So why the 3rd conductor ? The driver…Read More 


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…Read More 


What happen when 2 BACnet MSTP devices with the same device ID?

  • I was ask this question to day What would happen if you had two BACnet MSTP devices with different MAC address but the same BACnet Device ID (Node ID) on the same network and you send a Broadcast Who-is message. This would be an illegal configuration ( no two devices shall have the same Device ID..)…Read More 


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…Read More 


What can go wrong with 485 and BACnet MSTP?

  • Lets say you adopted all the best practices for installation of the network but you get intermittent or unacceptable performance because of packet loss, noise, collisions … Then you should consider hiring an expert to resolve your problems because now you are in the ‘Art’ part of RS485. These are some…Read More 


BACnet MSTP (RS485), Cable selection does make a difference.

  • All cables offer impedance (resistance). Some cables are designed so that the impedance is relatively independent of distance. You want one of these cables. A clue to knowing if you selected one is to look at the cable’s Nominal Impedance. If they quote a number such a 100Ohms you have a good cable….Read More 


BACnet MSTP Installation, RS485 and Cables

  • BACnet MSTP uses RS485 (also known as EIA485) as it physical layer. This is good and bad. Search the Internet on RS485 you will find Bob Perrins’s article called the “THE ART AND SCIENCE OF RS-485.” It is his reference to Art that makes RS485 bad. What he means is that RS485 is often non-trivial and…Read More 


What isCable Specifications for RS485 Networks – Profibus?

  • The PROFIBUS standard defines two variations of the bus cable. However it is recommended to use cable Type A in all new installations. Type A is especially recommended for high transmission speeds (>500 kBaud) and permits doubling of the network distance in comparison to Type B. Technical specification: Impedance:…Read More 


What is Cable Specifications for RS485 Networks – JCI Metasys N2 Open?

  • We have cable selection, installation and trouble shooting info on this page on our web site. Click Here Metasys® as used in this document is a trademark of Johnson Controls, Inc….Read More 


What is Cable Specifications for RS485 Networks – BACnet MSTP?

  • The BACnet spec defined the media for MSTP as follows (2004) ; “An MS/TP EIA-485 network shall use shielded, twisted-pair cable with characteristic impedance between 100 and 130 ohms. Distributed capacitance between conductors shall be less than 100 pF per meter (30 pF per foot). Distributed capacitance…Read More 


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….Read More 


Reducing latency on com ports

  • When a BYTE is sent from a serial device (BACnet MSTP thermostat) to a USB to 232/485 converters (USB-COMI-SI-M) it is cached on the USB converter until one of two things happened. Transfer size – More then x BYTES are collected or Latency timer – More then n Milliseconds (ms) expirer. By default…Read More 


RS485 on a Scope

  • Using an oscilloscope on a RS485 network. For reference purposes we present some scope captures of a typical RS485 line. In this capture we had our scope leads connected to the positive signal conductor and ground. Zone A: Idle State Zone E: Idle State with Noise This is the state of the network when…Read More 


What is RS485 Networks – Multiple Protocols?

  • RS485 defines the physical layer of the network. You may use it as you want. However, since the trunk is shared it is important that no two (or more) devices talk at the same time. If they do this will produce collisions and the corruption of both messages. The following applies for both half and full…Read More 


FieldServer 485 Connection Diagram to Isolator ICD102A (Black Box)

  • This diagram shows how to connect a FieldServer FS20 series to an ICD102A optical isolator using a 2-wire RS 485 connection. FieldServer’s only support 2 wire connections. Datasheet for the Black Box RS485 Isolator/Repeater can be found here: Blackbox Network Services…Read More 


USB-COMi-SI-M USB-to-Optical isolated RS-422/485 Adapter

  • Wiring diagram RS485. [USB-COMi-SI-M, pin 1] (-) Negative [USB-COMi-SI-M, pin 2] (+) Positive [USB-COMi-SI-M, pin 5] Ground [USB-COMi-SI-M, Sw1] OFF [USB-COMi-SI-M, Sw2] OFF [USB-COMi-SI-M, Sw3] ON [USB-COMi-SI-M, Sw4] ON Driver Downloads Download USBCom-CDM…Read More 


CAS Gateway UL RS232 to RS485

  • To switch the CAS Gateway UL box from its default RS232 to RS485 you will need to open the box and change some of the jumpers. For example this is a photo of the board set up in RS-232 …Read More 



RS485 Design Guidelines
A Quick Comparison of RS-232, RS-422, and RS-485 Serial Communication Interfaces
What is the “RS” in RS232/RS485/RS422?

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