Windmill Software
windmill.co.uk › rs485.html
Understanding RS485: Wiring, Connection, Monitoring Software
Usually though only the Transmit Data (TX) and Receive Data (RX) of a normal serial port are converted to RS485 or RS422. The other signals of the serial port are not used. Three arrangements are commonplace: Write only, 4-wire (full duplex) and 2-wire (half duplex).
NI
knowledge.ni.com › KnowledgeArticleDetails
Set Up 2-Wire (Half-Duplex) Communication with RS-485 Port - NI
August 3, 2023 - RS-485 is set up for four-wire communication by default. To connect a 2-wire device, you will need to short the transmit and receive signals together on the RS-485 port. Note that this is necessary to connect RJ45 (8-pin modular jack) ports to RJ50 (10-pin modular jack) ports.
RS485 Questions (Full Duplex vs Half Duplex, Bidirectional vs Unidirectional, Multi-drop vs multi-point)
I think you are asking for multi-master operation. Normally, RS-485 is a bus, and a single signal is sent over two wires (differential). This implies half-duplex. The physical level provides no bus contention (i.e. to determine which node should be transmitting on the bus). This must be done by a higher level protocol, with the simplest case being single master + N slaves. There are also multi-master protocols that use a token (for example) that are of course much more complex. For full-duplex, you need two signal path (transmit and receive) which means two wires for RS-232 or four wires (two signals, differential) for RS-422. But this still doesn't help regarding multi-master. You still need to have some kind of higher level protocol to decide who gets to transmit at a given time. As contrast, the CAN bus (which also is just a single signal, differential) provides bus arbitration at the physical level. So it inherently supports multi-master operation. More on reddit.com
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June 30, 2022Does RS-485 2-wire mode actually require only 2 wires?
Hi everybody, I am using the on-board RS-485 port on a cRIO-9035 to communicate with a temperature controller. The controller is using 2-wire RS485, terminated with a DB9 with three pins on it, (A-, B+ & GND). The pinouts for the cRIO RS-485 port shows connections for 4-wires; TXD-, TXD+, RXD- ... More on forums.ni.com
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February 19, 2018officially confused - in RS-485 shall we use ground or not?
Direct Answer You do not NEED a ground for RS485 networks. Medium Answer The measurement is the voltage difference between a pair of wires. If the voltage difference is too large, or if one leg is out of range of the receiver's decoders, the device will not understand what information is being modulated on the line. In some rare cases, adding a common 0V connection (not technically a GND) between control panels can solve the problem, but IMO this approach should be approached with extreme caution. Long Answer All senders and receivers must modulate/demodulate data in/out of the serial line. Each devices' manufacturer has chosen some chips for this purpose, but they may not all have the same limitations for what voltage range is acceptable. Adding to this, not every power supply will generate equal voltages. In the following example, some device on the network has a power supply that's running above the ground state (36V & 12V compared to earth ground). Everything seems fine, there's 24V between +/- on the power supply, and the panel does its job without any complaints. Another device on that network has been rigorously designed and tested and adheres to every code and standard. It's power supply is spot on (24V & 0V compared to earth ground). This panel too does its job without any complaints. But both panels have very restrictive serial chips, and they both expect a very narrow range of voltages on the serial line. Their specifications call for a common mode voltage between -7 to +12V, with a minimum bias of ±1.5V. When these two devices attempt to talk to each other, the first device modulates high to +5V and low to -5V COMPARED TO ITS REFERENCE 0V. This is clearly within the common mode voltage, and with sufficient bias to adhere to the standard. The problem is that the second panel interprets these voltages TO ITS OWN REFERENCE 0V. Which means that +5V, which is actually +17V compared to earth ground, is WAY outside the common mode voltage allowed, and the serial chip throws errors. In this case, if the two power suppliers were bound together (same 0V reference, at whatever value above earth ground it falls to) then they would communicate without issue. Binding the two power supplies in this way can cause a number of issues that I'm not going to get into because the PTSD is too intense. I'll leave it to someone else to escort that cat out of the bag. More on reddit.com
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April 15, 2024RS485: two twisted pairs better than one?
This submission has been allowed provisionally under an expanded focus of this sub (see column "G" in this table ). OP, also check if one of these other subs is more appropriate for your question. Downvote this comment to remove this entire submission. More on reddit.com
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October 11, 2023Videos
02:11
RS-485 Explained in 2 Minutes (Differential Signaling + Wiring ...
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RS485 Wiring: 2-Wire or 3-Wire? #shorts - YouTube
29:09
RS-485 – The Basics (2/2): Simple Master-Slave Half-Duplex Setup ...
28:41
RS-485 – The Basics (1/2): Principles, Topology, Specification, ...
06:03
How to wire modbus rs485 serial communications to a MOXA Mgate ...
RS-485 Arduino Communication: Simplex, Half-Duplex & Full ...
Control Solutions
csimn.com › CSI_pages › RS-485-FAQ.html
RS-485 FAQ - Control Solutions
The popular reference to “2-wire” goes back to the days when RS-422 was more commonly used than RS-485. Data flow is full duplex with RS-422, and half duplex with RS-485. In order to operate at full duplex, 4 data lines are needed rather than 2. Therefore, these became known as “2-wire” ...
Raveon
raveon.com › wp-content › uploads › 2019 › 01 › AN236SerialComm.pdf pdf
Serial Communications RS232, RS485, RS422
normal serial port are converted to RS485 or RS422. The other signals of the serial port · are not used. Three arrangements are commonplace: Write only, 4-wire (full duplex) and 2-wire (half duplex).
multidrop serial communication standard
Factsheet
TIA-485-A (Revision of EIA-485)
Standard ANSI/TIA/EIA-485-A-1998
Approved: March 3, 1998
Reaffirmed: December 7, 2012
Approved: March 3, 1998
Reaffirmed: December 7, 2012
Protocol information
TIA-485-A (Revision of EIA-485)
Standard ANSI/TIA/EIA-485-A-1998
Approved: March 3, 1998
Reaffirmed: December 7, 2012
Approved: March 3, 1998
Reaffirmed: December 7, 2012
Protocol information
Wikipedia
en.wikipedia.org › wiki › RS-485
RS-485 - Wikipedia
March 13, 2026 - Thus a 50-meter cable should not ... allowing individual transmitters to be deactivated. This allows RS-485 to implement linear bus topologies using only two wires....
NI
knowledge.ni.com › KnowledgeArticleDetails
Set Up 2-Wire (Half-Duplex) Communication with RS-485 Port
August 3, 2023 - Privacy | Terms of Use | Other Legal Info | National Instruments Corporation. All rights reserved
Embedded Related
embeddedrelated.com › showthread › comp.arch.embedded › 30532-1.php
RS485 is bidirectional does it mean it is fullduplex?
June 15, 2005 - If you use a 4 wire connection, ... only one drives the bus a any given time you can still have full duplex operation. RS485 allows you to work with a 2 wire connection between A and B....
SeaLINK
sealevel.com › home › blog › basics of rs-422 and rs-485 communications
Basics of RS-422 and RS-485 Communications - Sealevel Systems, Inc
A half duplex arrangement connects ... full-duplex network, a driver can communicate with one or more devices and then another driver can use the bus to report results or send commands to other receivers....
Advantech
advantech.com › home › support › resources › basics of the rs-485 standard
Basics of the RS-485 Standard - Advantech
August 28, 2018 - two-wire mode is limited to half-duplex and requires attention to turn-around delay. Four-wire networks, on the other hand, allow full-duplex
Renesas
renesas.com › en › document › rep › rs-485-transmitting-full-duplex-data-over-single-twisted-pair-cable pdf
Application Note R15AN0009EU0100 Rev.1.00 Page 1 Mar 4, 2022
March 4, 2022 - In this new type of full-duplex interface, each transceiver connects to the bus using a 4-to-2 wire converter that
Reddit
reddit.com › r/embedded › rs485 questions (full duplex vs half duplex, bidirectional vs unidirectional, multi-drop vs multi-point)
r/embedded on Reddit: RS485 Questions (Full Duplex vs Half Duplex, Bidirectional vs Unidirectional, Multi-drop vs multi-point)
June 30, 2022 -
I'm working with RS485 between a PI and a STM32. I thought I had a grasp on this until I decided I wanted full duplex so I could read and write at the same time so the protocol I'm making on top of the rs485 layer wouldn't have to wait for responses for validity. The wiring is confusing. Before, I had a half-duplex setup that required a DE pin to operate.
The (old) setup:
| PI | Transceiver | Transceiver | STM32 |
|---|---|---|---|
| TX --> | DI | DI | <-- TX |
| RX <-- | DO | DO | --> RX |
| DE --> | DE | DE | <-- DE |
| A <-- | --> A | ||
| B <-- | -- >B |
This worked, but it wasn't really what I needed. With the new chip (max3077e), I want to go full duplex. I'm confused though because it doesn't have a DE pin. It just has DI, DO, AB, YZ.
The (new) setup:
| PI | Transceiver | Transceiver | STM32 |
|---|---|---|---|
| TX --> | DI | DI | <-- TX |
| RX <-- | DO | DO | --> RX |
| A | <-- Y | ||
| B | <-- Z | ||
| Y --> | A | ||
| Z --> | B |
So now I supposedly have full-duplex capability. Here are my questions:
1.) Now that I have full duplex capability, shouldn't I be able to send and receive at the same time?
2.) This (I think) qualifies as a multi-drop network setup, but since I want a slave to be able to talk to the master at any time (ie not in response to a packet, asynchronously), how do I know a slave won't talk over another slave at the same time? Is this where RTS/CTS are needed?
3.) Am I misunderstanding multi-drop vs multi-point? I thought multi-drop meant 1 sender and X receivers on the line, does this require blocking read/writes to slaves in order to stop slaves from talking over eachother?
Multi-point should mean any device can communicate with any other device on the line, but I don't really need that. I just need the clients to be able to communicate with host at any time, not requiring the host to talk first. So I guess I'm sorta confused on where my use case fits into these standards.
Edit: After thinking about this more, I feel like I'm just recreating ethernet. Would UDP packets over ethernet be a better way to achieve what I want? I feel like the "being able to write while you read" part of a full-duplex rs485 would only be useful if I didn't have to wait for responses from clients. But I still have to wait for responses (use blocking write/reads) to make sure that clients don't talk over eachother. Am I missing something?
Top answer 1 of 5
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I think you are asking for multi-master operation. Normally, RS-485 is a bus, and a single signal is sent over two wires (differential). This implies half-duplex. The physical level provides no bus contention (i.e. to determine which node should be transmitting on the bus). This must be done by a higher level protocol, with the simplest case being single master + N slaves. There are also multi-master protocols that use a token (for example) that are of course much more complex. For full-duplex, you need two signal path (transmit and receive) which means two wires for RS-232 or four wires (two signals, differential) for RS-422. But this still doesn't help regarding multi-master. You still need to have some kind of higher level protocol to decide who gets to transmit at a given time. As contrast, the CAN bus (which also is just a single signal, differential) provides bus arbitration at the physical level. So it inherently supports multi-master operation.
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Just for extra precision: what you are doing right now is not really RS485 anymore but RS422. On RS485 transceivers you usually have DE and RE which are used to enable respectively the transmitter and the receiver in the chip. This is needed in an half-duplex setup because you can't have more than one transmitter driving a pair of differential wires at the same time (the RE is not strictly necessary but it's useful to avoid receiving what you are sending on the same device), in this case your line is bidirectional because both devices can drive it (not at the same time). For full-duplex RS422 communication with only 2 devices, you don't need theses as they can be left always enabled because you now have 2 unidirectional differential pairs: one is driven by one end and the other one by the other end, you don't have any risk of both devices driving the same pair. The RS422 chip you are using is actually the same as if you would have 2 RS485 transceivers on each side: one with only only the receiver enabled and the other one with just the transmitter EDIT: to answer your question yes, you can send at any time from any of the devices. This setup is neither multi-point nor multi-drop, it is composed of 2 point-to-point networks (1 emitter with 1 receiver each)
CommFront
commfront.com › products › usb-to-2-wire-rs485-adapter-converter
USB to 2-Wire RS485 Adapter / Converter – CommFront
The USB-485-1 is a port-powered, bi-directional, integrated USB to RS485 adapter / converter that can be used to convert any standard full-duplex USB port into a two-wire balanced half-duplex RS485 port and vice versa.
Renesas
renesas.com › en › document › apn › rs-485-design-guide-application-note pdf
Application Note R15AN0014EU0100 Rev.1.00 Page 1 Sep 12, 2023
same pair of wires, but not simultaneously. This means that devices on the network take turns transmitting and · receiving data. In other words, when one device is transmitting, the other is listening, and when the other is · transmitting, the first is listening. Half-duplex transmission is proper when only one device is required to ... In contrast, full-duplex transmission allows data to be transmitted and received simultaneously over two pairs of
Seeed Studio
seeedstudio.com › home › all about rs485 – how rs485 works and how to implement rs485 into industrial control systems?
All about RS485 – How RS485 Works and How to Implement RS485 into Industrial Control Systems? - Latest News from Seeed Studio
November 28, 2024 - This means data can be transmitted in both directions to and fro devices one direction at a time. By adding another 2 wires, making it a 4 wires system, it allows data transmission in both directions to and fro devices at the same time, also known ...
Schneider Electric
se.com › support · / › search faqs · / › fa221785
Wiring of RS485 Communications Networks | Schneider Electric USA
Full duplex implementation requires 4 wires. This may be necessary for some applications involving legacy devices. In half-duplex implementation 2 wires are used this is the recommended configuration for most Schneider Electric PMC devices. All products manufactured by Schneider Electric PMC ...
US Converters
usconverters.com › index.php
RS485 FAQ : U.S. Converters LLC, Serial Data Communication
With RS-422, two units cannot connect ... toggle them high/low. RS-422 is always full duplex (if the device is not listen only), RS-485 can be full (4-wire mode) or half duplex (2-wire mode)....
Analog Devices
analog.com › home › resource library › an-960
AN-960: RS-485/RS-422 Circuit Implementation Guide | Analog Devices
This configuration is also known as a 2-wire RS-485 network connected in a multipoint configuration and allows for data transmission in both directions, but only in one direction at a time. Figure 3. Half-Duplex RS-485 Bus Configuration. Figure 4 shows an example of an RS-485 bus connected ...
Industrialmonitordirect
industrialmonitordirect.com › industrial monitor direct › knowledgebase › rs485 4-wire full duplex industrial equipment applications
RS485 Full Duplex Equipment: Industrial Applications & Examples – Industrial Monitor Direct
April 29, 2026 - RS485 is a balanced serial ... repeaters). The standard specifies two-wire half-duplex operation as the default, but also defines a four-wire full-duplex configuration using separate transmit and receive pairs....
Reddit
reddit.com › r/plc › officially confused - in rs-485 shall we use ground or not?
r/PLC on Reddit: officially confused - in RS-485 shall we use ground or not?
April 15, 2024 -
hi All
I keep reading conficting info on the internet and it gets my head spinning clockwise or anticlockwise.
please explain to me in plain words - do we need a ground for RS485 networks or not?
it keep reading its a must where there is high noise but is tight intervowen pair of wires keeps canceling the the impact because both lines are going UP or DOWN and only the difference matters.
Then i read that the ground has to be used for decoding so if both A and B voltages are moved too high then decoding will not work or something will get burn.
Then i keep reading and connecting ground to all slave and master can be a recipe for the disaster because if there are different potentials there will be a ground loops and flowing current.
Can anybody explain me his or her understanding please?
ps. to cover the topic shall we use shield or not ? this is less confusing in that if it is connected only on 1 end there will be no ground loops.
Top answer 1 of 12
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Bond shield. Side note, if this is a new installation, please reengineer it, I'd like to stop carrying serial cables in the near future.
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Direct Answer You do not NEED a ground for RS485 networks. Medium Answer The measurement is the voltage difference between a pair of wires. If the voltage difference is too large, or if one leg is out of range of the receiver's decoders, the device will not understand what information is being modulated on the line. In some rare cases, adding a common 0V connection (not technically a GND) between control panels can solve the problem, but IMO this approach should be approached with extreme caution. Long Answer All senders and receivers must modulate/demodulate data in/out of the serial line. Each devices' manufacturer has chosen some chips for this purpose, but they may not all have the same limitations for what voltage range is acceptable. Adding to this, not every power supply will generate equal voltages. In the following example, some device on the network has a power supply that's running above the ground state (36V & 12V compared to earth ground). Everything seems fine, there's 24V between +/- on the power supply, and the panel does its job without any complaints. Another device on that network has been rigorously designed and tested and adheres to every code and standard. It's power supply is spot on (24V & 0V compared to earth ground). This panel too does its job without any complaints. But both panels have very restrictive serial chips, and they both expect a very narrow range of voltages on the serial line. Their specifications call for a common mode voltage between -7 to +12V, with a minimum bias of ±1.5V. When these two devices attempt to talk to each other, the first device modulates high to +5V and low to -5V COMPARED TO ITS REFERENCE 0V. This is clearly within the common mode voltage, and with sufficient bias to adhere to the standard. The problem is that the second panel interprets these voltages TO ITS OWN REFERENCE 0V. Which means that +5V, which is actually +17V compared to earth ground, is WAY outside the common mode voltage allowed, and the serial chip throws errors. In this case, if the two power suppliers were bound together (same 0V reference, at whatever value above earth ground it falls to) then they would communicate without issue. Binding the two power supplies in this way can cause a number of issues that I'm not going to get into because the PTSD is too intense. I'll leave it to someone else to escort that cat out of the bag.