RS485 DB9 standard connection

Thread Starter

bug13

Joined Feb 13, 2012
1,846
Hi team

I need to use RS485 and DB9 connector, is there a standard way to connect DB9? I have googled it already, it looks like there is not a standard of how to connect RS485 to DB9 connector??

Thanks guys!
 

Papabravo

Joined Feb 24, 2006
13,694
Hi team

I need to use RS485 and DB9 connector, is there a standard way to connect DB9? I have googled it already, it looks like there is not a standard of how to connect RS485 to DB9 connector??

Thanks guys!
With single ended connections, they are the same as for RS-232. AFAIK there is no standard for differential connections. Only standard I know of with differential connections is V.35, but you won't be crazy about the connector.
 

MaxHeadRoom

Joined Jul 18, 2013
20,649
I have never come across a RS485 'standard' socket, but the DB9 was used to replace the 25pin DB25 socket for RS232 COM port.
Max.
 

Analog Ground

Joined Apr 24, 2019
385

Thread Starter

bug13

Joined Feb 13, 2012
1,846
I have used RS-422/485 a lot over the years. There are some common pinouts but I wouldn't call them standards. Remember, standards are great. Everyone should have one.
How would you connect them, if you need to use RS485 on a DB9, with full duplex. I see some use 1,2,3,4. Some use 2,3,7,8. GND can be 4 or 5 or 6.

What would you use if you were doing it and want most compatibility?
 

Analog Ground

Joined Apr 24, 2019
385
How would you connect them, if you need to use RS485 on a DB9, with full duplex. I see some use 1,2,3,4. Some use 2,3,7,8. GND can be 4 or 5 or 6.

What would you use if you were doing it and want most compatibility?
I usually had a particular piece of equipment to connect with my custom boards. I would just make my pinout compatible with the other equipment. This allowed simple cables without a lot of crossovers. For example, Digi International makes terminal servers which support multiple signal level protocols. Here is an example. Again, nothing standard that I know. There may be an accepted practice in some application areas. Or, pick something like the Digi pinout and "claim compatibility".

Edit: For this reason, you won't find off-the-shelf cables like you do with RS-232. Just lots of custom cabling for RS-422/485.
Another edit: An example of accepted practice would be MODBUS.
 
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MrChips

Joined Oct 2, 2009
21,053
If anybody should know it would be B&B Electronics.
In their technical notes they state that there is no DB-9 wiring standard.
Read the last sentence of the RS-485 Connections FAQ.

However, they use
DATA(A)- pin-2
DATA(B)+ pin -3
GND pin-4

Alternative
DATA(A)- pin-8
DATA(B)+ pin -7
GND pin-6

Be aware that B&B Electronics use the opposite labeling of pins A and B versus the labeling of a 75176 package.

1591056941136.png

Model 485SD9R

Quick Start Guide
 

SamR

Joined Mar 19, 2019
2,096
ModBus is using 4,5,8gnd.
But this is more in line with the existing pin protocols.
It keeps TX, RX, and GND the same across all protocols. YMMV
1591057220787.png
 

dendad

Joined Feb 20, 2016
3,514
When I do boards that use RS485, I place SMT, and DIP pads there, as it is easier to put a good socket in and change the DIP driver when it gets zapped.

All my boards use half duplex RS485, and plug/socket screw terminals, never DB connectors.
Also, ALWAYS include an Earth/0V connection, not just the 2 data wires.

Another option is have a look at the Mornsun isolated drivers.....
https://www.mornsun-power.com/html/products/48/rs-485-transceiver-module.html shows the range.
They are quite cost effective and remove common mode voltage problems.
 

nsaspook

Joined Aug 27, 2009
7,280
I think I will stick a terminal block on it just like others.

What do you think of this chip? I will need to do 115Kbps, full duplex, 1000 meters max, point to point connection.
https://www.st.com/content/ccc/resource/technical/document/datasheet/45/b4/14/d4/eb/dc/4a/7e/CD00003152.pdf/files/CD00003152.pdf/jcr:content/translations/en.CD00003152.pdf
You will need a chip with enables (DE *RE) for half-duplex operation as that's usually a standard configuration with many devices.
1111.gif
Typical-RS-485-and-RS-422-pinouts-1439265187.png
https://www.eeweb.com/profile/intersil/articles/rs-485-transceiver-tutorial
 
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Analog Ground

Joined Apr 24, 2019
385
I think I will stick a terminal block on it just like others.

What do you think of this chip? I will need to do 115Kbps, full duplex, 1000 meters max, point to point connection.
https://www.st.com/content/ccc/resource/technical/document/datasheet/45/b4/14/d4/eb/dc/4a/7e/CD00003152.pdf/files/CD00003152.pdf/jcr:content/translations/en.CD00003152.pdf
Within the TIA-485 specification, all the drivers and receivers are about the same. That is the point of having an electrical specification. There are some devices which go beyond the spec. For example, there are some very, very fast devices from T.I. which I have used to transmit 30 nanosecond pulses for over 15 meters. Your requirement is within the spec and the device you cite should be fine. I suggest picking a device with functionality and pinout shared with other devices. This gives you a second source. BTW, the "input fail safe" feature is good and fairly standard now(?). More important is the cabling, shielding, grounds and termination.

Edit: My reply assumes you are going point to point. Otherwise, like on a shared bus, you will need an output enable.
 
Last edited:

Thread Starter

bug13

Joined Feb 13, 2012
1,846

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