possible to do loopback test on two-wire RS-485?

defcon.klaxon

Lifetime Supporting Member
Join Date
Feb 2015
Location
Far NorCal
Posts
616
Hello all,

I am working on getting a Hach particle counter set up over a serial to ethernet adapter. We need to move the counter to another building so that the sample point is before chlorinators, long story short. The particle counter data comes in over RS-485, two-wire and gets converted to RS-232 at the PC that runs the interface software.

Between my lack of experience with RS-485 and a mess of adapters that used to work but now they're not sure what does and doesn't work, I'm having a bear of time sorting out everything for testing.

To the point, I'm trying to determine is there is a way to perform a loopback test on RS-485 two wire. I can find everything in the world about RS-485 four-wire loopback, but not on two-wire. I'm thinking it could, in theory, be as simple as putting a jumper between the + and - terminals but wanted to verify with someone who knows more than me. The distinct lack of Google search results with this potential solution being suggested has me worried that I may damage something.
 
I dont think so. But you can use two computers running hyperterminal. Ive done this before to test the converters etc. Type message on computer 1 and it should show up on computer 2
 
Shorting + and - won't work. The pair alternates between 'send only' and 'receive only'.
It sends a message, switches over to receive, and waits for a response. You need hardware to do that.
RS485 - 232 converter and Hyperterminal. In Win10, use PuTTY. 32 or 64 bit. Link.
 
Shorting + and - won't work. The pair alternates between 'send only' and 'receive only'.
It sends a message, switches over to receive, and waits for a response.

That makes sense, since two-wire is half duplex. I can now understand why that wouldn't work.


You need hardware to do that.
RS485 - 232 converter and Hyperterminal. In Win10, use PuTTY. 32 or 64 bit. Link.

A bit confused about what you're suggesting. Right now I have a USB to rs232 converter, and then an rs232 (de9) to rs485 two-wire adapter. What are you suggesting I connect to the two-wire rs485 side of the adapter? BTW am using PuTTY, Win7.
 
you'll need 2 pairs of converters to test. Open two copies of putty, one looking at the comm port of adapter 1, the other at the comm port of adapter 2.

Send a character on one port it should show up on the other and vice versa.
 
Does the RS485 adapter have it's own power supply? The ones that 'steal' power from the port only work on true ports, usually on a desktop. Anything else is 'iffy'.

Test for voltage at RS485 lines. A to B. Should be about 4 volts when not connected to a load. That doesn't guarantee it works, just a quick test to make sure it's not totally dead.
 
Does the RS485 adapter have it's own power supply? The ones that 'steal' power from the port only work on true ports, usually on a desktop. Anything else is 'iffy'.

Test for voltage at RS485 lines. A to B. Should be about 4 volts when not connected to a load. That doesn't guarantee it works, just a quick test to make sure it's not totally dead.

I don't trust these. I usually either use a usb-485 adapter on my computer. Or I use the adapters on site in a scenario like yours. Or even both to find where the problem lies.
 

Similar Topics

Hello The plant is running and there is no shutdown nowadays therefore I can add 1734- AENTR and its card while PLC is in Run? I do not wanna...
Replies
1
Views
111
Folks, I have a client with an old ABB Advant / MOD300 system (v14.4). Around y2k I installed the ABB Industrial IT MOD300 OPC Server 1.1/2...
Replies
0
Views
65
Hi all so i have a robot project im working on were i need to set the layers. using the hmi screen i would like to use the numeric data display to...
Replies
11
Views
768
I have a FactoryTalk View Se project, Is it possible to export Direct Reference tags to edit in a CSV file or Excel? I know I can export HMI...
Replies
1
Views
266
If I have a PanelView Plus, is it possible to show historical data via historian on it? I have the application all setup in FT Studio Machine...
Replies
3
Views
453
Back
Top Bottom