Question : Problem: multi-drop RS-485 garbled

Hi,

I have two separate but related issues I’d like to seek advice about.

First, we have several Basler 951 relays that we are bringing using to feed a Harris D20M++ RTU.  

The Baslers talk RS-485, and are daisy chained together.

 The RS-485 cable then goes to a BB Electronics 485LDRC9 RS-485 to RS-232C converter.  

The RS-232C side of the converter goes to SIO3 on a D20 RTU with a D20M++ processor board.  

There is a ASE protocol analyzer between the BBElec and the D20.

Everything communicates fine if there is just one Basler.  Hang a second one on the RS485 line and  communications gets garbled.  The protocol analyzer (on the RS232 side) shows lots of framing errors and other similar errors.  I do not know whether the communications on the RS485 side is good or not.

If I slow down and stagger the poll rate, then the data on two baslers comes thru ok, with only a few comm errors; the sweet spot seems to be around 4-5 seconds poll rate.

We have not tried more than two baslers yet.

The cabling on the RS485 side is 3wire (one pair) RS485.  RTS is not being used (or even hooked up)

The cabling on the RS232 side is 9 pin RS232 non-null

1  - 1

2 – 2

3 – 3

5 – 5

7 – 7

8 – 8

4, 6 and 9 are not connected.

RTS in enabled on the RS-232 side.  CTS is disabled on the RS-232 side.

Are there any suggestions or general practices that anyone can recommend for using daisy chained RS-485 devices into an RS-232 based device?


 

Answer : Problem: multi-drop RS-485 garbled

I'm not familiar with that exact equipment. Will leave just some thoughts.
But it seems the baslers might be trying to send data at the same time, corrupting each other.

When you poll for data, do you identify which basler do you want the data from?
In that case, is each basler correctly setup with a unique "address"?

Hope that might help...