Host Engineering Forum

General Category => Do-more CPUs and Do-more Designer Software => Topic started by: Rosco on December 03, 2015, 07:11:20 PM

Title: Modbus RTU network read
Post by: Rosco on December 03, 2015, 07:11:20 PM

Hi Guys

Just wondering if I can get some help.

I have a do-more cpu running with a Modbus gateway over RTU, reading field inputs & outputs via ASI.  I am able to read and write single bits and words at any speed from 9600 baud rate to 38400.

However, when I bump the speed up to 57600k or 115k I get an error bit set on when reading any holding registers or input registers and my Modbus connection starts dropping in and out. Once I drop the speed back down to 38400 or lower everything is fine again.

I have adjusted the inter-packet delay with no luck, I also get no Modbus exception response codes when this happens.

Any help would be greatly appreciated

Rosco

Title: Re: Modbus RTU network read
Post by: plcnut on December 03, 2015, 07:59:01 PM

What are you using for cabling?
Did you use termination resistors?

Title: Re: Modbus RTU network read
Post by: Controls Guy on December 04, 2015, 12:50:15 PM

Agree.  You're probably on the wrong side of the cable length vs. baud rate tradeoff once you go past 38400.  Maybe upgrade to better cable or convert one segment to fiber if there is one that's the lion's share of the cable distance.