Internet PLC Forum

General => Technical support => Topic started by: Will Steed on December 29, 2003, 10:51:22 AM

Title: Modbus
Post by: Will Steed on December 29, 2003, 10:51:22 AM

Hello all at Triangle, I trust everyone has had a good holiday.

I am running a medium sized program on a MD2424 plc, I would like to interface a ultrasonic proximity switch as a modbus slave device.
I am monitoring a fairly fast moving conveyor, and I am wondering what would be the highest rate that I can quiry the slave device without risk of slowing down the program.  
I can also use a 4-20mA analog output from the proximity switch if I need to, but I thought I would try to use the Modbus protocol rather than rely on the 8-bit resolution of the analog inputs.

Any help would be appreciated.

Thanks,

Will Steed

Title: Re: Modbus
Post by: support on December 29, 2003, 12:22:09 PM

The maximum compatible baud rate is 38,400 bps with most third party devices.

Communication overheads mainly has to do with the time it takes to exchange a MODBUS command/response messages.  Of course you should avoid reading from the slave more often than necessary. Running a clock pulse of 0.2s to periodically read some data from the slave should be OK.