1734-485asc Ethernet Comm

I have only been at this 3 months and I recieved a curveball. My cabinet is finished and shipped but I was just informed that feedback which was supposed to be 6 4-20mA signals will actually be from a RS485 device. Since my cabinet is full and shipped I was thinking of using a 1734-AENT with a 1734-485asc (they also added digital feedback so the point I/O is needed for that as well) to send the feedback via ethernet to my main cabinet that has a compactlogix L35E. I have the point I/O added to my program in RSLogix 5000 and configured the cards. The issue I am having is with no delimiter how do I know if I have collected all of the string before I start using and manipulating it?

You've run squarely into one of the limitations of the 1734-___ASC modules; they really require a delimiter to function correctly.

I've worked around this, inelegantly, using "overflows" and looking for patterns in the stream of incoming data.

Can you describe in more detail the feedback data that's coming in over RS-485 ?

Hi.

I work for HMS and on our gateway (AB7007), see link below, one can use a meassage delimiter to determine end-of-frame. For example, setting it to 20ms will make the gateway wait 20ms after last recieved character and if no other character is recieved end-of-frame is set.
One can also combine this function with a trigger byte, to indicate to the E/IP scanner that a new serial telegram is recieved.

http://www.anybus.com/support/support.asp?PID=110&ProductType=Anybus Communicator

//Patrick L

The feedback will be 1 of 4 possible messages 9 char long. The format I have been told is as follows:

4KWATxxxx
4FREQxxxx
4COVOxxxx
4LKDKxxxx

The x's being the actual number value of the feedback and I don't know if there will be a delay between messages.

I have set up a lab to test different ideas using a USB to rs485 adapter. I can recieve the serial data and see it in my 1734-485asc's tags but can't seem to transfer it into a string tag nor can I clear the buffer so I can recieve new data. I am trying to find a way to count incoming bits so that I can then pull the data and clear the buffer but I am not sure what commands to use since I can't use the serial port ASCII commands. Any advice would be appreciated.

A gateway device is going to save you a lot of development time; if your schedule is pressed, I recommend punting and getting an HMS or RTA gateway.

Real Time Automation (RTA)'s ASCII gateway is extremely easy to use; an hour from start to success is common. The tradeoff is that it's bulkier and a little more expensive than some other options.

The HMS Communicator is a great little device. It fits my preferred sweet spot for cost, complexity, and size. They have a significant USA presence and Kendall Electric stocks them in Michigan.

If you're going to stick with the 485ASC, send me a PM and I'll send you a snippet of the code I used with a similar protocol stream.

One thing I wish the 485ASC did was have an output bit that allowed you to flush the incoming buffer. But it doesn't. I attempted to do this by sending a Reset Service to the module, which worked but required several seconds to reboot, making it impractical.