adccommunitymod (AutomationDirect) asked a question.
Created Date: June 11,2000
Created By: Fluoronator
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We have a gas chromatograph that automatically performs an analysis every 15 minutes. I would like to pass the analysis results to our DCS system as a 4-20ma signal. The chromatograph has an RS232C port and a limited BASIC interpreter which can be programmed to “Print� the data to the serial port as ASCII text. The approach I have in mind is to purchase a DL05 and an analog output card, program the PLC to receive the serial data from the chromatograph and output the scaled data to the analog output as a 4-20ma signal. Is the DL05 capable of receiving this ASCII data from the serial port? If so, what is the procedure for monitoring the port for incoming data and placing it on the stack or somewhere that it may be used?
Created Date: June 11,2000
Created by: Fluoronator
We have a gas chromatograph that automatically performs an analysis every 15 minutes. I would like to pass the analysis results to our DCS system as a 4-20ma signal. The chromatograph has an RS232C port and a limited BASIC interpreter which can be programmed to "Print" the data to the serial port as ASCII text. The approach I have in mind is to purchase a DL05 and an analog output card, program the PLC to receive the serial data from the chromatograph and output the scaled data to the analog output as a 4-20ma signal. Is the DL05 capable of receiving this ASCII data from the serial port? If so, what is the procedure for monitoring the port for incoming data and placing it on the stack or somewhere that it may be used?
Created Date: June 11,2000
Created by: Russ
Hi,
If you haven't already seen this, you should check out my posting "DL05 and DL250 Port 2 ASCII Input " back on March 22. There are also several other forum threads on ASCII input to PLC's around that time as well.
Basically, while it is technically possible to receive ASCII into the DL05, I don't recommend doing so because it is an unsupported feature and most simple ASCII plain text transmissions lack error detection and correction capabilities.
Since your chromatograph has on-board language support, you may be able to solve the data integrity issue by developing your own protocol and transmission scheme. Another possibility is that you can write your BASIC program to send data via a protocol the PLC already understands like MODBUS, k-sequence or DirectNet. If you are interested in the DirectNet protocol, it is described in the the DA-DNET-M manual. This manual is downloadable in PDF form from the documentation section of the main web site. Specifically, you may want to read chapter 6 ( ftp.automationdirect.com/documentation/dadnet/ch6.pdf ).
Hope this helps you out, and good luck with your project!
Regards,
Russ O 'Rourke
Created Date: June 11,2000
Created by: Fluoronator
Since only a few characters are to be transmitted every 15 minutes speed is not an issue. Could I achieve reasonable reliability if I transmit the data repeatedly to the PLC until the PLC see the same data, say 2 or 3 times in a row before it transmits it out to the DCS?
Created Date: June 11,2000
Created by: Tom Jenkins
1) The DirectNet protocol is not hard to set up and write. We have done it in Quick Basic, but the fundamental code would work in interpretive Basic as well. The DirectNet manuals are quite good, and I suspect that it would be just as fast to put this or Modbus in your Basic as to come up with your own ASCII code scheme. Your chromatograph serial port should be compatible.
2) I don't think that the DL-05 has an analog output, just analog inputs. You may have to go to a DL-205 to get analog output.
3) What protocol does your DCS use? A lot of them have Modbus RTU drivers available, and this is a public domain protocol. You could probably write directly from your chromatograph by setting it up as a Modbus slave on your DCS or else using the Modbus port on a DL-05 to bridge the networks. This is probably easier than creating your own protocol.