1734-485ASC

[ratcliffe_ic 2]

I have a project and was hoping to use the above module, I need to send some strings to a Control Techniques Unidrive, now these are in the following format: Drive Read, drive node 11, parameter 13.07 EOT11111307ENQ and I will get the reply STX1111+0.489ETXBCC So I will not get a delimiter at the end, just the BCC value Drive Write, drive node 11, parameter 13.07 EOT1111STX1307+0.490ETXBCC and I will get the reply ACK for a valid message or NAK for an invalid message Therefore I,m dealing with different length replies, no delimiter. Has any one been successful in dealing with strings of different length, I believe you can't change the config in the plc code (for the received length/new delimiter) as you need to reboot the module to do this. Any ideas anyone?

[Ken Roach 194]

I have been through a very similar application with the 1734-485ASC, and I think you will not be able to use it for your application. The 1734-232ASC and -485ASC are great for ASCII applications that have fixed lengths or a delimiter byte like CR or LF or ESC. But they cannot deal with variable-length incoming data that doesn't have a delimiter. They cannot use a period of time as the 'end of frame' indication. Does the Control Techniques have any 'Echo' or 'Ping' commands that will always return a NAK or ACK (and are those literally three-character ASCII or are they single byte codes) ? Maybe you could send both a Read and a Ping, so you always ended with an ACK or NAK. The device I chose instead (I spent a week trying to find a workaround with the 485ASC) is the HMS Fieldbus 'Communicator'. Those modules can use time as a delimiter, and can even check a BCC or CRC for you. You can use different 'parser' definitions for different outgoing data, so one can look for a complex frame (start, variable data, end, checksum) and another can just look for ACK or NAK followed by a few milliseconds of silence.

[ratcliffe_ic 2]

The ACK and NAK are single byte codes, Ctl F and Ctl U There is no PING facility The most important thing here is that I can write to the inverter, so I will have to use the ACK as my delimiter. However I wanted to read some values off it as well, so, what if I was to read a value then write a value (to a parameter of no significance) so I got my ACk, the data after the ACK would then be my read data. Bit messy I know, but I've ordered these things last week and could be stuck with them. Edited 22 Apr 2013 by ratcliffe_ic

[Ken Roach 194]

Your idea of performing a Write after each Read is a good one, but what if one of the data bytes is 0x06 or 0x15 ? That's the basic problem with trying to implement a binary protocol with an ASCII device. When I went through this, I did a similar workaround where my data stream sometimes stops until the number of received bytes coincides with the buffer length. I wrote apologies to future users in the rung comments. You can return unopened modules, re-use them on another project, or sell them on the aftermarket to recover part of their cost. But wasted time is gone forever.

[IO_Rack 178]

You are too kind Mr. Roach! I usually just write excuses.

[ratcliffe_ic 2]

Thanks for the reply. Is there a way to reboot the module from within the PLC code? I take your point about wasted time.

[Ken Roach 194]

I tried resetting the module programmatically as part of my investigation, but it just took too long to reboot. You send a CIP service code 0x05 to the module using a MSG instruction (target the Identity Object, Class/Instance/Attribute = 0x01/0x01/0x01) , and that performs a Reset.

[ratcliffe_ic 2]

I have plenty of time to do a reboot so I think it could work for me, how did you locate the information to do a reboot??

[Ken Roach 194]

It's one of those 'Advanced CIP Programming' things. The Reset service is similar to any other CIP service, like Get Attribute Single or Apply Configuration Parameters. I've had to experiment a little with the actual target object, but Identity usually gets the job done. I've learned a lot about CIP under the hood by connecting Wireshark and pressing various buttons in RSLogix 5000. If you go to the status tab for a 1734-485ASC in RSLogix 5000 and click the 'Reset Module' button, the software sends that same service code.