modbus communication on CLX

[nileshsurya 0]

hi thr all plc ppl i have a ControlLOGIX L63 processor with 8mb ram i want to use the rs232 port on the CPU for modbus communications to DCS. i would like to know what all things are to be done to do this. is it necessary to write any special logic in the PLC to perform this function also can sumbudy tell me how modbus data is mapped in controllogix processor what ladder exactly has to be written for this purpose if anybody has some nice tutorial on this or can tell me the URL of the tutorial,then it would be of gr8 help also i would like to know how to program a msg instruction to pass on the data from one CLX processor to another via ENBT module (EtherNET / IP ) it would be of gr8 help if anybody can tell me this Thanx in advance Regards NileshSURYA

[TConnolly 13]

To talk modbus on CLX you add a Prosoft modbus card to your rack. http://www.prosoft-technology.com/ Prosoft is an AB encompass partner, so contact your AB distributor.

[Contr_Conn 4]

Starting with ver 13 processor can handle limited Modbus communications via serial port. If you have v13 installed, look at RSLOGIX5000 projects sample folder. You will find ACD files and PDF with details. Why it is limited? Because processor services all comms during overhead time slice and Modbus messages with be in queue will all other comm requests. For higher performance use Prosoft card recomended above.

[nileshsurya 0]

hi thr Contr_Conn i have tried the modbus slave program given in the samples folder of rslogix5k i have included it as a periodic task in my program with the interval of 9 ms i tested the same modbus communication with modbus simulator (modscan32) i faced some problems of modbus query received an invalid response,modbus message timeout etc etc but in my controlLOGIX, a lot of minor faults are logged due to this periodic task (for eg :- in some 5 minutes the number of minor faults can go upto 50000, as seen in the project properties) the minor error is an arithmatic overflow i would like to ask whether these minor errors (though they not take the CLX into fault mode) can hamper the performance in long run(say several years) can i force off/or unconditionally unlatch the arithmetic overflow bit will this cause any further problem it would be of gr8 help if anybody tests the same program at your own side and let me know the observations,comments and suggestions made by you (the program can be found in the RSLOGIX5k samples folder as a file modbusslave.acd) waiting eagerly for reply thnx in advance regards NileshSURYA

[Contr_Conn 4]

As far as I know this program used by many people during last 2 years. I did not try it and have no plans to do this. Just keep in mind that serial port service will be done during overhead time slice and will be in queue with all other messsages and comminications, including HMI and computers. If your cont task is 40ms, then making comm tasks every 9 ms makes very little sence, because processor will service com port only every 40ms in the best case. If you have time critical Modbus application, you should look at Prosoft Modbus module.

[nileshsurya 0]

hi thr contr_conn iam getting that lot of minor errors when the program modbusslave.acd is run getting a lot of minor errors will force my cpu into halt the periodic task in my modbus program executes every 9ms if i increase that period more than that of my continouous task execution period then will i still get those minor errors? is there any alternative ladder code to accomplish this task,it wud b of gr8 help,if u suggest me to it i need some help and comments regarding this,it will be of very much help and i will be very gr8 ful to you.i will request you to try that modbusslave.acd file from your side if it is possible for you. it will be of gr8 help. eagerly waiting for ur reply and valuable comments regards NileshSURYA

[Contr_Conn 4]

nileshsurya, What minor errors you getting? What code? you may have a different reason, like task overlap. And check task priorities. Minor errors do nothing to the processor. Slow code may be a sign of excessive use of communications, as main processor handles them during overhead tile slice, try to increase it up to 40%. You can ask me with word PLEASE, but you can't REQUEST... No I will not do it, I always thought this was a big mistake to have this code in a first place instead of Prosoft module. It is a waste of time to debug slow serial comms because they ARE slow. Edited 18 Sep 2005 by Contr_Conn

[Contr_Conn 4]

OK, I looked at the code - I have no processor to run it right now. They have no continous task! Overhead set to 50% Priority for Periodic task is 1 - highest priority!!! What it tells me that as soon as you add cont. task logic or any other logic performance will change dramatically.