CJ1 & Modbus RTU

[ECSI 17]

Hi I need to read/write to 55 (and counting) VFD's configured as Modbus RTU slaves. I am thinking of using the CJ1M PLC using Serial Gateway instead of Protocol Macro. A tech support guy mentioned that there was a limitation of 31 slave nodes maximum using either of these methods. I don't really believe him and don't see any mention of this in the documentation. Can anyone confirm or deny this? Thanks

[Jay Anthony 118]

The Modbus RTU protocol allows for 254 nodes (0 is for broadcast). The hardware is what limits the number of nodes to 32(31 + master). What you need is a RS485 repeater which gets you another 32 nodes. Try Blackbox or B&B Electronics.

[MPM 1]

if you use a scu31 you have 2 ports with 422/485. this makes 62 nodes, if in the future you need more addresses you can add more scu31 in the rack. adding 62 nodes for each scu 31 v1. regards, MPM

[ECSI 17]

OK. That explains it. I'll use an SCU31 instead of SCU41. Thanks Guys!

[lostcontrol 23]

May I ask why you want to use Serial Gateway as opposed to PMCR? Is that so you can add/subtract nodes without having to add to the CX-Protocol file??

[ECSI 17]

No. I just find that using the CMND instruction is a little easier than using the PMCR instruction. Maybe I'm wrong. I've only done a few projects using Modbus RTU on a CJ1. I'm open to any and all suggestions. This project only involves starting/stopping the VFD's and also setting the speed reference.

[lostcontrol 23]

There are many options.. 1. You can create a command string for each node using CX-Protocol, then call the command from ladder 2. You can create a common command string, and substitute the node no from ladder 3. there is more, maybe some others will suggest some. Creating the command strings in CX-protocol is easier than ladder, IMO, and you can store them in the SCU, as opposed to using up ladder memory. What VSD's are you using, I can provide some assistance if you want to look at the PMCR option?

[BobB 58]

I prefer to use Protocol Macro quite frankly. The last guy I was with that used Serial Gateway had problems with bytes having to be manipulated into correct words. Have never had a problem with Protocol Macro.

[PMCR 66]

Not trying to plug my own work, but have you looked at this? http://forums.mrplc.com/index.php?autocom=...mp;showfile=867 It may be too simplistic for your application. A repeater would need to be used, as my code only supports 1 port. Attached is the pdf that explains the code. CP1LCP1HCJ1CJ2CS1ModbusSolutionV207.pdf Edited 23 Jul 2009 by PMCR

[ECSI 17]

PMCR, I downloaded your project a couple of days ago. Very impressive and well documented! I'll be looking at it in more detail soon. The consensus here seems to be with Protocol Macro. That kind of surprised me so I'll have to look at it a little closer. The good news is I have plenty of time to explore my options. Thanks everyone!

[PLCnuts 0]

Any chance you change to Device Net?

[ECSI 17]

Not available on the VFD's