modbus

I have to control 53 omron inverter via modbus with a omron CS1 !
Is that possible ? Is it a good solution ?

There are a number of ways to do Modbus on a CS1 PLC:
1. TXD/RXD
Advantages:
No additional equipment needed.
Ladder is pretty simple.
Disadvantages:
No good way to calculate Modbus CRC16 error checking routine
Must use precalculated strings.
CPU RS232 Port is the only port that can use TXD/RXD

2. C200H-ASC02
This module will work with CS1 and provides both Modbus Master and Modbus Slave capabilities.
Advantages:
Very Robust and bulletproof
No ladder necessary
Code already exists. (See Omron Europe)
Disadvantages:
Moderately Expensive
Must manage several programs

3. Protocol Macro Communication Boards and Modules
Accessory Communications Board that slides into the CPU. Has two serial ports which are customized with CX Protocol Software and Custom Communication sequences.
Advantages:
Fastest possible Communications (Data Rates to 38K)
Code already exists for Omron Inverters (See Omron North America)
Disadvantages:
Somewhat expenseive, although not as much as C200H-ASC02
Some Ladder necessary to trigger the sequences

53 is not a good number for Omron inverters? Give us more of a number. Tell us more about your needs.

If its Start/Stop and parameters setting it's OK but... calculate the time between updates and you should find this time a little long for controls like PID and synchro.

It is a good solution if your comm link is reliable. Use separate conduit for your communicaitons, and put an optical isolator at each drive and PLC. Make sure your drives (and evrything else for that matter) are grounded properly. And finally, make sure you have a HARD WIRED E-STOP for each drive.

modbus protocol

report about modbus protocol

I'm not that familiar with Modbus but isn't it limited to addresses 0-31? do you have more than 1 communication port?

I'm not that familiar with Modbus but isn't it limited to addresses 0-31? do you have more than 1 communication port?

Click to expand...

The Modbus RTU protocol defines nodes from 1-254 with 0 being reserved for broadcast. If implemented on RS422/RS485, then you are correct. You are limited to 32 physical nodes, but they can be in the range of 1-254. If implemented on radio modems or other wireless media, then this physical limitation is moot.
Since we are assuming Manu is going to use copper between all nodes, he will need at least two communication ports.
:nodi:

Jay, regarding number of nodes, you are correct that there are limitation in the protocol and by hardware. (According to EIA standard: RS485 can have max 32 drivers/receivers on line).

Some devices with Modbus support, use the whole 8 bit address range 1-255 (0=broadcast), others a limited number of adresses, like only 1-31 or 1-247.

The hardware/electric restriction can be avoided with use of repeaters.

A RS485-repeater can be used to:
- increase the lenght of a network
- increase number of nodes
- make network-segments
- make a legal ”tree” or ”star” structure. (Branched- and star-networks are not allowed with RS485, only line structure with a limited lenght of stub cable)

The driver/receiver on each side of the repeater must be counted according to the 32 limit, so with 7 repeaters on a straight line a maximum of 2*31 + 6*30 = 242 nodes can be used!

This is the hardware limitation, but as Pierre stated, the communication turnaround-time increases fast with increasing number of nodes. For best performance, more than 1 comm.port should be used.


Regards
Karl Egil Liaset