Communication gets slower in DH485 network

[Sebu 0]

I have 18 SLC 5/03 PLCs. I have connected 12 PLCs in DH485 network. If I try to add one more PLC to existing network the speed of the network is becoming very slow. Could anyone suggest me what can be the problem? What are the parameters should I look at? Sebastian C

[b_carlton 61]

1. Make sure your network addresses are sequential starting at 1. 2. Set the 'Max Station Number' on all stations to the highest number in your network. If this still doesn't help consider dividing your network.

[BobLfoot 302]

You might also look at what data you're exchanging and how you're exchanging it. Scattered blocks across different files and the use of writes rather than reads can eat up the available bandwidth. Also define slow. For a 12 SLC DH485 network to pass a number from SLC A to SLC B and Back to SLC A anything under 10 seconds would be fast in my experience.

[Sebu 0]

Hi Bob, There is no messaging between any PLCs. All PLCs are for independent process. These are connected to DH485 network for the reason the data from all PLC should be displayed in SCADA.

[BobLfoot 302]

Minor Sticking point but there is messaging on your 485 network. 1. The DH485 token is passing station to station whether or not the plcs are messaging. 2. There is at least one SCADA to PLC message and possibly several happening. Referencing item 2 above - It is best practice to package you SCADA Data into one Integer File for example N77 and pack that file tight. So you read N77:0 thru N77:31 for example to transfer data to SCADA. Example Easy versus Efficient SCADA Easy SCADA Scada reading N7:1, N7:100, N7:299, I:5/3, I:6/4, O:7/2 and O:8/8 that is seven message packets for one PLC. Efficient SCADA Scada reading N77:0 thru N77:3 after PLC moves N7:1 to N77:0 ; N7:100 to N77:1 ; N&;299 to N77:2, I:5/3 to N77:3/0, I:6/4 to N77:3/1, O:7/2 to N77:3/2 and O:8/8 to N77:3/3 is one message packet for the PLC. Given you are using DH485 you must be efficient.