Using ASCII on DH485 channel in Allen Bradley ML1200

[schembm]

Hi,
I am using an Allen Bradley micrologix 1200 plc with two serial channels. I need to connect to a General Electric Motor Drive (VAT-23D) which connects serially using RS-485 but accepts instructions in packets of 48 bytes of ASCII code.

I have tried to set the plc channel to ascii but the drive did not recognise the protocol I think because I recieved nothing and when I set the plc channel to DH485, an error developed in the ASCII Write (AWT) command stating that the local modem is not ready.

Any help is much appriciated!

Thanks!

 

[sthompson]

Post your PLC Code RSS File (Zip it first - Forum Rule) and we will have a look at it.

Stu....

 

[schembm]

Thanks for your reply Stu.

Here is the PLC code attached, the relevant programming is in ladder 4 (COMM).

I do not know if maybe there is another way how to send an ASCII string on RS-485 protocol.

 

[sthompson]

I see a few problems here.

Hardware:

You stated the GE VFD uses RS-485, but the two Ports on the ML-1200 are both RS-232. The DH-485 selection on the ML-1200 is a Protocol only. You will need a RS-232 to RS-485 converter such as a 1761-NET-AIC to convert between the two Serial Standards.

ML-1200 User Manual:

http://literature.rockwellautomation.com/idc/groups/literature/documents/um/1762-um001_-en-p.pdf

PLC Program:

You might want to use Trigger Timers, as you are calling the Comm Subroutine every Scan. Also, you are not allowing enough time between your Write and Read. (1mS?) Some Interlocking between the Write and Read Instructions would be nice as well. You stated the ASCII is 48 Bytes, but you have an 11 Byte Write, and you don't specify a length for the Read.

ML-1200 Instruction Reference:

http://literature.rockwellautomation.com/idc/groups/literature/documents/rm/1762-rm001_-en-p.pdf

ASCII Instructions are in Chapter 20.

Hopefully Ken Roach will chime in here. He is the Guru when it comes to AB Communications.


Stu....