ControlLogix Serial Port - Not Echo'ing back

[skyfox]

Does anyone know what would cause this or how to get it to Echo the received data?

Controllogix 1756-L55.

My understanding is if Echo option is checked, Controller is supposed to "Echo" the received data back to the originating device. Somehow my controller is not doing that (ver 16).

Any ideas as to what to look for?

Thanks

 

[Ken Roach]

I admit I've never tried to use either the Echo or the Delete Mode selections.

Maybe the controller only echoes when it receives a Termination character; try changing Termination Character 1 back to carriage return ($0d) and then test it with a terminal program that sends Carriage Returns.

 

[ctipping]

Make sure that you are not set for Half-Duplex, this mode "disables" the echo.
Echo will send back exactly what was received, it does not append anything regardless of the controller settings, so if the originating device requires a termination character to "see" the command, it must send one to the Contrologix.

 

[ctipping]

Temporarliy using "hyperterminal" in place of your field device, will allow you to see what is being "echoed" (ie,.. you should get back what you type and send)

 

[skyfox]

Thanks Ken/Ctipping,

Well, found an anomally and not sure why this works this way.

Starting With PLC set at;

ECHO: OFF
Term Character: OFF
Buffer Size: 1
Transmitting from Test Program to PLC "$FF"

No Echo As expected:

Now turn only ECHO ON and hit "Apply": Still no Echo from PLC

Then did the following;

1) Sent Characters several times - Still No Echo
2) Adjust PLC Buffer Size from 1 to to 2 and Hit Apply - Echo starts working.
3) Turn PLC Buffer size back To 1 : Echo still continues to work
4) Turn "OFF" Echo and Hit Apply: Echo Still Works.
5) Adjust PLC Buffer Size and hit apply: Echo Stops
6) Return Buffer back to the original value where Echo Started working: No Echo
6) Turn ECHO back on and Hit apply: Still No Echo
7) Adjust Buffer Size again (to some random #): Echo starts working again.
8) Put the buffer size back to "1", where it did not echo prviously: Echo Still Continues to Work.

What Gives? I tried various combo's. ie. Going from "Run-Program back to Run" after each change, and then only hitting "Apply" after each change. Same behaviour in both cases. It seems Once echo starts working, turning it on/off has no effect and it stays in the last state that it was in. That is until buffer size is changed. Changing the termination charactor or disabling it had no effect on same sequence of steps above.

One thing I did not try was to Powercycle the PLC between changes. Just a bit puzzeled by this behaviour.

 

[ctipping]

I am curious as to why your buffer size is so low ?
$FF is 2 bytes of data.
I have never had the need to change the buffer size from the default of 82.
Not sure how it would handle the "echo" but if the number of bytes received is larger than the buffer, the extra characters are "dropped" by the Contrologix (on a read), on a write (from Contrologix) this will cause a minor error.

 

[skyfox]

I am curious as to why your buffer size is so low ?
$FF is 2 bytes of data.
I have never had the need to change the buffer size from the default of 82.
Not sure how it would handle the "echo" but if the number of bytes received is larger than the buffer, the extra characters are "dropped" by the Contrologix (on a read), on a write (from Contrologix) this will cause a minor error.

I am kind of experimenting with this. I need to set-up the controllogix to communicate with the other device where the TX/Rx are based on a single byte transfer between the two devices (which actually takes two bytes to transmit it's decimal value).

1: CLGX - Sends One Byte

2: Device Echo's it and then send the Data byte Clgx requested.

3: CLGX echos the received data byte.

There are no Termination characters (CR/LF) used by the devices.

Echo is used for confirmation of TX/RX between the two. Please see the attached image. With the buffer size set to one in logix, I still get the two-byte transmission in to my destination tag.

So is the buffer size of 1 actually two bytes?

Trying to make sense of all this.