Siemens PLC w/ CP340 connected to Sartorius CP scales

Hello,

We have a Sartorius CP scale connected via RS232 to the PLC. All was working well but when the scale was replaced with a newer version (still of Sartorius CP series), the reading on the PLC has changed. A reading of zero grams is read as -2.016 g on the PLC. What could be the possible reason for this? The scale installed now is not a direct replacement of the old one since we couldn't find the old model. Could there be a possible issue on the pin connections of the RS232 cable? We've connected only three pins for both devices, Tx, Rx, and GND.

But when the old scale is put back, the reading is OK again.

Can someone please assist us?

Thank you & best regards,

dbm

Check the documentation of your old Sartorius and your new one. Perhaps the protocol and/or settings are different between the 2 versions.

Hi sigmadelta,

Well that's the problem.. There doesn't seem to be any documentation for the old Sartorius. The casing of the old scale was all worn out and after inspecting it, checking inside and out, no part number was present (or maybe I didn't see any coz the body is all worn out). From what my superiors told me, they acquired the replacement scale by viewing the device catalog and looked for a unit that had a similar appearance with the old scale. However, according to them the replacement scale was a successor of the old one. Though they definitely have the same casing and buttons, there is no surefire way to say that the latter is a successor of the prior. Opening the case of the two scales, we determined that the new scale has some additional circuits.

Hope someone can shed some light on this.

Thank you again,

dbm

Do you have access to your STEP 7 software? You can check the RS-232 settings for the CP340 in the hardware configuration, and then make your new scale's settings match.

(ex: 9600 baud rate, 8 data bits, parity: none, 1 stop bit)

Yes I do, I recently acquired the Parameterization software for the CP 340 and upon checking they seem to have similar settings with that of the scale, baud rate, data bits, stop bit and parity. I'm not so sure, however, with the handshake mode, I'll still have to verify it. May I ask, is it necessary to connect the DTR and CTS pins of the RS232 cable? Coz apparently in the setup we have now only TxD, RxD and GND are connected.

Thank you for such quick response.

b/r,

dbm

What happens to the -2.016 value when you put weight on the scale?

dbm said:

only TxD, RxD and GND are connected.

Click to expand...

It is enough with 3 wire connection it is what you usually see.
If 3 wires was connected before and it was working then it will work with 3 wires also with the new scale.

If you are sure that the settings (Baudrate,Parity,Flowrate etc) are the same on both scales then the protocol (Data that the scale is sending) has probably changed

The protocol is probably in ascii (Readable text) so you can connect the scale to a computer using usb to serial interface and use Putty or hyper terminal to see what the scale is sending. Hopefully the scale sends data all the time but it can also be the case that the plc sends some sort of trigger command and the scale responds to that. If you have access to the PLC program then you can check if the send block is called or if only the receive block is called in the program. If only the receive block is called then the scale sends data without trigger.

If you have access to an old laptop with XP then it most likely has a built in serial port and hyper terminal. Hyper terminal was shipped default with win XP but it was removed in windows 7

I deal a lot with heavy truck scales and serial comms. From your description, it may be that there are settings in the old scale that were not transferred to the new scale.
First thing I would try is putting a known weight on the scale and seeing what the PLC reads. If the value read by the PLC changes (even though it may be wrong) , and it is formatted correctly, then you know the comms are correct and something else needs to be setup in the scale. If the value doesn't change or the formatting changes when the weight changes, then you may have comm settings that are not correct.

You should not have to set up handshaking unless the PLC requires it since the scale is probably output only. If the scale is an on demand type (you have to send a command to get the weight) then handshaking MAY be required depending on how fast you are asking for the weight.

One last thing, if the value read by the PLC changes as the weight changes, but is always off a certain amount compared to the scale display then there may other settings in the new scale that have to be set correctly. I would contact the scale manufacturer and ask for assistance.

Hi all,

To answer the first question, when the weight on the scale changes, the weight read by the PLC also changes but apparently it is not the same with the one in the scale. We are having quite the trouble with this because weight reading displayed on the plc is not in real time. The weight reading is displayed after the process is done filling the container placed on the scale. And we can't test this properly since the filling is started only when the PLC reads a zero (o) on the scale, which doesn't happen since the zero reading on the scale is -2.016 on the PLC, this activates an alarm which halts the program sequence. Apparently this is a PLC program issue. We're still trying to bypass this condition for testing purposes.

Handshake mode for the scale is of 3 types: (1) Software, (2) Hardware, 2 char. after CTS, (3)Hardware, 1 char. after CTS. On the PLC "Data Flow Control also has 3 types: (1)None (2) XON/XOFF (3) RTS/CTS. From the setup done by the on site technicians, "Hardware, 1 char. after CTS" was set on the scale while on the PLC "None" is set. The setup on the PLC wasn't changed at all, retained the original settings from the old scale. The only variable parameters in our setup was that of the scale.

Do these above information have any effect on the data read by the PLC?