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Messages - ccdubs

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91
Technical support / MODBUS RTU and PLC address
« on: February 05, 2008, 06:20:02 PM »
Hello again.

I am having some difficulty using MODBUS RTU where I have 20 remote PLCs connected to a SCADA system running DAQFactory.

All comms works OK apart from the PLC's with addresses in the A to F range. It seems that when these PLC's are addressed for reading that all the others on the line also try and respond as if they all had the same address! This doesn't happen when polling the other PLC's.

Any ideas on why this may be?

92
Technical support / Re:Output State on Power Off
« on: January 21, 2008, 02:50:59 PM »
Thanks for the suggestion.

One last question. Where is the low impedance path from +24V to GND coming from? Is it the power status LED only? If so I could cut this and/or its resistor off.

93
Technical support / Re:Output State on Power Off
« on: January 20, 2008, 11:52:58 PM »
Thanks for getting to the bottom of this. I am already committed to using the PLC's I have (30 of them!) and by using the 2 mosfets I have a workaround that I am comfortable with. I really like the TriPLC and have enjoyed learning how to use them.

It may be worthwhile looking at another darlington driver cct that won't cause this issue....but I guess if I am the first customer to have this issue it may not be.

If anything comes up to help me with my other 2 outputs (also driving a PIC input but slightly less critical) on pins 6 and 5 please let me know.

94
Technical support / Re:Output State on Power Off
« on: January 20, 2008, 10:57:41 PM »
Hi Support,

I have already performed this test, please refer to my post on 19 Jan 05:17:

"I have done a further test where I completely disconnected the PLC from everything and applied 3.88V (3 x AA cells) to output 4 and 0V. The measured current was 4.5mA and I noticed that the LED next to the power supply connector was faintly glowing."

I know that there is at least one other component in my system that is contributing to the issue and I am dealing with that seperately.

I disagree with your first comment, as it would be expected that an output would be off (open cct) when a unit is depowered otherwise there is no failsafe capability. If the PLC and all loads are on the same cct failsafe is automatic but I am sure there would be many applications using seperate supplies especially with remote IO apps.

Perhaps you could perform the test I have conducted on a PLC you have there. It may be that the batch I have has a faulty component.

95
Technical support / Re:Output State on Power Off
« on: January 19, 2008, 07:45:00 PM »
Yes the 0V is common.

I can't measure the exact output to gnd impedance but it is higher than 3Mohms.

This doesn't explain why when I put the 3.88V on the output I measured 4.5mA which equates to ~860ohms or why the green LED next to the power connector marked P1(i think) goes on during the same measurement.

Surely the leakage current of the darlington transistors wouldn't be this high at such a low voltage?

It is very important in this application that false activation risk is almost eliminated, hence all the questions  :)

96
Technical support / Re:Output State on Power Off
« on: January 19, 2008, 02:17:44 PM »
Upon doing more testing I have found the following:

With PLC DC-DC power supply completely isolated:
- PIC V = 2 V
- Remove PLC +V: PIC V = 2V
- Remove PLC 0V: PIC V = 3.24V
- Remove both PLC +V & 0V = 3.24 V

With PLC DC-DC power supply outputs connected but no inputs, therefore depowered:
- PIC V = 1.05V
- Remove PLC +V: PIC V = 2V
- Remove PLC 0V: PIC V = 1.05V
- Remove both PLC +V & 0V = 3.24 V

This would tend to show that there are 2 paths that lead to ground:

One within the DC-DC converter output (measured at 470ohms between + and - on output)

The other between PLC output and it's 0V power terminals (measured at 760ohms between ouput and 0V).

Ironically it is only when both of these paths are enabled that the PIC voltage drops low enough to cause and activation, however it surprises me that it seems like the PLC ouputs when depowered are not isolated from 0V.

I have done a further test where I completely disconnected the PLC from everything and applied 3.88V (3 x AA cells) to output 4 and 0V. The measured current was 4.5mA and I noticed that the LED next to the power supply connector was faintly glowing.

When repeating the above test I found that outputs 7 and 8 don't draw any current and when I connect them to the PIC there is no action. So these seem to work as predicted.

97
Technical support / Re:Output State on Power Off
« on: January 19, 2008, 01:12:16 PM »
When the PLC is depowered (by turning off its supply) the voltage on the PIC input pin is 1.05V. If I remove the PLC ouput connector this raises to 3.24V, the same happens if I disconnect the PLC power connector.




98
Technical support / Re:Output State on Power Off
« on: January 19, 2008, 11:32:28 AM »
The PLC is a T100MD888+.

Yes the input pins of the PIC are pulled up by 10k resistors, they also have a 10k resistor in series to limit the current driven by the PLC from its outputs in the OFF state due to the PLC's pull up resitors. This also means that the PLC output LED's are always ON.

Is it possible that the issue is being caused by the pull up resistors in the PLC. What were to happen if they were removed?

Thanks.

99
Technical support / Re:Output State on Power Off
« on: January 18, 2008, 03:28:10 PM »
Hi Support,

The code in the PIC requires the input to remain low for the action to occur so I don't think it is anything to do with momentary glitches during power down. The undesired action will be continuous upon PLC power down hence showing the PIN is pulled to ground idefinetely.

What I would like to know is via what "low impedance" components in the PLC are the outputs connected to the +ve supply?

Is it the pull up resistors, LED's?

Thanks

100
Technical support / Output State on Power Off
« on: January 17, 2008, 02:59:14 PM »
Hi All,

Here's another one for you related to the power loss topic posted previously.

I have a PLC whose outputs pull down input pins (5V high) on a PIC micro to perform certain actions. The PLC is powered by a seperate supply which shares the same gnd as the PIC supply.

When the supply feeding the PLC shuts down, the pins on the PIC get pulled low and unwanted action occurs. I found that if I disconnect the output connector on the PLC that the PIC action stops. The same is true when I installed the diode in the +ve supply to the PLC.

The path to gnd is probably through some other load but are there any extra measures (beyond the diode in the +ve line) I can take within/on the PLC to stop this. I can't put diodes in each ouput as the Vdrop across them doesn't let the PIC recognise a low on the input pin.

Once again any and all help appreciated.

101
Technical support / Losing program with power outage - revisited
« on: December 22, 2007, 03:17:45 PM »
Hello,

I am experiencing some issues not unlike the other recent posting. I have a TriPLC being powered at 12V from a 24 to 12 V DC-DC converter which is fed from a 24V battery.

When the mains fails and the battery begins to drain the PLC code corrupts (goes into pause, loses ID and requires download password). I think this is due to the DC-DC converter which seems to cycle when it's input votage gets too low. According to the data sheet I thought the DC-DC converter unit would completely shutoff at 19V but it seems more like 10-12V input.

Can this cycling be causing the PLC code to corrupt? I have installed a 1000uF capacitor and it made no difference.

Short of buying a new converter are there any other recomendations to prevent this from happening.

The converter is a MeanWell SD-50B-12 which I would have thought was a reasonable product.

Any help is greatly appreciated.

Thanks

102
Technical support / Re:RS485
« on: December 08, 2007, 03:50:02 PM »
Thank you for the explanation.

103
Technical support / Re:RS485
« on: December 07, 2007, 02:27:16 PM »
Yes thanks all has worked well.

Apart from spending half an hour trying to get comms with my SCADA to work with the PLC. This turned out to be another Duh! moment where I forgot that the PLC ID is in Hex and the SCADA works in decimal.

That begs the question....why does the PLC ID need to be entered in HEX. Can't the interface perform a conversion on the users behalf?

104
Technical support / Re:RS485
« on: December 06, 2007, 06:00:56 PM »
Whoops, posted too soon.

I forgot that I was setting only comm1 to 9600 Baud but comm3 was still at default of 38400.

Duh!!!

105
Technical support / Re:RS485
« on: December 06, 2007, 05:26:30 PM »
Hi support,

I received my Auto485 today and it worked immediately off my PC's serial port.

But, I ideally want to be able to use my laptop so i can reprogram in the field. As my laptop doesn't have a serial port I need to use a USB to serial converter. I have tried to use a Prolific USB-RS232 converter which then goes to the Auto 485 and it just won't work.

Do I need to change any jumpers on the Auto 485? Have you tried with USB-Serial converters before?

I would have thought this would have been a common situation amongst automation engineers.

Thanks.

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