Micrologix Devicenet limitation

Hi guys,

I'm configuring an architecture using MLX 1500 (LRP processor) with Devicenet (1769-SDN), communication with 3x Flex IO (1794-ADN). I've about 200 IO points (80% are digital) over a DNET link 100m from the MLX 1500. Is there any limitation using MLX 1500 with Dnet?

Thanks guys....

You can run up to 500 meters at 125 kb/s with DeviceNet, and about 100 meters at 500 kb/s, so your distance is fine.

The 1769-SDN scanner has a network data memory map of 180 Words (16-bit each) of Input and 180 Words of Output.

When you create a 1769-SDN in the I/O configuration for the MicroLogix 1500, the I/O sizes are 66 words of Input and 2 words of Output data. Those are the Status/Diagnostics and Command data elements. You add to those for the I/O data map area. These are very well described in the 1769-SDN User Manual, publication number 1769-UM009.

Approximation of I/O data size: If 80% of your 200 I/O points are discrete (total 160) and you are using 16-point modules, that's just 10 words of data. Adapter diagnostics is 1 word per FLEX adapter. 40 analog channels at 1 word per is only 40 more words, so we're well below the limit for I/O data size.

One mistake I see often: you can't pass through a MicroLogix 1500 to the DeviceNet like you can with a CompactLogix or ControlLogix. You will need a 1770-KFD, 1784-PCD, or 1784-U2DN to connect to and configure the DeviceNet and the FLEX I/O modules.

Also, even though it is tempting, do not use the same 24V power supply for the FLEX adapters and I/O and the DeviceNet network. The DeviceNet must have a dedicated 24V power supply that only supplies power to the network ports.

Ken Roach said:

You can run up to 500 meters at 125 kb/s with DeviceNet, and about 100 meters at 500 kb/s, so your distance is fine.

The 1769-SDN scanner has a network data memory map of 180 Words (16-bit each) of Input and 180 Words of Output.

When you create a 1769-SDN in the I/O configuration for the MicroLogix 1500, the I/O sizes are 66 words of Input and 2 words of Output data. Those are the Status/Diagnostics and Command data elements. You add to those for the I/O data map area. These are very well described in the 1769-SDN User Manual, publication number 1769-UM009.

Approximation of I/O data size: If 80% of your 200 I/O points are discrete (total 160) and you are using 16-point modules, that's just 10 words of data. Adapter diagnostics is 1 word per FLEX adapter. 40 analog channels at 1 word per is only 40 more words, so we're well below the limit for I/O data size.

One mistake I see often: you can't pass through a MicroLogix 1500 to the DeviceNet like you can with a CompactLogix or ControlLogix. You will need a 1770-KFD, 1784-PCD, or 1784-U2DN to connect to and configure the DeviceNet and the FLEX I/O modules.

Also, even though it is tempting, do not use the same 24V power supply for the FLEX adapters and I/O and the DeviceNet network. The DeviceNet must have a dedicated 24V power supply that only supplies power to the network ports.

Click to expand...

Ken,

Thanks a lot for your information and advice. I'll invest on a 1784-U2DN and take noted on the power supply that you highlighted. Really appreciate that.

Care to explain, any particular reason we are not advice to share the 24V DC power for the DNet? Just curious, if we double up the 24V DC power supply Amp, will it help (it can reduce overall cost and size of the panel).

DeviceNet uses a power pair and a signal pair for three reasons:

1. To provide power to small devices with a single connector (i.e. photeyes, limit switches, motor overloads).
2. To provide isolation between the power circuitry for the network interface and the often-noisy power circuitry for the rest of the system.
3. To reduce common-mode voltage drop across the DeviceNet length, a common drawback to CAN network buses.

When you have a device with a DeviceNet network port and a power plug right next to one another, like the 1794-ADN, it's very tempting to just use the same power supply for both. And when there's 24V DC or analog I/O nearby, it's tempting to use the power supply for all three !

But if you do, you open yourself up to problems with Reasons #2 and #3. It might work at first, or it might work sometimes, but if you have a problem it's the first thing you have to fix and it's usually a lot more expensive to add a power supply later instead of adding it during design.

I've seen a great many first-time DeviceNet users ignore the fundamental wiring rules and have a lot of trouble that caused them to have a poor opinion of the network. These were guys who I'd already seen front and center during my lectures on DeviceNet so I knew they had heard me explain voltage drop and isolation. But the temptation was too much.

If your I/O is analog or 120V and you will be providing a separate power supply for the I/O circuits, you could use a DeviceNet-rated power supply at each 1794-ADN for just the Adapter and the Network, and then not connect the red wire on the DeviceNet trunk. This would eliminate common-mode voltage drop and reduce potential noise problems to a minimum.

If your I/O is analog or 24V DC, you could power both the 1794-ADN and the I/O with the same power supply, then put a good 4A DeviceNet-rated power supply back at the MicroLogix/Scanner location or (ideally) in the middle of the DeviceNet trunk.

Since this is your first DeviceNet system, I still strongly recommend following the DeviceNet cable system design rules and dedicating a DeviceNet-rated power supply for the network only.