AB DeviceNet


Thread Starter


I am setting up a DeviceNet card with a SLC 503 processor. I am having trouble getting RSNetworx to go on line. A 79 fault appears all the time. Yes, I have checked the cable connections 100 times. Help...

William Sturm

Try changing the baud rate of RSNetworx. Also, if you have more than one slave device, try bringing them up one at a time. You will have to match baud rate with each one. Then you can change them. It can be a very difficult task.
Error code 79 indicates that the 1747-SDN has not successfully exchanged a message with any other device on the network. There can still be a valid network attached, but the 1747-SDN might have no reason to communicate with any of the devices on it.

This can show up if there are no scanlist entries, or if there is a damaged receiver section of the CAN interface on the 1747-SDN.

Are you trying to "go online" with an external tool and 1770-KFD or 1784-PCD, or are you trying to use the 1747-SDNPT passthrough driver?
79 Module has failed to transmit a message. Make sure that your module is connected to a valid network. Check for disconnected cables. Verify baud rate.

RA tech support should be able to solve this quickly. Give them a try.

wire in one node only if possible ensure it is default programmed or set the dip's to auto baud etc. Set address to something other than scanner.

check you have 24Vdc on both scanner and node in the correct places.

I found this somewhere years ago and it is good to know.

It needs to be understood that DeviceNet is actually a three wire Differential Voltage network. communication is accomplished by switching the CAN-H ( White wire ) and CAN-L ( Blue wire ) signals relative to the V- line ( Black Wire ). The CAN-H swings between 2.5 VDC (Recessive State) and 4.0 VDC (Dominant State) while the CAN-L swings between 1.5 VDC (Dominant State) and 2.5 VDC (Recessive State)

Without a network master connected to the DeviceNet, the CAN-H and CAN-L lines should read
between 2.5 VDC and 3.0 VDC relative to V- and the voltages should be identical. ( Recessive State ). Measure these voltages right at the SDN scanner. Use a voltmeter in DC mode.

With a network master connected and polling the network, the CAN-H to V- voltage will be around +3.2 VDC. The CAN-L to V- voltage will be around 2.4 VDC. The reason these values appear a little different than the ranges shown on the scope trace, is that the signals are switching, which slightly affects the DC value being read by the VOM.

If Can-H to V- and Can_L to V- are too low; less than 2.5 V dc and 2.0 V dc respectively the issue is probably a bad transceiver or bad wiring. To find a bad transceiver remove one node at a time measuring Can-H and Can-L to V- each time a device is removed.

To check a transceiver (rough test) with everything removed from a device use an ohm meter to measure resistance between V+ and Can-H V+ and Can-L V- and Can-H V- and Can-L. These impedances should all be greater than 1 M ohm.

Good luck.