We are using an Allen Bradley SLC-5/05 in Chassis 0 with a 1746-SN Scanner to communicate with three 1794-ASB/D Remote I/O Adapters (Racks 1-3) along with their associated Flex I/O Modules. Periodically the last rack (Rack 3) faults for no apparant reason and indicates that the PLC is in Program/Test mode even though it is still in Run Mode (Local Fault LED = Off; Adapter Fault LED = Off and Adapter Active LED = Blinking).

RIO Communications can operate flawlessly for long periods of time between faults or sometimes can fault several times within hours. It is totally unpredictable (intermittant). The baud rate is set at 57.6k and a 150 ohm terminating resister is installed at both the scanner end and the last physical device end (I/O Adapter for Rack 3). Initially, since the last chassis was always the one to fault, we suspected that the problem might be the setting of dip switch S2-6 which defines which Chassis is the last one. Allen Bradley said that this dip switch setting is irrelevant with all 1794-ASB RIO Adapters above Series B. Since all of our adapters are Series D we eliminated this as the potential cause. Any/all suggestions would be appreciated. Thank you!

 

Take a close look at your RIO media. Usually intermittent RIO problems like this can be traced to your wiring or wiring routing, especially if there is some distance between racks or you have noise sources in close proximity.

 

Remove the resistor at the Scanner end.

 

> Remove the resistor at the Scanner end. <

According to the Installation and Wiring diagram that Allen Bradley faxed us, from their Publication 1747-6.6 - July 96; Page 3-5, a terminating resistor must be used on both ends. Have you personally tried operating with only a resistor on the adapter side? Please reply again! Thanks!

 

Any RIO system needs two 150 ohm resistors on the physical ends of the network to function properly at 57.6 kb. Leave those termination resistors alone.

Check your shield and drain connections carefully. I've seen lots of RIO networks where a "whisker" of stranded wire from the clear or blue wire shorted to the shield drain causes noise induction and random faults.

Are all the segments of the daisy-chain more than 10 feet long? You can set up some reflected-wave trouble with segments less than that minimum length.

Check also the grounding of the shield drain at the 1746-SN scanner. The recommendation for where to tie this shield (at the module or at the chassis) has changed over the years; refer to the installation instructions for your Series of scanner.

Make sure all of your FLEX blocks are well seated together. More than half of the FLEX problems I've seen result from poorly seated bus connectors or bent pins on the FLEX bus packplane.

 

Thanks for your suggestions. I'll let you know what we eventually find out!

 

The adaptor is in test mode not the scanner modual. When you first put power on an ASB modual it scans it's flex I/O and logs what it finds. If for any reason the connections between any of the TB-3 terminal blocks is disrupted and the ASB looses one of it's flex units it will fault. At this point if the ASB is powered down, when power is returned the ASB will rescan, removing the flex units it can't see. The Adapter Fault LED will go out but the Adapter Active LED will continue to blink because the MO and MI files will not match the ASB memory. If this is in fact what happened to you, when the Adapter Active LED is flashing remove the last flex I/O modual by unpluging it from the terminal block(with out turning off the power, it's OK to do this). If the Adaptor Fault LED does not flash then the unit you unpluged is not on the ASB's active list. Go down the Flex I/O from last to first until you find a modual that does make the Adaptor Fault. Your connection problem is in this area. Hope this helps.

 

Ken:
Is the 10 ft. minimum length per segement an A-B specification? I have never heard of this. If it is a specification, please tell me where it is specified. I have frequently installed two or more chassis in the same enclosure with only a few feet of cable between them and have never had any problems.

 

Hello
I had have i problem like yours for halfyear ago. The problem was the power supply and the commmon connection, my suggestion is to messure all powersupplies, both 5V and 24V and the main power supply. Measure the different bettwen ground and common.
Best regards fredrik

 

Thanks Ed! You raised some very interesting possiblities in your response to my plea for help. It appears as though you have experienced a similar problem in the past. Since you are essentially anonymous, I cannot send you a direct e-mail or telephone you, so would you please be so kind as to e-mail me your telephone number. Then we (there are two other fellows that are directly involved in this issue and they have questions as well) can discuss this matter with you personally. Thanks again Ed! Hope you have/had a great weekend!

 

A few suggestions.

1. Try swapping the the ASB that is failing with one of the other ones that does not and see what happens. If the problem stays with the same adapter, replace it. If it stays at the same rack go to suggestion 2.

2. Check the cable to the last rack. Might be worth even replacing it. Make sure it does not get near any motor or VFD power wiring. Make sure you put the correct value of terminating resistor on - check it with a meter.

Make sure its the correct value for the communications rate you are using (150 Ohm sounds right for 57.6kbaud). One time I saw an intermittant RIO problem that turned out to be that the terminating resistor had some plastic
wrapped around one lead so it was insulated from the cabling, might as well not been there.

Bob Peterson