ecotechinstructor

Allen Bradley 1746 specs frustration

6 posts in this topic

Is it common for vendors to list the wrong specs for products? I bought a 1746 IV16 on eBay dirt cheap because it was listed as being a digital sourcing input module. When I go to Rockwellautomation.com , it agrees with this. I've only checked two other vendors, and they both have contrary info listed. PLCProducts.com says it receives analog via current or voltage, and qualitrol.com says it's a digital sinking module. Oh, and the text I'm teaching this from by Rehg and Sartori have it listed in a table of "discrete i/o modules" and call it a sinking module.

Im assuming that qualitrol, eBay, Rockwell, and the text have it right that it receives digital and not analog inputs. But the text and qualitrol may be wrong in their labeling it a sinking one and not a sourcing one as Rockwell and eBay (not usually my most trusted source) have it listed. 

In the end, it was $8 and I don't care if it isn't what I thought it was, but in your experience, do you find such wild inconsistencies in specs? Thanks!

Edited by ecotechinstructor
More detail needed

Share this post


Link to post
Share on other sites

Not typically, but sometimes. In this case, I believe the Rockwell site. Their 24VDC sinking modules are usually "-IBxx", where "xx" is the number of points (usually 16 or 32). When I need something on E-bay or PLCCenter (or etc....) I identify the part number I need first from the manufacturer's site and then search for that part number. Very rarely do I have to rely on the descriptions on the third-party vendor sites.

1 person likes this

Share this post


Link to post
Share on other sites

The terms "sink" and "source" probably cause more confusion than any of the other jargon in our area of expertise. My personal rule of thumb is to look at the connection diagram for the component.

In the case of a PLC module, if the terminal that is common to all of the inputs (or to each group of inputs) is DC+, the module is a sourcing module. If the common terminal is DC-, the module is sinking. Some modules allow you to connect either DC+ or DC- to the common terminal, meaning they can act as either sources or sinks.

The confusion arises with input modules because you connect a sourcing field device to a sinking input module and a sinking field device to a sourcing input module. Also, if you check the voltage at the PLC terminal with your multimeter, a sinking input shows a positive voltage when the input is true while a sourcing input shows a positive voltage when the input is false.

It is also possible with a pull-up or pull-down resistor to connect a sourcing field device to a sourcing input module or a sinking field device to a sinking input module. If you do that, you find that the input point in the PLC program is true when the sensor signal is false and false when the sensor signal is true.

DC output modules can be sourcing or sinking, but the field devices connected to them generally don't care. A DC solenoid will operate equally well connected to  a sourcing or sinking PLC output. However, if you use a diode as a suppression device, you need to wire it following the correct polarity.

The documentation I have for the 1746-IV16 module (Publication 1746-5.3 dated June 1996) calls it a sourcing module and the wiring diagram shows the module's common terminal as DC+.

1 person likes this

Share this post


Link to post
Share on other sites

Thank you Steve. This is good info. But really my concern is that if vendors can't get specs right, how often is time and money wasted on ordering the wrong item? Do you run across vendors with incorrect specs often?

Share this post


Link to post
Share on other sites

My personal experience is that you are more likely to find mistakes or misleading statements in advertising copy. Less likely in specification sheets and wiring diagrams. OEMs are less likely to have incorrect information about their products than resellers.

Share this post


Link to post
Share on other sites

I'm just sitting here hoping no one is using an SLC in a new application.

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!


Register a new account

Sign in

Already have an account? Sign in here.


Sign In Now