A reader recently asked what the difference was between Point I/O and Flex I/O, and in today's article I'll share my thoughts on the subject.

Note: Those not familiar with Rockwell's I/O products may also want to consider 1756 I/O, 1769 Compact I/O, 5069 Compact I/O, and 5094 Flex 5000 I/O, all of which also support being used as distributed I/O.

How Point I/O and Flex I/O are similar

Let's start by comparing features common to both lines.

Distributed I/O Focus

Both Flex I/O and Point I/O are modular, in-cabinet distributed I/O systems that are designed to be used as distributed I/O.

Flex I/O System with Ethernet Adapter
Point I/O System with Ethernet Adapter

While in years past up to sixteen local Flex I/O modules could be controlled by a local FlexLogix controller, those controllers have long been obsolete.

And while it's also true that 5371 CompactLogix controllers can use Point I/O for local expansion, that support tops out at eight modules with the L18.

In contrast, 1769 and 5069 Compact I/O not only supports being used as distributed I/O, but can also be used as local I/O modules in CompactLogix systems with up to thirty modules.

So today it's safe to say that both Flex I/O and Point I/O are predominately used in distributed I/O applications, as neither has it's own dedicated controller supporting large numbers of local modules.

Plug-In Modules

While Flex I/O and Point I/O physically look quite different, each line consists of I/O modules which plug into terminal block bases.

Flex I/O Module Terminal Bases
Point I/O Module Terminal Bases

This design allows modules to be removed and replaced without disturbing field wiring, and many panel designers have taken advantage of this design to eliminate interposing terminal blocks commonly used with standard “rack based” PLC modules.

Flex I/O and Point I/O modules also support “Removal and Insertion Under Power” (RIUP,) a must have feature in continue process applications.


Flex I/O and Point I/O both support being mounted either horizontally or vertically, while most of Rockwell's other I/O Modules (aside from the Flex 5000) only support being mounted horizontally.


Today, both the Flex I/O and Point I/O support Ethernet, ControlNet, DeviceNet, and Profibus DP distributed I/O networks.

Flex I/O DeviceNet, ControlNet, and Ethernet Adapters
Point I/O DeviceNet, ControlNet, and Ethernet Adapters

How Point I/O and Flex I/O are different

Modules and Bases of a Flex I/O System:

The first thing you'll notice when you compare Point I/O and Flex I/O are how different they look.

Flex I/O modules are wide and rectangular, and plug into a terminal base that looks very similar to a standard three level terminal block.

Flex I/O Modules

Before I/O modules can be installed, the module bases must be connected from right to left, starting with the right most terminal base and ending with the adapter.

Flex I/O Module Terminal Bases

These terminal bases support being mounted directly to a back-panel, or being installed on din-rail.

Once mounted, to connect each terminal base a bus connector must be slid from right to left, which is actually one of the weak points of the Flex I/O design.

If not lined up correctly (which typically happens when the bases are not mounted on a flat surface,) the pins in the connecting socket bend quite easily.

In addition to bent pins, the little plastic “hooks” on the top left of the terminal bases also break off quite easily.

As far as the total number of Flex I/O modules supported by the above mentioned netowrk adapters, each supports up to eight modules which can be split into two rows with the use of a Flex I/O bus extension cable (right to right) as shown below:

Keep in mind that if you do split your Flex I/O modules in two, unlike 1769 systems the slot numbering of the modules follows the physical path.

This means that while the first bank of I/O has slot numbers which increment from left to right, the second back of I/O has slot numbers that increment from right to left.

Modules and Bases of a Point I/O System:

In contrast to Flex I/O, Point I/O modules (sometimes referred to as “Slice I/O,”) are tall and narrow.

To assemble a system, users start by sliding the first module's terminal base in down into groves on the right side of the network adapter.

Then subsequent terminal bases are added to the right of the preceding base, with the last terminal base's bus connections being covered with a cover that comes with the network adapter.

Compared to Flex I/O, Point I/O systems are much easier to assemble, however Point I/O only supports din-rail mounting.

And while Point I/O also supports bus extension cables (right to left,) they are extremely bulky and not nearly as popular as the Flex I/O versions.

More Modules, but Less Terminals

Unlike Flex I/O's limit of eight modules per adapter, Point I/O supports up to sixty three modules per adapter.

That said, once you get above thirteen modules an Expansion Power Module (1734-EP24DC) is required to allow additional power to be provided to the module power bus.

Large Rack Of Point I/OAdditionally, since the terminal bases also carry field power, if you change from DC to AC modules (or need to isolate module power from one set of modules to another) you'll need a Field Power Distributor Module (1734-FPD) to break the field power bus to the left of the FPD, as well as provide terminals for the new field power bus to the right of the FPD.

And while Point I/O supports many more modules per Adapter, each module typically has less I/O points per module with less terminals per point.

With Flex I/O, it's common to have sixteen point modules sitting on top of three level terminal block bases with fifty two terminals.

Flex I/O Module Terminal Bases

With Point I/O, the largest input module only has eight points, and the standard terminal base also only has eight unique terminals.

Point I/O Module Terminal Bases

And while you can get Point I/O blocks with up to twelve terminals, the additional terminals are internally tied to four of the original terminals (4 to 8, 5 to 9, etc.)

This leaves users with either having to add an additional terminal strip to wire the remaining connections of his two and three wire devices, or requires the addition of the relatively expensive 1734-CTM (Common) and/or 1734-VTM (Voltage) Point I/O modules to provide additional terminals.

To Hold (Or Not To Hold) Last State

The final topic I'll cover is how the two platforms differ when it comes to holding last state.

In many mission critical applications it's important for remote I/O modules to maintain their last state if they lose connection with the Programmable Controller.

This includes applications where the outputs are controlling fans that remove toxic or flammable gases from an area.

In those applications, even if the connection to the PLC is lost, any outputs that were on may need to hold their last state and remain on to continue to remove the unwanted gasses from the area.

And while Flex I/O will always maintain last state when set to do so, oddly enough Point I/O will not hold last state if a user downloads a program to the programmable controller that is controlling the Point I/O even when the Point I/O is set to maintain it's last state.

Because of this, Point I/O is typically not considered for applications that require holding of last state, including most systems designed to utilize ControlLogix redundancy.

So, do you have a preference between Point I/O, Flex I/O, or some other distributed I/O system?

Let us know along with any thoughts, questions, or opinions you have by clicking on the “post a comment or question” link below.


Shawn Tierney
Automation Instructor and Blogger

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