1769-L16ER-BB1B: why it takes POINT I/O instead of 1769 modules, and when it needs a second power supply

The question I get most about a 1769-L16ER-BB1B is which 1769 modules plug into it. None of them. The second most common one starts with "the controller is green, it's online, and none of the I/O works." That one is usually an empty FP+ terminal, not a bad controller.

Below is the hardware side: what plugs in, how it gets power, where the expansion limits sit, and what to check on a used unit before it goes in your panel.

The 1769 in the name, the 1734 modules on the side

The 1769-L16ER-BB1B is the smallest controller in the CompactLogix 5370 L1 family. The catalog prefix says 1769, but the bus on its right side is POINTBus, and the only local expansion it takes is 1734 POINT I/O. 1769 Compact I/O belongs to the L2 and L3 controllers. Different bus, different modules, and they don't mix.

On board: 16 sinking 24V DC inputs, 16 sourcing outputs, 384 KB of user memory, and two Ethernet ports on an embedded switch that share one IP address and support DLR. The USB port is for temporary connection only. No serial port, no DeviceNet. RS-232 goes through a 1734-232ASC at firmware 4.002 or later, and that module won't work without field power.

The rest of the L1 family looks the same from the front. The limits are what differ:

Catalog number	User memory	Local 1734 modules, max	EtherNet/IP nodes, max	Integrated Motion
1769-L16ER-BB1B	384 KB	6	4	No
1769-L18ER-BB1B	512 KB	8	8	No
1769-L18ERM-BB1B	512 KB	8	8	1 or 2 axes
1769-L19ER-BB1B	1 MB	8	8	No

 

All four are 100 mm (3.94 in) wide, 144 mm (5.67 in) tall overall, 105 mm (4.13 in) deep and about 0.66 kg (1.5 lb). DIN rail mount only.

The node limit catches more people than the module limit. Four EtherNet/IP nodes go fast once a drive, a remote I/O rack and a couple of other devices land in the I/O tree. As of November 2026, its lifecycle status is still Active.

Series A, B or C: read the label before you wire anything

The side label on a 1769-L16ER-BB1B: SER B next to the catalog number, the same letter after the slash on the barcode line, and BACKPLANE OUT: 5V, 1A, which is the entire POINTBus budget.

 

The series sits in the SER field on the side label, and the barcode line repeats it after a slash, so a /B at the end means series B. It matters mostly for one thing: power.

On a series A L16ER, L18ER or L18ERM the embedded power supply is non-isolated. That controller needs its own dedicated Class 2 or SELV supply on VDC, in the same enclosure, plus a second, fused supply for FP+ and everything else. Series B and C got an isolated supply, and one 24V supply can feed both VDC and FP. The L19ER is the odd one out, its series A is already isolated.

Series A vs B and C (L16ER, L18ER, L18ERM)	Series A	Series B and C
Embedded power supply	24V DC, non-isolated	24V DC, isolated
External supplies	Two: one dedicated to VDC, one fused for FP	One can feed VDC and FP
Line requirement, min	50 VA	30 VA
Field power fuse, user supplied	4-6 A	4-5 A
Input off-state current, max	1.5 mA	1 mA

 

B and C are direct replacements for A, so a B drops into a two-supply panel without rewiring. Not the other way around. A series A in a panel wired around one shared supply is out of spec until a second supply goes in.

Then firmware. Series B units carry a yellow notice on the front: 20.014 or later within v20, or v24 and later, otherwise you can't program the controller. The FW box on the label is not what you'll run, either. On a 2022 series B I looked at, it reads 1.005, the out-of-the-box revision. Every 1769-L16ER-BB1B needs real firmware loaded before first use, and an interrupted flash drops it back to 1.xxx. Which software goes with which firmware I covered in RSLogix 5000 vs Studio 5000.

Five terminals on the power plug, and the one that gets the switch

The removable power plug has five terminals. Top to bottom: VDC+, VDC-, NC, FP+, FP-. Nothing goes on NC.

VDC powers the controller and the POINTBus backplane, nothing else. The label even says it: BACKPLANE OUT 5V, 1A. Field power for the embedded I/O and every POINT I/O module comes in on FP+ and FP-. Leave FP empty and the controller boots, goes online, and the I/O side does nothing. Analog modules such as the 1734-IE2C show it with solid red channel LEDs.

One supply or two depends on the series. In both cases the switch goes on VDC+, the fuse and the MCR sit in the FP+ feed, and NC stays empty.

 

If a relay or selector switches the controller, its contact goes on VDC+. Put it on VDC- and the controller can have trouble powering up and down cleanly. The field side has an internal fuse you can't replace, so keep field current at 3 A total and add your own fuse between the supply and FP+. The supply has to be Class 2 or SELV, 24V DC nominal, 10 to 28.8V DC.

For an E-stop or master control relay, the MCR goes in the feed to FP+. Inputs wired straight from the supply, upstream of the MCR, keep reading while it's open, as long as FP- stays connected, since the inputs use it as their reference. The ones powered from the V terminals on the embedded I/O drop out with the outputs. If the program has to see the E-stop circuit with outputs dead, wire the inputs the first way.

Power plug wire: 30 to 12 AWG copper rated 90 °C, up to two wires per terminal, 10 mm (0.39 in) strip, 0.5 to 0.6 N·m (4.4 to 5.3 lb·in).

What the embedded 16 in and 16 out will drive

The inputs on a 1769-L16ER-BB1B sink, so they want sourcing devices: PNP sensors or dry contacts back to +24V, two-wire or three-wire. The outputs source 0.5 A per point and 3 A per module, and 0.5 A is also the pilot duty rating. A small relay coil is fine. A contactor coil pulling more than that needs an interposing relay. No analog on the controller at all, that comes from POINT I/O.

Embedded I/O	16 inputs	16 outputs
Type	24V DC, sinking	24V DC, sourcing
Operating range	10-28.8V DC	10-28.8V DC
Current	On-state 2 mA min; off-state 1 mA max (B and C)	0.5 A per point, 3 A per module
Surge	n/a	1 A for 100 ms, repeatable every 2 s
Off-state	5V DC max	Leakage 0.5 mA max at 24V DC
Timing	0.5 ms hardware filter plus 0-65 ms selectable	0.1 ms off to on and on to off
Isolation	50V continuous, tested at 500V AC for 60 s, system to field; none between channels	Same
Wiring	24-16 AWG, one wire per terminal, 10 mm strip, spring clamp	Same

 

Six POINT I/O modules, if the budget lets you

Six is the maximum number of POINT I/O modules on a 1769-L16ER-BB1B, and it only holds while the embedded I/O plus every local module stay within 1 A of POINTBus current and 3 A of field current. I add up each module's 5V backplane draw before I promise anyone six.

Thermocouple and RTD cards have their own rule. Say a machine needs eight RTDs, so four 1734-IR2 cards. No more than three 1734-IR2 or 1734-IT2I modules can draw from one power source, because their inrush is more than the DC to DC converter can handle. Off the controller's own supply that makes three the ceiling, so the fourth card goes in a remote rack, where the adapter is a separate power source.

A 1734-FPD only helps with the field half. Slide it in between modules and everything to its right runs on a new field supply, up to 10 A, at 24V DC or 120/240V AC, while the modules to its left keep sharing the controller's 3 A. POINTBus passes straight through. It adds nothing to the 1 A.

The local budget: six modules, 1 A of POINTBus for all of them, 3 A of field power unless a 1734-FPD takes over part of the rail. Past that, I/O moves to a remote rack that uses one of the four Ethernet nodes.

 

Past six modules or past the 1 A, the next step is a remote POINT I/O rack on Ethernet behind a 1734-AENT or 1734-AENTR. Each adapter takes one of the four node slots on a 1769-L16ER-BB1B. Setting those adapters up on the bench, IP address and chassis size included, is in my 1734-AENT setup article. When racks, drives and everything else go past four nodes, that's an L18ER or L19ER job, with eight.

When the whole local rack goes quiet

The controller properties in the project carry an Expansion I/O setting, a plain count of local modules. At every power-up the 1769-L16ER-BB1B compares it with what's on POINTBus and won't open a single I/O connection until the two match. Pull a module for a spare, forget the number, and the whole local rack goes quiet.

POINTBus can't see an empty terminal base. So no gaps on the rail, and a slot that has to stay open gets a 1734-ARM address reserve module.

Module firmware is the one nobody checks. Every 1734 module on the local bus should be on a current series and firmware, because an old revision can make the entire bus inoperable. They also have to sit on Autobaud, the factory default, which you can't change once the module is on an L1. When the bus does fall over, the controller logs a BUS OFF minor fault, type 03, code 01. Check the module count first, then that every base is locked, then Autobaud.

Pulling a module under power is supported and doesn't stop the others. Two traps, though. If that module is set to major-fault the controller on a lost connection in Run, pulling it does exactly that. And two module types swapped into each other's slots fail the keying check, unless keying is disabled. Then nothing stops them, and unintended control can follow.

No battery, a clock that wanders, and somebody else's SD card

There's no battery in a 1769-L16ER-BB1B. An internal energy storage module keeps the CPU alive at power-down long enough to write the program and tag values to onboard nonvolatile memory. After that the real-time clock can keep running on whatever energy is left: typically about 40 days at 0 °C, 35 at 25 °C, 28 at 40 °C and 16 at 60 °C, and typical is not guaranteed. A unit that sat on a shelf for months comes up with the wrong date. If timestamps matter, set the clock at commissioning and again after long outages.

A new unit ships with a 1 GB 1784-SD1 card already in the slot, and the 1784-SD2 is the 2 GB option. Leave a card in even if you never use it. After a major non-recoverable fault, extended fault information gets saved there.

On a used controller, pull the card and look at what's on it before the first power-up in your panel. A card with Load Image set to On Power Up loads its project at every power cycle, and that load can change the firmware and the IP address too. There's no setting on the controller that turns it off.

Before a used 1769-L16ER-BB1B goes into your panel

1.    Read SER on the label and plan the power to match: one supply for B or C, two for series A.

2.    Pull the SD card, check what's on it, then decide whether it goes back in.

3.    Use zinc-plated, chromate-passivated steel DIN rail. The controller grounds through the rail, and aluminum or plastic rail can give you a bad or intermittent ground. Leave 50 mm (2 in) clear on every side.

4.    Switch on VDC+, fuse ahead of FP+, nothing on NC.

5.    Set the Expansion I/O count to the modules actually installed, no empty bases, end cap on the last base.

6.    Load firmware before downloading a project, and on a series B check it against the yellow notice on the front.

Questions that come up

What I/O modules work with the 1769-L16ER-BB1B?

Locally, 1734 POINT I/O only, up to six modules, as long as they fit the 1 A POINTBus and 3 A field power budget. 1769 Compact I/O doesn't connect to it at all. Anything beyond that runs on Ethernet through a remote adapter, which counts toward the four-node limit.

What software do I need to program a 1769-L16ER-BB1B?

RSLogix 5000 version 20 for firmware 20, or Studio 5000 Logix Designer version 21 and later for firmware 21 and up, plus RSLinx Classic 2.59 or later. ControlFLASH, which loads the firmware, installs along with either one. A series B needs at least 20.014 if you stay on v20; otherwise go to v24 or newer.

Can one 24V supply run a 1769-L16ER-BB1B and its I/O?

On series B and C, yes, the same supply can feed VDC and FP. On series A, no. VDC gets its own Class 2 or SELV supply inside the same enclosure, and the field side gets a second, fused one.

Does the 1769-L16ER-BB1B have a battery?

No. The energy storage module handles the save to nonvolatile memory at power-down, so there's nothing to replace. What suffers after a long outage is the clock.

Getting a 1769-L16ER-BB1B and the parts around it

We keep the 1769-L16ER-BB1B in stock. If six modules or four nodes look tight already, the 1769-L18ER-BB1B and 1769-L19ER-BB1B give you eight of each, and the 1769-L18ERM-BB1B adds one or two motion axes. For the rail: 1734-IB8 and 1734-OB8 modules on 1734-TB bases, a 1734-FPD for a second field supply, and a 1734-ARM for a slot you need to hold. Remote racks start with a 1734-AENT or 1734-AENTR, and a spare 1784-SD1 is worth keeping in the drawer. If your panel was built around a series A, or around one shared supply, tell us which series you need when you order.