Allen Bradley1756-OB16DControlLogix 16-Point Diagnostic DC Digital Output Module, 10–31.2 V DC Sourcing, 2 A per Point

Product Description

What this part is

An Allen-Bradley ControlLogix 1756-series diagnostic digital output module with 16 solid-state DC outputs. The outputs are sourcing over a 10 to 31.2 V DC range, rated 2 A per point, electronically fused, and each point reports its own field-side condition back to the controller as diagnostic data. That per-point reporting is what the D in the catalogue number buys: the program sees which output failed and how, rather than only that a group has tripped. The module occupies one slot in a 1756 ControlLogix chassis.

Decoding the catalogue number

1756 is the ControlLogix I/O and chassis platform, O an output module, B the DC voltage category and 16 the point count. The trailing D is the diagnostic variant. Read against its siblings the letter is the whole specification difference: 1756-OB16E is electronically fused without diagnostics and rated 1 A per point, 1756-OB16 is neither fused nor diagnostic, and 1756-OB16I is individually isolated. SER A is the hardware series letter and tracks revision rather than function.

What the diagnostics actually detect

Diagnostic outputs monitor the field circuit in both states. With an output commanded off, a broken wire or missing load shows as an open-load or no-load fault, which is the condition an ordinary output module cannot see at all — a solenoid that has been disconnected looks identical to one that is simply not called for. With an output commanded on, the module verifies that the point actually pulled up and flags an output-verify or short-circuit fault if it did not. Each condition arrives as a bit in the module’s input tag structure, so the fault reaches the HMI without extra field wiring or monitoring relays. That is the case for paying the premium over an OB16E: the diagnostic content replaces separately wired proof circuits.

What these specs mean in practice

Two amps per point is double the OB16E figure and covers larger solenoid valves and valve islands directly, which often removes the field-mounted fuse block and interposing relay that a 1 A module would need. Electronic fusing still responds to inrush as well as steady-state current, so a large safety contactor or a heavily inductive load can trip a channel whose running current is comfortably inside rating.

Quantity and configuration

1 Pc. Module only. No removable terminal block, no chassis, no chassis power supply, no wiring arm and no field 24 V supply. The RTB is a separate 1756-TBNH or 1756-TBCH item.

Installation and setup notes

The module goes into any I/O slot of a 1756 chassis, and its slot, catalogue number and revision must match the entry in the controller’s I/O configuration tree; electronic keying decides whether a differing series or revision is accepted on a swap. Diagnostic modules produce a larger tag structure than plain output modules, so replacing an OB16E with an OB16D is a program change as well as a hardware change. Field wiring stays on the removable terminal block through a module swap.

Interchange guidance

Later series letters of 1756-OB16D are the substitution path and use the same terminal block. The 1756-OB16E is a related module rather than a replacement, since it halves the per-point current and drops the diagnostic tags the program may be reading. Sinking DC output modules in the range reverse the common rail polarity and are not interchangeable on existing field wiring.