Multi-brand
PLC Module Repair — I/O, Analog, Communication & Specialty Modules — Mail-In
Component-level repair for PLC modules: digital and analog I/O, communication modules, and specialty cards for SLC 500, ControlLogix/CompactLogix, PLC-5, MicroLogix, and Siemens S7. Free evaluation, typical repairs $250–$650. CPUs get honest test-and-verify service — most CPU faults are not economically repairable, and we say so.
Coverage: SLC 500 (1746) · ControlLogix (1756) · CompactLogix (1769) · PLC-5 (1771/1785) · MicroLogix · Siemens S7-300/400
✓Fixed — or it's free
Unrepairable units are refunded; evaluations cost $0
✓24-month warranty
Every repair covered, parts and labor
✓3–5 business days bench time
Rush (1–2 day) and emergency service available
✓Program preserved
Your application and settings stay in the unit
Symptoms this repair fixes
- ✓Dead or stuck I/O channels
- ✓Analog channels reading wrong or drifting
- ✓Comm module link failures
- ✓Modules killed by field-wiring events (evaluated)
Honest limits
- ✕Most CPU internal failures — there is no honest component-repair market for dead CPUs; we test, verify, and tell you rather than sell false hope
- ✕Safety-rated (GuardLogix class) controllers — out of scope by policy
What you get
- ▸Free evaluation and firm quote
- ▸Channel-level I/O repair (drivers, optos, relays, terminals)
- ▸Communication port and PHY repair
- ▸Corrosion remediation where viable
- ▸CPU test-and-verify with an honest verdict
- ▸24-month warranty
Your program is safe
Programs live in the CPU/memory card, not I/O modules — I/O repair never touches your logic. For CPUs, always maintain your offline project backup.
Typical modules we see
Part-number pages for this service
How PLC Module units actually fail
Most units we receive fail in one of a handful of well-understood ways. Here is what each fault looks like from the machine, what is happening electronically, and what we do about it at the bench.
Failed output points
- What you see
- Specific outputs no longer switch, or are stuck permanently on.
- Why it happens
- Output drivers fail from switching inductive loads without suppression, from short circuits in field wiring, or from sustained operation near their rated limit. A stuck-on output usually means the driver has failed short.
- What we do
- Failed drivers are replaced and every point on the module is retested individually — not just the ones reported faulty.
Failed input points
- What you see
- Inputs that no longer register field devices, or that read as permanently on.
- Why it happens
- Input opto-isolators fail from overvoltage or from surges arriving on field wiring, which is exactly the job they exist to do.
- What we do
- Isolation components are replaced and every input verified against specification.
Backplane interface failure
- What you see
- The module is not recognised in the rack, or faults the rack, despite appearing otherwise sound.
- Why it happens
- Damage to the bus interface circuitry, which can occur independently of the I/O section.
- What we do
- The interface is tested and repaired and correct communication confirmed.
Connector and terminal damage
- What you see
- Intermittent points, or channels that work when wiring is disturbed.
- Why it happens
- Terminal blocks and connectors suffer mechanical wear and thermal cycling, and loose connections mimic module failure convincingly.
- What we do
- Terminals and connectors are inspected, repaired or replaced, and reliable connection verified.
What happens on the bench
The same sequence on every 5-stage repair, so nothing gets skipped because a fault looked obvious.
- 1
Intake and channel mapping
The module is logged and the reported fault mapped to specific points. I/O modules rarely fail as a whole unit — far more often a subset of channels has failed while the rest work, so we establish exactly which points are dead and in what way.
- 2
Point-by-point testing
Every input or output point is exercised individually against the module's own specification. This is what separates a real repair from a visual inspection: a module can pass a power-on self-test and still have three dead outputs.
- 3
Driver and isolation repair
Failed output drivers and input opto-isolators are replaced. These are the components that take the damage when an inductive load is switched without suppression or when a field wire shorts to a supply, and they are replaceable at board level.
- 4
Backplane interface check
The backplane or bus interface is verified, since a module that appears dead in the rack while testing correctly in isolation usually has interface damage rather than I/O damage.
- 5
Full functional retest
Every channel is retested after repair, not only the ones that were reported faulty, so the module returns as a fully proven part rather than a partially repaired one.
Check these before you ship
Some of these will tell you the unit is fine and the fault is elsewhere. We would rather you find that out now than pay to ship a good unit.
- □Move the suspect module to a different slot. If the fault follows the module it is the module; if it stays with the slot, the problem is the rack or backplane.
- □Identify exactly which points have failed and whether they are stuck on, stuck off, or intermittent. Stuck-on outputs usually mean a shorted driver; stuck-off can be the driver or its supply.
- □Check the field wiring on the failed points for shorts and for inductive loads without suppression diodes or snubbers — otherwise the repaired module fails the same way.
- □Confirm the rack power supply is healthy under full load. A sagging supply produces faults that look like several modules failing at once.
- □Send the module only. We do not need the rack, processor or field wiring.
Repair or replace?
I/O modules are worth repairing mainly when the platform is discontinued or when the alternative is an unplanned migration of a working control system. Many older module families are no longer manufactured, and the surplus market for them is inconsistent in both price and quality — a repaired module of known provenance, tested point by point and warranted, is usually the more predictable option. Where a module is current, inexpensive and readily stocked, we will say so.
What to ship
- ▸The module(s) in anti-static packaging
- ▸Slot/symptom description per module
Full packing guidance: mail-in instructions.
Ship your unit to
USPS
Control Board Lab
PO Box 120241
Arlington, TX 76012
UPS / FedEx
Control Board Lab
1009 Oakwood Ln # 120241
Arlington, TX 76012
Write your RMA number on the outside of the box and include it on a note inside. Units over 50 lb (large VFDs, spindle drives, industrial PCs in enclosures) ship by freight — contact us before ordering and we will arrange dock-to-dock shipping.
Frequently asked questions
Can you repair my dead SLC or ControlLogix CPU?
Usually no — and anyone who tells you otherwise deserves skepticism. CPU failures are dominated by proprietary silicon and memory that has no component-level repair path. What we CAN do: verify whether the CPU is actually the problem (it often isn't — check the power supply first), test-and-verify your spare, and repair the power supplies and I/O around it.
The module is obsolete and unavailable — is repair realistic?
Obsolete I/O and comm modules are exactly where repair shines: the failures are usually in generic components (drivers, optocouplers, relays, capacitors) that remain available even when the module doesn't.
Start with a free evaluation
Ship it in, get a firm quote, approve before any work — pay nothing if it can't be repaired.
The Fixed-or-Free Guarantee: if we can't repair it, you don't pay for the repair.
Call (817) 799-7332 · 📱 Text us a photo of your unit · Email repairs@controlboardlab.com