Description
Key Technical Specifications
| Parameter | Value |
|---|---|
| Product Model | 3500/20 Rack Interface Module |
| Part Number | 125744-02 (Standard RIM) |
| Manufacturer | Bently Nevada (Baker Hughes) |
| Product Type | Standard Rack Interface Module (RIM) |
| Host System | Bently Nevada 3500 Series |
| Installation Position | Rack slot 1, adjacent to the 3500/15 power supply |
| Communication Interface | RS-232 / RS-422 serial |
| Max Serial Data Rate | 38.4 kbaud |
| Internal Modem Rate | 14.4 kbaud (where applicable) |
| Power Supply | Supplied by 3500 rack backplane; no separate external supply |
| Typical Power Consumption | 4.75 W |
| System OK Relay | Yes — rack-level health output |
| Rack Control Inputs | Reset, Trip Multiply / Alarm Inhibit support |
| Operating Temperature | Typically -30°C to +65°C; verify against OEM datasheet |
| Dimensions | Approx. 241.3 × 24.4 × 241.8 mm (standard 3500 full-height module) |
| TMR Capability | No — 125744-02 is the standard RIM; TMR requires 125744-01 |
Product Introduction
A 3500 rack without a functioning RIM is hard to configure, harder to diagnose, and impossible to integrate cleanly with upstream systems. The Bently Nevada 3500/20 125744-02 is the standard Rack Interface Module that anchors the rack — it sits in slot 1 next to the 3500/15 power supply, handles rack configuration, maintains the system event list, and provides the System OK relay that many DCS/ESD systems monitor for rack health.
It communicates over RS-232/RS-422 at up to 38.4 kbaud and draws about 4.75 W from the backplane, so there is no separate power feed to wire. This is the standard RIM, not the TMR version. If the rack uses triple-modular-redundancy logic, the 125744-02 will not satisfy that requirement — you need the 125744-01 TMR RIM instead. Bottom line, it is a straightforward drop-in replacement for existing 3500 racks that were built with serial communications rather than a 3500/22M TDI.
QA/QC Transparency SOP
Intake and origin verification: Record the full model 3500/20, part number 125744-02, serial number, hardware revision, and any rack labels. Photos are taken before power-up.
Live functional testing: The module is installed in a compatible 3500 test rack and checked for normal boot, OK LED, RX/TX activity, and System OK relay operation.
Communication verification: RS-232/RS-422 serial communication is tested at up to 38.4 kbaud using 3500 Rack Configuration Software. Rack configuration upload/download and event list access are verified.
Electrical and backplane checks: Backplane contacts, power rails, and relay outputs are checked for continuity and correct operation. Power consumption is measured under load against the 4.75 W typical reference.
Final QC and anti-static packaging: Firmware/configuration registers are read out. The module is sealed in anti-static packaging with test notes. Confirm whether the site requires standard RIM or TMR RIM before dispatch.
Field Engineer Gotchas
Standard vs TMR RIM confusion: The 125744-02 is the standard RIM. If the rack is configured for TMR, this module will not perform monitor voting. Check the rack architecture before swapping.
Wrong slot placement: The RIM must go in slot 1, next to the 3500/15 power supply. Putting it in any other slot will prevent proper rack communication and configuration.
Serial cable and baud rate mismatch: The 125744-02 uses RS-232/RS-422 at up to 38.4 kbaud. Using the wrong cable, wrong port, or wrong baud rate will look like a dead module when it is really a communications setup issue.
System OK relay wiring: The System OK relay is often wired into the DCS/ESD permissive chain. A miswired or untested relay output can cause nuisance trips or mask a genuine rack fault. Verify relay state before returning the rack to service.
Configuration lockout: If the rack is in RUN mode or the configuration key is locked, you may not be able to download changes. Check the programming key position and security settings before attempting a config update.
BENTLY 3500-20 125744-02
Application Scenarios
Legacy 3500 rack communication restoration: A plant running an older 3500 rack with serial communications needs to replace a failed RIM without migrating to a 3500/22M TDI. The 125744-02 restores RS-232/RS-422 connectivity and rack configuration access.
DCS health supervision via System OK relay: The rack System OK output is wired to the DCS to confirm the 3500 rack is healthy. The 125744-02 provides this rack-level status signal for continuous supervision.
Standard (non-TMR) machinery protection rack: For turbines, compressors, or large motors protected by a standard 3500 rack, the 125744-02 handles configuration, event logging, and serial communications without TMR voting overhead.
Anonymized case note: A refinery replaced a failed 3500/20 RIM with a surplus 125744-02. The rack powered up, but the DCS showed a persistent rack fault. The root cause was that the original rack had been migrated to TMR logic, and the 125744-02 standard RIM could not support monitor voting. After confirming the architecture, the team sourced a 125744-01 TMR RIM and restored full functionality.
FAQ
What is the difference between 125744-02 and 125744-01?
125744-02 is the standard RIM. 125744-01 is the TMR RIM, which adds monitor-channel voting for triple-modular-redundancy racks.
Which slot does the 3500/20 RIM go in?
Slot 1, immediately next to the 3500/15 power supply.
Does it need an external power supply?
No. It is powered from the 3500 rack backplane. Typical consumption is about 4.75 W.
Can I use it with a 3500/22M TDI rack?
No. A rack uses either a RIM (3500/20) or a 3500/22M TDI in slot 1, not both. The 3500/22M replaces the RIM plus external communication processor.
What serial speed does it support?
Up to 38.4 kbaud over RS-232/RS-422.
Is this module part of the protection trip path?
The RIM itself is not in the critical monitoring path. Monitor modules and relay modules handle trip logic. The RIM handles configuration, communications, and rack-level status.
Can you supply refurbished units?
Yes. Surplus and refurbished stock can be provided. Part number, rack type (standard vs TMR), and serial communication requirements should be confirmed before purchase.
