TL;DR: The 3500/20 Rack Interface Module (RIM) is the module that gives a Bently Nevada 3500 rack its communications backbone and its rack-level timing. Every 3500 rack requires one RIM or one TDI, always in slot 1 next to the power supplies. Baker Hughes has since replaced the /20 with the 3500/22M Transient Data Interface, which “combines the function of a 3500/20 Rack Interface Module (RIM) with the data collection capability of a communication processor such as TDXnet”. The RIM is still a live spare part on legacy racks, and swapping one for a TDI has a specific sequence that matters – done wrong, it damages the new module.
What is the 3500/20?
The RIM is the interface between the 3500 rack’s backplane and the outside world. It sits in the reserved left-most position, to the right of the two 3500/15 power supplies, and it is the module that rack firmware, configuration and host communications pass through. In Baker Hughes’ own framing the RIM function still exists today – it has simply been absorbed into the TDI: “Every 3500 rack requires one TDI or RIM, which always occupies Slot 1 (next to the power supplies).”
What the RIM does not do is carry field wiring. Sensor cables land on the monitor module’s own I/O module (internal termination) or on an external termination panel wired back to that I/O module. Nothing from the machine goes through the 3500/20, which is why a failed RIM shows up as lost communications rather than as lost channels.
One caveat on sourcing: Baker Hughes no longer publishes a standalone 3500/20 datasheet. The module appears in the 3500/22M TDI manual as the predecessor, but it has dropped out of the current 3500 System Datasheet (162096) component list. Sizes, LED definitions and ordering sub-codes for the /20 itself are therefore documented only in legacy literature. We will verify a specific RIM against the rack drawing before quoting; we do not want to guess a suffix.
3500/20 versus 3500/22M
Use this table to identify what is in slot 1 of your rack and what a replacement implies.
| Function | 3500/20: rack interface only. 3500/22M: RIM function plus data acquisition and gateway processor |
|---|---|
| Status | 3500/20: legacy, not in the current system datasheet. 3500/22M: current, required component of a new 3500 rack |
| Slot | Both occupy slot 1, immediately right of the power supply positions |
| Waveform capture | Not a RIM capability; the TDI collects transient data for monitor modules |
| Config storage | The RAK configuration file lives in the interface module, so it must be moved or re-applied when you change generations |
| TDI size / weight | One full-height front slot + one full-height rear I/O slot; 241.3 x 24.4 x 241.8 mm, 0.91 kg, 10.5 W |
| TDI indicators | OK, TX/RX, TM and CONFIG OK; CONFIG OK lights when a module is unconfigured, misconfigured, when stored configuration does not match hardware, or when a security option is unmet |
| Rack OK relay | Housed in the interface module’s I/O: SPDT, normally energised, 5 A at 24 Vdc / 120 Vac; indication only, never a shutdown path |
The equivalent TDI ordering pattern, for reference, is 3500/22-AXX-BXX-CC with A = 01 (standard), B = 01/02/03/04 and C = 00/01/02. No equivalent option-code table is published for the 3500/20.
Upgrading from a RIM to a 3500/22M TDI
The upgrade is the right call on most legacy racks – you keep rack communications and gain transient data capture, Ethernet and a modern configuration path in the same slot. But it is not a like-for-like pull-and-swap, and Baker Hughes flags the reason in the TDI manual: the RIM’s rear I/O module and the TDI’s rear I/O module are different pieces of hardware occupying the same rear position.
The documented sequence is: remove the 3500/20 main module, then remove the RIM’s I/O module, then install the TDI I/O module, then install the TDI. Leaving a RIM I/O module in the rack and pushing a TDI in on top of it will damage the TDI. Two further checks before you start:
| Monitor compatibility | The manual warns that when moving from a 3500/20 RIM to a 3500/22 TDI, some “M” series monitors (for example 3500/40M) may not be compatible – confirm the module list in the rack first |
|---|---|
| Configuration | Back up the RAK file before removal; you will restore or rebuild it against the TDI |
| Software level | Plan on updating 3500 System Configuration Software and TDI firmware as part of the job |
| I/O hot swap | Main modules and power supplies are hot-swappable; I/O modules are not – the rear work is a de-energised job |
Symptoms that point at the interface module
There is no published RIM fault-code table, so read the rack rather than the module. Host communications lost while every monitor channel still shows valid readings and local Alert/Danger behaviour is intact is an interface-module class failure, not a sensor failure. Loss of configuration retrieval, or a rack that shows stored-configuration mismatch after someone has been in the software, is the other common RIM/TDI signature – on a TDI that is exactly what the CONFIG OK LED is for.
Before condemning the module, rule out the cheap causes: the Ethernet or fibre run (Cat5 shielded, RJ-45 limited to 100 m; MT-RJ fibre to 2000 m), the USB lead – which must be the specified 123M4610 cable if you want chassis isolation to survive, and the laptop’s own grounding, since a mains-powered laptop through a USB connection creates a ground loop that looks like a communications fault.
3500/20 parts we can supply
Part number
| 125840-01 | 3500/20 Rack Interface Module · In stock · View |
|---|
Frequently asked questions
Does a 3500 rack need a 3500/20?
It needs an interface module, but not necessarily a 3500/20. Baker Hughes states that every 3500 rack requires one TDI or RIM in slot 1. On a rack built or refreshed since the TDI became current, that module is a 3500/22M; on older racks it is a 3500/20. A rack with neither will not configure or report.
What is the difference between a 3500/20 RIM and a 3500/22M TDI?
The TDI does everything the RIM did and adds data collection. The TDI manual describes it as combining the function of a 3500/20 Rack Interface Module with the data collection capability of a communication processor such as TDXnet. In practice that means transient waveform capture, a USB and Ethernet path for configuration software, and rack status relay contacts in its I/O module.
Can I put a 3500/22M TDI in place of a 3500/20 RIM?
Yes, and it is the normal upgrade – but you must change the rear I/O module as well, in the documented order: pull the RIM, pull the RIM I/O, fit the TDI I/O, then fit the TDI. Fitting a TDI while the old RIM I/O module is still in the rack will damage it. Also confirm your monitor modules are compatible, because the manual flags that some M-series monitors may not be.
Which slot does the 3500/20 occupy?
Slot 1 – the position immediately to the right of the two half-height power supply slots, at the left-hand end of the rack. The left-most position is reserved for power plus the interface module, which is why a full-size 19-inch rack gives you 14 usable module slots and a mini rack gives you 7.
Does sensor wiring go through the RIM?
No. Proximity probes, accelerometers, RTDs and thermocouples land on the I/O module belonging to their own monitor, either directly at internal terminals or through an external termination panel and a multi-core cable. The RIM carries rack communications and configuration, not field signals.
If the 3500/20 fails, do I lose protection?
You lose visibility and configuration access, not the protection logic itself. Monitors keep evaluating their own setpoints and relay modules keep driving alarms from the rack backplane. What you lose is the path an operator uses to read the rack and a history, which is why a failed interface module is urgent but is not normally a trip event.
Is the Rack OK relay in the interface module a shutdown output?
No, and this is worth stating plainly. The Rack OK relay sits in the interface module’s I/O, is SPDT and normally energised, and Baker Hughes specifies that it should not be used as part of a machinery automatic shutdown circuit. Shutdown paths go through the relay modules – 3500/32M or 3500/33 – which are driven by the alarm logic.
Can you still supply a 3500/20?
We list the RIMs and RIM I/O modules we hold with live availability above. Because the /20 is a legacy item with no current public datasheet, we confirm the revision and the rack drawing against your label before shipping, and we will tell you honestly when the sensible answer is a 3500/22M upgrade kit instead of another RIM.
Talk to us about a RIM or an upgrade
Send the label from slot 1 – module code plus I/O module code – and we will confirm whether your rack is RIM or TDI based, whether the rear I/O matches, and what your options are for repair versus upgrade. We quote prices per enquiry rather than publishing them, and we will always show you the datasheet revision a part was verified against.
Related: 3500/22M Transient Data Interface, 3500/15 power supply and the 3500 system complete guide.