4ms RCDBO (2012–)
A 4HP breakout for the RCD. Once you have the RCD, this module gives you front-panel access to features that live in its microcontroller.
Overview
The RCDBO is not a standalone module. It is a sliver of black panel with six little toggles. Plug it in beside the 4ms Rotating Clock Divider (RCD) and the RCD gains features that used to be buried in its firmware. Before the RCDBO, you had to reflash the RCD's chip just to get Spread mode or set a maximum division ratio. Now a switch does it. You can spread pulses across the divisions, flip the counting direction, or lock in auto-reset at 16 steps.
You need to own the RCD already. The RCDBO draws no power, has no inputs or outputs of its own, and does nothing if plugged in alone. It is a control interface, linked by a short ribbon cable to the RCD's expansion header. The RCD itself rotates division ratios across multiple outputs, which makes evolving, cascading rhythms. The RCDBO puts the controls on a panel. Want to switch from dividing by 64 down to 2 in Gate mode, counting upward, with Spread enabled? Done. Need Auto-Reset off for a stuttering, unresolved loop? One switch. What used to be a firmware tweak is now something you can do live.
4ms sold the finished unit and also a kit version for $45, with through-hole parts and beginner-friendly assembly. The depth differs between assembled (29mm) and kit (48mm) versions, mostly because of different jack placements. Functionally they are identical. The kit is popular with tinkerers, and building it shows how the hardware connects to the firmware. If you like reflashing chips, the 4ms GitHub repo hosts the latest firmware, including experimental versions like the Tracking Gate Sequencer. Most users won't need it, since post-2012 units ship with Spread mode already enabled - no hex files required.
Specifications
| Manufacturer | 4ms Company |
| Production Years | 2012– |
| Original Price | $95.00 (assembled), $45.00 (kit) |
| Module Size | 4HP |
| Depth | 1.14" (29mm) for assembled; 1.85" (48mm) for kit |
| Power Consumption | +12V: not used, +5V: not used, -12V: not used |
| Power Connector | 16-to-16 pin ribbon cable (included) |
| Control Interface | 6 DIP switches |
| Switch Functions | Max Divide-by (up to 64), Gate/Trigger mode, Counting mode (Up/Down), Spread mode, Auto-Reset (16/off) |
| Compatibility | Requires 4ms Rotating Clock Divider (RCD) with expansion header |
| Faceplate Material | FR4, 1.6mm thickness, satin black with white artwork |
| Included Accessories | 16-to-16 pin power cable, 2 M3 knurled screws (assembled version) |
| Assembly Options | Pre-assembled or DIY kit |
| Firmware Requirements | Version 1.1 or later recommended for full Spread mode support |
| Expansion Type | Breakout module for RCD microcontroller settings |
Key Features
Switch-Driven Firmware Control
Most modules hide advanced functions in menus or make you reflash from a computer, which is tedious in a modular system. The RCDBO uses six DIP switches that directly set the RCD's internal state. No menu diving, no USB cables, no risk of bricking during an update. Want to change from Gate to Trigger mode? Flip a switch. Need to reverse the counting direction for a backwards rhythmic cascade? Another flip. In a live set you can reconfigure the RCD's behavior on the fly.
Spread Mode Unleashed
Before the RCDBO, Spread mode was only available by reflashing the RCD's chip with custom firmware. When engaged, Spread mode distributes pulses evenly across the entire division range. Instead of stepping sequentially from /2 to /64, the module spaces out divisions to maximize rhythmic variation. The result is less predictable and more organic. It works especially well with the Max Divide-by switch, which lets you cap the range at /16 or /32 for tighter patterns. Without the RCDBO you'd need a laptop to get at this. With it, you are two toggles away.
Kit-Build Flexibility
The DIY kit isn’t just a budget option—it’s a design statement. By offering a build-it-yourself version, 4ms invites users into the module’s inner world. The kit uses through-hole components, making it accessible even to beginners with basic soldering skills. The deeper profile (48mm) comes from using panel-mounted jacks instead of PCB-mounted ones, but that also means easier repairs down the line. For educators and hobbyists, the RCDBO kit is a perfect intro to embedded control in modular synthesis. And because it includes the same firmware-capable microcontroller interface as the pre-built version, you’re not sacrificing functionality—just time and precision. That inclusivity, that refusal to gatekeep, is part of what made 4ms a cult favorite in the Eurorack underground.
Historical Context
The RCDBO emerged in 2012, as Eurorack was moving from niche curiosity to global phenomenon. Many modules still treated firmware as an afterthought, flashed once at the factory and forgotten. 4ms saw microcontrollers as reprogrammable brains that could evolve. The RCD already had its rotating division logic, but it was limited by front-panel space. The RCDBO treats the RCD like a motherboard and the breakout like an expansion card, a concept more common in computers than synths.
It also fit a broader trend toward open-source hardware in the modular world. 4ms hosted firmware on GitHub and encouraged user modifications. Competitors like Doepfer or Intellijel offered polished modules, but few invited users to tweak the code. The RCDBO and its sibling SCMBO (for the Shuffling Clock Multiplier) came out of that approach. It wasn't the first breakout module, but it was one of the first to make firmware-level features performable.
Collectibility & Value
The RCDBO has a stable, modest collector's niche. As of 2026, used pre-built units trade between $75 and $90, while the kit version fetches $40 to $50 depending on condition. It draws no power and has no exposed circuitry, so failure points are few. Most issues come from the ribbon cable connection or bent pins on the RCD's expansion header. The DIP switches are durable, but years of toggling can cause intermittent contact, especially if dust builds up. Contact spray usually fixes this. The faceplate is FR4, a fiberglass composite. Unlike aluminum, it won't scratch easily, but it can crack if overtightened.
For buyers, the real risk is compatibility. Early RCD units (pre-2012) may lack the firmware to support Spread mode, even with the RCDBO attached. Verify the RCD's PCB version (1.0.2 or later recommended) and firmware before pairing. Some users report success reflashing older units, but it takes an AVR ISP programmer and technical confidence. If you're buying used, ask whether the seller has tested the Spread function. The kit version requires assembly, so don't assume it works out of the box unless specified. The wiring guide is well documented, but miswiring the ribbon cable can damage the RCD's header, so double-check pin alignment.
On its own, the RCDBO does nothing. Paired with an RCD, it is the way to reach those settings, and demand has stayed steady among rhythm-focused modular users.
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