EX-MotorShield8874 vs the L298N motor shield

The L298N-based motor shield is the board most people start DCC-EX on, because it is cheap, everywhere, and was the recommended option for years. The EX-MotorShield8874 is DCC-EX’s own board, built around a DRV8874 driver.

They are not the same class of component, and the difference shows up in three specific places.

1. Voltage drop

The L298N is a bipolar design. Its output transistors drop somewhere in the region of 2–3V under load — that voltage never reaches the rails, and it leaves the board as heat instead.

Put 15V in and the track may see 12V or so. Everything still works, but locos reach a lower top speed than they should, and the board is warm before you have run anything.

The DRV8874 is a MOSFET design with very low on-resistance. The drop is a fraction of a volt. The rails get close to what you feed in, and the board stays cool because it isn’t burning the difference.

2. Current capability

The L298N’s practical continuous rating is modest — around 2A per channel in ideal conditions, and rather less in a warm enclosure with no airflow. For one or two locos that is fine. Add sound, lit stock, and a few things standing with their lights on, and it becomes marginal.

Marginal is the worst place to be, because the failures are intermittent. Everything works until the moment three things move at once.

The DRV8874 handles substantially more current with proper thermal management behind it.

3. Current sensing, and why CV reads fail

This is the one that turns a mild preference into a real reason to switch.

Reading a CV works by listening for an acknowledgement pulse — the decoder answers a read request by briefly raising its current draw by about 60mA for a few milliseconds. There is no data coming back down the rails. The command station is, quite literally, watching for a small current blip.

The L298N’s current sense output is coarse and electrically noisy. A 60mA pulse against the standing draw of a locomotive sits close to, or under, its noise floor. So:

• Writing CVs works, because writing needs no reply
• Reading CVs fails, or succeeds one time in five

That produces one of the most misdiagnosed faults in DCC-EX. People replace decoders, rewire the programming track, and reflash firmware, when the board simply cannot hear the answer.

The DRV8874 provides a proper analogue current-sense output with far better resolution. The acknowledgement pulse stands clear of the noise, and reads become reliable.

Is it worth changing an L298N that works?

If you only ever drive trains, never programme decoders on a programming track, and run one or two locos at a time — the L298N will keep doing that job.

Change it if any of these apply:

• CV reads fail or are unreliable
• The board gets hot, or the track cuts out and recovers by itself
• Locos feel down on top speed for the supply voltage you’re providing
• You are adding sound decoders, which draw more and are fussier to programme

Swapping it over

Mechanically it is close to a drop-in on a Mega 2560. The firmware needs to be told which board it is talking to, because the pin assignments and the current-sense scaling differ — get that wrong and current limiting will be calculated against the wrong reference.

Set the motor shield type in the firmware configuration, rebuild, and upload. Then confirm the change took by checking what the command station reports on startup, and by reading a CV back from a known-good decoder on the programming track. If the read succeeds first time, the swap has done what it was supposed to.

Where to go next

• Choosing DCC-EX command station hardware — the full hardware picture
• Decoder programming — what to do with a working programming track