SMD84 - Switch Machine Driver
with Serial Bus

The SMD84 is an eight (8) output DCC accessory decoder (HO & N scale) specially suited to drive 3 wire solenoid type switch (turnout) machines. It features 13 local inputs and 8 routes.

MSRP - 99.95 USD
Manual in PDF format

Board size: 2.75" X 4.25"

Works with most DCC systems that supports accessory decoders (see below)

Provides 8 outputs
• Drives twin coil solenoid switch machines
• Drives Tortoise™ switch machines for turnout control

Provides 13 inputs
• For local turnout control.
• For occupancy sensors
• For turnout feedback sensors
• Remote input lockout option

Control routes on your layout
• Eight routes

Simple inter-board wiring
• Serial bus reduces system wiring
• Digitrax LocoNet® compatible
• DCC gateway to serial bus

Simple basic setup
• "Smart Programming" in conjunction with self programming

The SMD84 is capable of driving eight twin coil solenoid and/or stall motor type switch machines. It contains a Capacitor Discharge Unit to store energy. A discharge of this energy is used to energize the solenoid to change the switch state. Using a CDU reduces the current drawn from the track to a small level sufficient to recharge the CDU. Once the CDU has been discharged (a switch state changed) a time period is required for the CDU to recharge. This is typically about three seconds. Since more than one switch may be requested to change state at the same time, as in a route, the requests are held so that each switch may change state in turn. A stall motor type does not use the CDU and therefore, no recharge time is required when activated.

There are four output configurations: short pulse for solenoid type machines, continually on for stall motor type machines, eight second pulse (variable) for stall motor type machines where reduced track current is desired and manual control.

The eight outputs of the SMD84 can individually be assigned any address from 1 to 2040. There are 13 inputs available for local control. An addition eight inputs are available with the use of some resistors.

Since the SMD84 is an accessory decoder, it can be controlled via DCC commands. It can also be controlled via the serial bus (LocoNet® compatible). A DCC gateway feature allows DCC switch or accessory commands to be passed directly to the serial bus. This allows DCC switch commands to control devices connected to this serial bus. Other devices (SRC16s or SIC24ADs for example) connected to this “accessory” serial bus can communicate with each other. This can provide several advantages for a DCC system.

“Smart” programming is a term used to describe a method to program Configuration Variables (CVs) using switch addresses. This method is a much easier way to program CVs. With “Smart” programming, you can set the output addresses and the output configuration for solenoid or stall type switch machines.

The SMD84 is not fully compatible with the Atlas, Backmann or MRC (except Prodigy Advance) DCC systems because they do not support accessory commands or can not program the number of CVs in the SMD84. The SMD84 will not drive SwitchMaster, Cobalt, Scale Shops, Switch Tender or Fulgurex motors. Note: 1/22/15 the Cobalt iP Analog has a much lower current requirement and should work oK.

smd84 diagram