Protothrottle Receiver

Protothrottle Receiver

The PTRx is a small board that allows you to control Large Scale locomotives using battery power and the Protothrottle. It can operate in three dead rail modes- High power DCC, Electronic Speed Control and Servo mode.


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Theory of Operation

The Protothrottle uses an off-the-shelf wireless communications device called the Xbee. Unlike Airwire or other R/C designs, the Xbee is closer to an internet sort of communications network. There are broadcast and directed messages that can be formulated and sent in a defined protocol.

The PT sends out a broadcast message to all Xbees on the network once per second. However, if a control is changed, a message is sent immediately. This message contains all of the information needed to construct a DCC stream or drive an Electronic Speed Control or Servo. Every packet has the locomotive address, the throttle value and a bit for each of the function codes 0-28.

The receiver examines each of these broadcast messages from the PT. If the locomotive, PT and base address match, the message is passed to the DCC/ESC/Servo generation code inside the receiver. The receiver uses this info to format a DCC output stream and set the proper timing for the R/C outputs.

In all modes, Servo 0 follows the DCC throttle dependent on the servo mode setting. In ‘ESC’ mode, servo output 0 is combined with the direction to produce a ‘center-off’ signal compatible with Electronic Speed Controllers. In ‘Steam mode’ Servo 0 follows the throttle directly and instead of a ‘center-off’ signal, the direction is used to control servo 1. It is set to the low limit or high limit depending on the direction setting from the PT.

DCC Output Mode

Below is a block diagram of the components when used in DCC mode.

The PTRx intercepts the Xbee broadcast packets sent by the PT. It uses this info to generate the proper DCC stream which is output on the ‘D’ pin. This is fully NMRA compliant DCC, however it is only at a logic level (5v). That signal is then used to control a motor driver which effectively ‘boosts’ it up to whatever voltage your battery is (typically 14-18v). The 5v supply shown is a small ‘buck’ converter that draws power from the main battery to produces a clean, steady 5v to power the receiver. The Motor driver (Amp) outputs then directly drive the DCC decoder. (Note that the Programmer is shown in this diagram, however it is not required for the operation of the Receiver)

ESC Output Mode

The hookup for the ESC mode is similar, however in this case we are using the Servo 0 R/C output signal. Below is a block diagram. Here we are using the BEC circuit to power the receiver so we don’t need the 5v converter.

Most Electronic Speed Controls have an R/C input and this is (almost always) a ‘center-off’ input.   An R/C pulse is always between 1000 and 2000 micro seconds. The center off mode sets the pulse to exactly in the middle of that range (1500us) to stop the motor. As the throttle is advanced, the pulse width increases until it gets to 2000us which is full speed forward. Likewise, when the reverse lever is set on the PT, advancing the throttle then decreases the pulse width from 1500 until it gets to 1000 which is full speed reverse.

Note the relay outputs. The PTRx has two digital outputs that can be assigned to DCC Function Codes from the PT to drive relays – Relay Module

Connectors Description

Below is a diagram of the pins on the Receiver. These are standard R/C connections, the pins closest to the edge of the board are ground, the pins in the middle are +5v and the pins nearest the chip are the signal pins.

   Z   X   D   2   1   0   P
   |   |   |   |   |   |   |________ Power Connection (+5v)
   |   |   |   |   |   |____________ Servo 0 (Throttle)
   |   |   |   |   |________________ Servo 1 (Direction in steam mode)
   |   |   |   |____________________ Servo 2
   |   |   |________________________ Logic Level DCC signal
   |   |____________________________ Digital Output X
   |________________________________ Digital Output Z

Power Connections

The schematic below shows how I wire all my installations. A DPDT switch is used to connect the battery to either the charge jack or power the system. This makes a nice clean break of all power to the system. DO NOT forget a fuse! If you use Lipo batteries, they can dump a TON of current if they are shorted and with no fuse it will melt wires and cause all sorts of havoc if you make a mistake and short them.

Cables, Speed Controllers and other Accessories


Wiring LEDs

Using LEDs with DCC decoders

Upgrading the Firmware

Firmware Upgrades