Example_USB

Current Loop Example with Compact USB Interface

Hardware

The idea is to connect a single current teletype to a PC running Windows or Linux using a TTL<->USB adapter in its basic functionality. This minimal setup could be expanded to include a high-voltage power supply and polarity reversal relay, enabling true TW39 and allowing an FSG to be connected. However, the hardware requirements are higher and, due to the expanded possibilities, this is more likely to be implemented using a Raspberry Pi GPIO controller.

The example shown here can be used with a Windows-PC, Linux-PC, Mac (not tested) and Raspberry Pi.

Block diagram

Besides the CH340-USB-Adapter and the RX and TX data from the TTY interface circuit, a current regulated power supply for the line loop is required.

Schematics

The more detailed schematics show a step-up converter to generate a line voltage of about 24V, an LM317 in conjunction with a resistor as current regulator, an optocoupler to extract the RX data potential-free from the line and finally a BC337 transistor for switching the line for TX data.

Assembled circuits

Version 1

Here is the first approach with a USB to TTL adapter (middle), a DC/DC converter (bottom) and a self-made-board (top) for current regulator, reading and controlling the loop.

Version 2

This version uses the base PCB from the Raspberry Pi project part which is mounted in a lasered plywood case together with a DC/DC converter 5 to 30V and the CH340 board.

RTS/CTS Hack for TW39

To decode the TW39 protocol correctly, it is necessary to detect a permanent low and high level on the RXD pin. Neither the CH340 chip nor the drivers drivers can handle that. The hack is to use a separate input pin (CTS) to detect the state of the line.

Therefore the pin RXD has to be connected to pin CTS of the CH340 chip.

Most PCBs with the CH340 chip do not route the CTS pin to a connector. Use a thin wire to connect the pins 3 and 9 directly on the chip.

On some CH340-Adapter boards, the CTS line can also be accessed via a soldering point like here:

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Software

For Installation see SW_Install

This electronic is handled by the software module CH340TTY.

Start the program with arguments -W TTY on Linux:

$ ./telex.py -W /dev/ttyUSB0

or on Windows:

$ ./telex.py -W COM3

Adapt the com/tty port name to your PC/RPi setup.