SW_DevCH340TTY

Device Module "CH340TTY"

Description

Using an adapter board (or adapter cable - for V.10) based on the chip CH340 the device is detected as normal (virtual) serial interface. In Windows the device is shown as COMx:. In Linux it is often shown as /dev/ttyUSBx (may vary depending on the OS version...). With the Python library "PySerial" all necessary settings can be done to handle 50 baud, 5 data-bits and 1.5 stop-bits. Also, the handshake pins RTS and DTR can be set by this library. The TTL output signals of the CH340 PCB can drive some interface hardware for a TW39 / TWM / V.10 / EDS teleprinter. Examples can be found here.

Caution

There are several USB/Serial adapters that use different chips. Be aware that ONLY adapters with CH340 chip are compatible with this project. Only the CH340 chip supports a baud rate of 50 Bd!

System

System	Comments
RPi	OK
PC Linux	OK
PC Windows	OK
Mac	OK

Dependencies

Python Module	Install	Anaconda
pyserial	pip install pyserial	conda install pyserial

Command Line Arguments

USB-Serial-Adapter (CH340-chip) with base teletype (without dialing device):

-Y TTY
--tty TTY

USB-Serial-Adapter (CH340-chip) with TW39 teletype (pulse dial):

--ttyTW39 TTY

USB-Serial-Adapter (CH340-chip) with TWM teletype (keypad dial):

-M TTY
--ttyTWM TTY

USB-Serial-Adapter (CH340-chip) with V.10 teletype (FS200, FS220):

-V TTY
--ttyV10 TTY

Example Windows:

./telex.py -W COM3

Example Linux:

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

Config File Parameter

Param	Default	type/range	Description
mode	"TW39"	"TW39", "TWM", "V10", "EDS"	"TW39": current loop / Number Switch Dial "TWM" : keypad Dial, "V10" : FS200/FS220 mode "EDS" : ED1000 ""(empty): disable hardware handshake
portname	"/dev/ttyUSB0"	string	serial port used (COMx: in Windows)
baudrate	50	50, 75, 100	baud rate
bytesize	5	5, 6, 7, 8	number of databits
stopbits	1.5	1, 1.5, 2	number of stopbits
coding	0	0, 1, 2, 3	0: ITA2 = CCITT2 1: US 2: MKT2 3: ZUSE (see txCode.md)
loopback	true	false / true	if true, sent characters are removed from receive buffer
loc_echo	false	false / true	if true, echo back all characters received from teleprinter
inverse_dtr	false	false / true	If true, use inverted signal
inverse_rts	false	false / true	If true, use inverted signal

Tip

With mode set to any explicit value e.g TW39 or TWM, the module uses hardware handshake to control the TTY equipment connected to the matching interface hardware, to switch to WB mode, to A or Z mode.

Leaving modeempty, i.e. defining "mode": "", lets the module assume that there is no equipment with AT or ST button connected. The AT and ST buttons can then be emulated by special keystrokes at the teletype:

keystroke	button
5x Zi (Figs)	AT
5x Bu (Ltrs)	ST

Example config file section:

        # Module type "CH340TTY"
        # requirements: pyserial (Linux: sudo apt install python3-serial)
        # TW39 or V10 teletype over USB-serial-adapter with CH340 chip or UART which supports 50 Baud
    
        "CH340TTY": {
          "type": "CH340TTY",
          "enable": true,
          "portname": "COM5",          # Windows:"COMx" Linux:"/dev/ttyUSBx"
          "baudrate": 50,              
          "mode": "TW39",              
          "coding": 0,                 
          "bytesize": 5,
          "stopbits": 1.5,
          "loopback": false
        },
    

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. Some adapter boards provide a soldering point for the signals RTS and CTS, so you don't have to solder directly at the chip