[GreenKeys] TELEX Emulator for TTY 32

[Paul]

I decided I wanted to use my M32 Telex machine exactly as I received it from a food company where it was used to transact commodity purchases. I didn’t want to cut into the CCU to tap into the SMD or jam pointy objects into it to stop it from running open. Thus I determined to build me a Telex Exchange Emulator, the design of which I offer to other owners of WU Telex (and TWX?) machines. 


The TEE implements the originate function of a Telex connection as described in Teletype document 574-173-100TC, “32 Call Control Unit General Description and Operation”. My CCU is a UCC-9. 


The attached documentation consists of four files: a PDF of the Express PCB schematic, a .h header file, a .c program file, and a JPG. The Express PCB .sch is available on request but not the .pcb. This was my first use of the Express PCB tools and it does not deserve to be replicated. I have my one-off working system hidden in the pedestal of the 32 so I have no interest in “product-izing” it. With the exception of the PCB, the design used only parts that I had on-hand. 


I took a couple liberties in the circuitry. I get the high voltage loop supply by wiring a power transformer “backwards” and driving it from a similar step-down transformer. The pseudo RS-232 interface on the D-connector gets the job done. Otherwise, the hardware is straight-forward. 


The ATtiny26 Atmel AVR processor costs about $3.50 in singles and has more than enough resources to do the job. An IDE, AVR Studio 4, can be downloaded free from Atmel. AVR Studio links seamlessly with WinAVR, the compiler tool-set available at sourceforge.net. I use Atmel’s AVRisp to program the device in-system. 
The code is written in C and has three sections: a simple interrupt handler to service a free-running timer, a state machine to do the exchange emulation, and an update section to maintain the diagnostic LEDs and the data transfer between the D-conn and the telegraph loop. Even though the telegraph loop is half-duplex, it appears as full duplex at the D-conn. The entire program uses about 8% of the available program space. With some additional effort the ATtiny could be pressed into service as a speed and code converter but I leave that as an exercise for someone else. 


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