- PA2ION - Amateur Radio Station -

- QTH: Zeist, The Netherlands (JO22PC) –

A simple RS232 interface for use between Personal Computer and RTTY convertor and external AFSK generator.


Warning: this webpage is for personal use by PA2ION only! Anything you might do with this information is your own responsibility!

In the seventies many RTTY convertors have been designed for use with mechanical printers like the T37 from Siemens. In the period of the eighties came 'intelligent' terminals with a UART’s  and then the famous CBM64 from Commodore with its "COM-IN 64" program for RTTY-ASCII-CW-SSTV. During the nineties IBM compatible PC's were used  frequently for communications and then COM-ports were used often for in- and output of RX/TX data. The interface described here is useful for COM-port I/O in the neighbourhood of higher rated RF fields.
Only few electronic parts are needed to assemble an adaptor for RS232 ports.
The objective of this design was based on a opto-electric isolation of the RTTY convertor, computer and (external) AFSK generator from RF currents of TX/RX equipment.

o        CONVERTOR:
RX-audio signals are amplified by the DJ6HP telex convertor, several times filtered and amplified and until they shaped as blocks that form the Baudot signals.

Note: many convertors are designed to work with mechanical printers. Due to the mass of those type of printers fast spike pulses were ignored! Computers have their own rules! 
By 'trial and error' spikes were removed by a R-C filter placed over the output of convertor. The filter was optimized for 45/50 Baud. The RX quality for computer communications was improved now significantly.

The AFSK generator was switched by an invertor circuit.
TX-signals from the computer are sent via the RS232 to an optocoupler. Optocoupler output is sent to the input of the AFSK generator.

I used an 25 pol. female D-connector to build the interface. By scraping i removed unnecessary plastic parts inside the plastic caps of the connector.
With a jig saw two prints were made from 'island print material' both fitting inside the plastic cap of the connector.
The IC Max232, resistor and condensators were placed on the epoxy-side. Optocoupler and resistor were fixed on the copper side.
(a program for fine drawing  jobs was failing to me, hi)

o        RESULTS:
”This simple design proved to work very well for UHF and HF bands”.

PA2ION, October 1999.