Baudot to ASCII Converter Tech details

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Technical details

General Description
The Baudot to ASCII Converter accepts current loop baudot data from a WF44 radar and drives an ASCII serial or parallel printer. The units achieves this by converting the current loop baudot data to parallel data, applies it as the address to an eprom, which functions as a conversion table. The parallel data out of the EPROM is used for the parallel printer output and also it is converted to serial data for use by a serial printer.
The attached printer would only print each line of radar data after the radar has sent a carriage return at the end of that line.

These units were assembled in commercial sheet metal cases. Each unit was largely constructed in isolation as the need arose, meaning that they were very much one off's.
A simple transformer, bridge diode, filter capacitor and TO220 regular based power supply scheme was used, once again the layout and form of this section was fairly fluid over the few unit manufactured.
Standard D-way connectors were used for the inputs and outputs to and from the converter. A DB15S connector was used for the parallel printer output, DB25S connector for the serial data output and a DB9S connector for the serial input data connection from the parent radar.

Trendcom 100 Thermal Printer
The First printer used was the Trendcom model 100 thermal printer. It was a very neat looking unit built in a buff coloured metal cabinet. This printer, instead of using solenoids driving pins in a print head, used a thermal printer-head but it needed a heat-sensitive paper. Their operation was very quiet, about as loud as sliding your finger across a piece of paper. They were inexpensive compared to other printers of the day (most of which cost over $1,000), although the printing looked very much like that produced by a dot-matrix printer.
The Trendcom Model 100 printed 40 characters per line on paper that was about 4 1/2 inches wide and came in rolls 80 feet in length. The print colour was black.

Circuit Detail
A National Semiconductor MM5303 universal asynchronous receiver/transmitter (UART) converted the serial input data into parallel to feed the address lines of a 2716 EPROM. This was programmed with the look-up table between baudot and ASCII codes. The output data lines of the EPROM then fed a pair of 3341 FIFOs (First In - First Out buffers) to provide a parallel data output to a parallel printer. The same EPROM output lines are used as the parallel input to another MM5303 UART whose serial output was clocked out at a 300 baud rate for a serial connection.
Two 555 timer chips provided the input and output clock rates of 50 and 300 baud. The later also fed a diode voltage doubler to generate a neagitive 12v supply for the output UART.
The Eprom address ranges were setup so that with an external toggle switch the input Line Feed Baudot character could generate either two line feeds, line feed/carriage return or two carriage returns - as required by the model of printer used.

Block Diagram

Input Data Format
The input data had to be in the following form:- 50 Baud, 5 Data bits, 1 1/2 stop bits and input as a 40mA current loop
Output Data Formats
The output data took the following forms:-
Parallel - 8 bit parallel data with negative going strobe pulses
Serial - 600 Baud, 8 bit, no parity, 2 stop bits, ASCII data
Problems, Faults, and their End
Experience showed that it was imperative that all seven teleprinter drive pulses were the correct length and that the driving source was setup for single current working. Most difficulties with the converter eventually were proven to be caused by either of these being incorrect.
The units being such a simple circuit, rarely if ever failed and lasted until better technology became available. So the Baudot to ASCII converters were eventually replaced by the Radar Printer Interface as it was able to printout calculated wind data.

Normal Operation - no letters
0000 - 001F Letters translated to Figures
0000 00 35 0D 39 20 24 2C 2E 0A 29 34 40 38 30 3A 3D
0010 33 2B 06 3F 21 36 25 2F 2D 32 07 81 37 31 28 82

0020 - 003F Figures
0020 00 35 0D 39 20 24 2C 2E 0A 29 34 40 38 30 3A 3D
0030 33 2B 06 3F 21 36 25 2F 2D 32 07 81 37 31 28 82

0100 - 011F Letters, normal
0100 00 54 0D 4F 20 48 4E 4D 0A 4C 52 47 49 50 43 56
0110 45 5A 44 42 53 59 46 58 41 57 4A 81 55 51 4B 82

0120 - 013F Figures, CR translated to LF
0120 00 35 0A 39 20 24 2C 2E 0A 29 34 40 38 30 3A 3D
0130 33 2B 06 3F 21 36 25 2F 2D 32 07 81 37 31 28 82