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38k4 on every ST

Some people using a HST modem on a atari ST wished their computer had a 38400 Baud modem port for optimal arc transmission.A 38400 baud ,locked port can be created with some extra hardware.

I,writer of this document,and drawer of the picture do not feel responsable for any damage made to your equipment while building or using this 38400 baud serial port.

The file SER38400.PI3 contains a picture file wich gives you some information ,how to create a locked 38400 kbaud rs-232 serial port.

The principal is quite simple.Normaly Timer D creates the Baudrate of the USART in the MFP.In order to give the USART an external clock we have to disable the connection between the output of timer D (pin 16) and the reciever/transmitter clock (pin 7 and 10) of the USART. The easest way to to that is to make a little scratch op the platine.

At this point we have no clock for our usart.To get a new clock signal we take a clock signal of 2.4576 Mhz (pin 18 MFP) and devide it by 4 To do this,we use a 4040.The result on pin 7 of the 4040 (614400HZ) will be our new USART RC/SC.The frequency is this high because the USART will devide it by 16.Result is a serial port of 38400 KBaud.

I hope this explanation will give you enough information to create such a baudrate (if you need it)

resolution in gif is better than currently represented.