English is not my native language and i did only translate the most important parts of the document. If there are any problems in understanding this text, please contact me (Arno Welzel) and not Jurgen Mayer. Thanks.
MausNetz: Juergen Meyer@HH2
E-Mail : Juergen Meyer@hh2.maus.de
Altough this circuit is working properly in three different machines, you can never exclude faults. Because of that i can not be responsible for any damages caused by the usage of this circuit.
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This circuit consits of a TTL-chip, the 74LS74 (and a switch to choose the high baudrate).
Every ST or STE can only use 19200 bps at the serial interface. This limitation is caused by the clock-frequency of 2.4576 MHz wich is used for the MFP-timers. The baudrate is set by the timer D of the MFP. There where programs wich switched off the "divider" (sorry, i do not know the correct word for german "Vorteiler") for the RC/TC-input of the MFP.
The problem of this modification is, that the syncronization does not work any more - so that there will by errors after every 16th, 32th or 64th byte. Because of that is not recommended to switch of the "divider".
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The circuit does the following: It gets the main-clock of 2.4576 MHz and divides it to produce 614.4 KHz. This may be done very simple, by using a RS-flip-flop to divide the clock by 1:4.
The 74LS74 consists of two RS-flip-flops. The first one is used to get 1.2288 MHz. The second one gets the 1.2288 and produces 614.4KHz, wich is needed for the RC/TC-input to use 38400 bps for the serial interface.
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The translator (Arno Welzel):
If you still want to use lower baudrates, you have also to build in a switch. Normaly pin 7 and 10 are connected to pin 16 of the MFP for "normal" baudrates. I hope this helps you.
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