gwr@gomez.uucp (Gordon W. Ross) (07/26/89)
In article <539@vice2utc.chalmers.se> d5kwedb@hacke9.dtek.chalmers.se (Kristian Wedberg) writes: >I've written a Videotex program (Prestel type) for the Amiga, a kind of >communication program. Trouble is, here in Sweden some folks have 75/1200 >modems instead of the more normal 1200/1200. >[...] is it possible to transmit 75 bit/s and receive 1200 bit/s over the >serial port? At the same time? Or would you need some kind of transformer? > wonders kitte d5kwedb@dtek.chalmers.se Yes, it should be possible to transmit at a low speed using a serial port capable only of matched transmit and receive rates. The best way to do it is probably to write a special driver which generates each bit of the 75 bps data under software control. This is made relatively easy if the UART (Universal Asynchronous Receiver/Transmitter) can be programmed to generate a "break" signal of any desired length or to generate a "continuous break" until software tells it otherwise. Most UART's can do this. A software driver would have to handle timing for the 75 bps output and generate a "break" of the correct length for each zero bit to be transmitted (the start bit is also a zero bit). If the UART cannot generate "break" signals as described above, one could use some other, controllable output signal for the TD (Transmitted Data) line to the modem. A special, non-standard cable would be required to connect (for example) DTR to the modem TD pin. Again the driver software would have to handle the timing and generation of each bit in the 75 bps transmitted data, and would generate the transmitted data on the (for example) DTR line. Are these dual-rate modems very common? Gordon W. Ross gwr@gomez.mitre.org (617) 271-3205 (daytime) The MITRE Corp. (M/S E025) Burlington Road, Bedford, MA 01730
claudio@forty2.UUCP (Claudio Nieder) (08/03/89)
As I know somebody who has the MultiTerm Videotex program, I took my oscilloscope and attached it to that Amiga to see how the 1200/75 bps are produced there. What that program does is, it produces from every character a stream of character in such way, that when you send them out at 1200 bps it will result in a waveform very similar to that required by a 75 bps output. Of course there remain some spikes, because the start and stop bits are not under the programmers control, but probably they don't disturb, or you can filter them out. claudio
Claudio.Nieder@mamab.FIDONET.ORG (Claudio Nieder) (08/06/89)
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