Hardware PropertiesInterfacing devices
Types of UARTs
Loop Back Plug
We have already spoken briefly of DTE & DCE. A typical data receiver is computer and a typical data transmitter is modem. Often speaking of DTE to DCE or DCE to DCE speed. DTE to DCE speed is the speed between your computer and modem, sometimes this speed is referred to as limit speed. This speed must be faster than DCE to DCE speed. DCE to DCE speed is the speed between modems, sometimes this speed is referred to as the line speed.
Today most people have 28.8K or 33.6K modems. That's why we should understand that DCE to DCE speed would be 28.8K or 33.6K. To use such modem speeds we should use aproximate DTE to DCE speed of 115,200 BPS.(Maximum Speed of the 16550a UART). These speeds are sometimes confusing, sometimes people think that 9.6 KBPS, 14.4 KBPS etc. are modem speeds.
Today modems must have back-end data compression. This is analogous to PK-ZIP, but the modem software automatically compresses and decompresses the data. When the compression is set correctly you may achieve 1:4 compression ratio, and sometimes even more. Compression works especialy well with text files. So if you transmit a file at 28.8K (DCE-DCE) compressed by the modem, the DCE-DTE speed you actually get is 115.2 KBPS.
Some modem manufacturers speak of maximum 1:8 compression ratio. Wit such a compression at the modem speed of 33.6 KBPS the max baud rate we get between the modem and UART is 268,800 BPS. If you only have 16550a with max speed of 115,200 BPS that would cause downtime in work. The purchase of 16C650 should solve the problem of max speed up to 230,400 BPS.
However mind the way you work with your modem, if you don't get these speeds. These are MAXIMUM compression ratios. In some cases when trying to send already compressed file modem wastes much time on compression attempts, so the max baud rate between the modem and UART is reduced. If this occurs try turning off your data compression. This should be fixed on newer modems. Some files compress easier than others thus any file which compresses easier is naturally going to have a higher compression ratio.
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