Many end users tend to equate baud rate and bit rate. We believe you, the reader, know better but we felt the urge to be more precise…so here it is:

Baud rate and bit rate are not the same. Baud rate means the number of symbols or characters per second sent over a serial link. Bit rate means the number of bits per second over the link.

The relationship between Baud Rate and Bit Rate depends upon the total number of bits per character. Perhaps the most commonly used set of communications parameters used are No parity, 8 data bits and 1 stop bit. Adding this up, you always have 1 start bit, so 1 + 8 + 1 = 10 bits / character total. Whatever time period required to send a bit, in this example, it will take ten times as long to send a complete character. Therefore the bit rate is ten times the baud rate.

The table below shows the relationship for all parameter settings you can get with a 16750 UART which has a specified maximum data rate of 1 Mbps — we use 921.6 Kbps to illustrate actual baud rates. Note that it is not possible to have 2 stop bits with a 5-bit word (the UART produces 1.5 stop bits instead).

Note that depending upon parameters, you will get a throughput of from a lowly 76 Kbaud to a raging 137 Kbaud. Yet in all cases the UART is working at top speed, 921,600 bits per second.

The general rule about data rates is: “It’s the system, stupid.”

I hope you don’t take the above personally. Often we tend to forget that todays computers run at rates of an instruction every few dozen nanoseconds, that what really gets run is threads, not programs. Everything depends upon everything else.

In trying to characterize a serial port as “921.6” or “115.2” Kbps, we see from the table above that there’s a huge difference between bit rate and baud rate. On that, we’ve met the enemy and he’s us. We did this. We casually rattle on about how this board or that UART performs to such-and-so “Kbps” and it’s easy to forget that this doesn’t represent a transfer rate of so many characters or symbols per second.

It is not possible for us or anybody else to predict exactly how fast our hardware will work in your system. Therefore, the data rates specified for all hardware and software on this site is in Kbps maximum each port that the product can achieve under optimal but realistic conditions. You can translate this into baud rate per the above table if you wish.

Here is a partial list of items that can, independently or in combination, affect the actual performance of your software and/or hardware.

• Processor(s) speed and multiprocessing capability
• Operating system and service pack level, hotfixes, etc.
• Overall software load on the system – other threads, applications, services, and drivers consuming CPU time
• Use or non-use of flow control protocols: RTS/CTS, DTR/DSR, etc.
• Efficiency of the software using a port. Burning CPU cycles checking buffer content, reading a character at a time from the buffer – reduce the efficiency.
• Cable length and design – extra capacitance can reduce effective performance and introduce errors. More errors equals reduced effective data rate.
• Electrical load (resistance, capacitance) on hardware outputs. Consider daisy-chaining RS-485 devices as increasing the load; at some point this can degrade performance.
• Thermal effects: Digital circuits do not function the same over the range of temperatures they can tolerate.
• Use of surge suppression options. The IND option (available for some Quatech hardware) adds enough capacitance load to limit performance severely. Some Quatech hardware specified for 921.6 Kbps will, when the IND option is installed, function well only to 115.2 KBps in typical installations.