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TTL: What is it and How Does It Relate To RS232?

USB, Ethernet and RS232 are terms that many people are aware of. A good percentage of people also know of RS422 and RS485 and how those standards relate to RS232. Many have used converters when dealing with those serial standards. What's amazing is that comparatively few know what TTL stands for, and what is meant by TTL communication and TTL devices.

To be honest, unless you're an electrical engineer or hobbyist, information on TTL is not that easy to come by. If you look it up you actually find two definitions for TTL, both referring to data communication. One is "Time to Live" and refers to a data field in the Internet Protocol that indicates how many more hops a data packet will travel on the Internet before it is returned or discarded. A second one, and the one we're interested in, is "Transistor-Transistor Logic" but even that requires extra explanation.

Transistor-Transistor Logic, or TTL, refers to a way of constructing digital circuits. It was invented in the early 1960s when first Sylvania and then Texas Instruments created TTL-based integrated circuits. In TTL circuits, there are two transistors responsible for driving chip output, one of them generating the logical zero and the other the logical one. With TTL, a logical "zero" is defined as ground (or usually 0 to 0.4 Volts), and a logic "one" as plus 5 Volt. That made a lot of sense since it is simple and microcontrollers are usually run on a single supply voltage.

RS232, on the other hand, defines a logic "zero" as larger than plus three Volts and a logic "one" as less than minus 3 Volt. Voltages around zero, which is usually ground, are not considered a signal at all. Why use positive and negative voltages when circuits are natively generating either a voltage or no voltage? Because signal degradation and interference are much more likely to render a TTL signal unusable than a RS-232 signal where a "one" is very clearly negative and a "zero" very clearly positive. This means that TTL signals are generally used for fast communication within a device whereas RS232 signals are used for slower external connections over longer distances.

Do note that there is a difference between actual TTL circuits, and using the term "TTL" to simply refer to the voltage levels used for communication. "TTL" communication can happen without any actual TTL circuits or devices present.

Anyway, you can see the difference between TTL and RS232 now: one is based on signals generated by a certain type of integrated circuit, the other refers to signals that are specifically defined for reliable data communication.

Equally clear should be the need for a converter when you're dealing with both standards. Voltages need to be properly "translated" between the standards. More precisely, signals ranging from ground to 5 Volts must be translated into signals that can be up to plus or minus 10 Volts. That requires additional power sources and voltage inverter circuitry. Modern converters usually don't need an external power supply as they are powered by the RS232 data lines themselves. Also note that serial to TTL adapters are available between RS232 and TTL as well as RS485 to TTL, and that TTL devices can be 3.3 Volt or 5 Volt, so make sure you get the proper converter.

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