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This is mostly an educated guess but I'm assuming TTL Serial port means that the line level outputs of the serial port are at TTL level. TTL level defines zero as from 0 to 0.8 V and a one as from 2 to 5 V. Now, normal RS232 Serial Ports have an operating range from -15V to 15V for their output. The TTL Serial Port is a serial port that is specifically designed to interface with either TTL or TTL-compatible circuits. This means that it's much more effective for controlling a custom circuit than a regular RS232 Serial Port.
Matt
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