I am recently moving from C programming into automation engineering. I have learned about RS232 and TTL standards, and I understand that they are based on the same concept of serial communication, however TTL uses logic voltages (3.3 V or 5.0 V) to be compatible with microcontrollers, while RS232 uses higher voltages for historic reasons (signal-to-noise-enhancement).
To start connecting to the "real world", I planned on using a simple printer to start practicing sending bits via the COM-Port. Here is a link to an example: https://de.aliexpress.com/item/JP-QR203-58mm-Micro-Receipt-Thermal-Printer-RS232-TTL-USB-Panel-Compatible-with-EML203/32693670343.html
This device seems suitable for my needs, however, some questions remain, as I am very eager to learn, but still a newbie in electrical engineering:
The interface is stated as "Serial (RS-232,TTL)" – Is there any way to find out which standard is implemented on the PCB exactly, RS232 or TTL?
Another suitable device seems to be this one here:
https://www.amazon.de/WELQUIC-Thermodrucker-Bondrucker-Bluetooth-Standard-Art-1/dp/B075GG7VJT/ref=sr_1_2?ie=UTF8&qid=1533543476&sr=8-2&keywords=welquic+printer
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Again, how can I figure out whether RS232 or TTL is implemented?
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This device seems to feature a Mini-USB port for USB-communication and another Mini-USB port for RS-232/TTL communication… Which cables would be needed? Is there something like a serial/Mini-USB adapter cable? As I understand, USB standard has to be converted to RS232 or TTL – So my second question basically is, how can there be a port in Mini-USB-format for RS232/TTL?
Best Answer
The problem lies in the fact that the data stream as produced and consumed by a UART does not have an official name.
In the 'good old days' the UART data was always send through line drivers* before they came out of the equipment. The connector and the signal levels outside the equipment where formalized in the "RS232" standard.
Nowadays we use a lot of micro controllers en single board computers and the signals out of the "UART" interface are used directly. Which raises problem: how do you call that type of interface?
You will find that the interface is often referred to as "R232-TTL". With which the mean the RS232 data format but with TTL signal levels.
Therefore there is a high probability that the equipment has TTL level signals. But you can only find out for certain after you bought one or talked to the vendor.
To connect such an interface to a standard PC there are USB-RS232 converters/adapters. Yes, they are called as such although they are not conform to the RS232 standard in neither the signal levels nor the connector pin-out. Which proves the point I made above.
*The line drivers also inverted the UART signals which gave raise to more confusion: is the idle state high or low?