I recently salvaged a thermal printer saying "EM205_101122" on it's circuit board. I also got success in accessing to all it's datasheets. It seems that it has a RS232 as well as TTL ports, but both of the ports are given two extra pins saying "RTS/DTS and DTR/DSR".
While I read about them, they are the flow control pins and are to be kept +12V or -12V.
For communication with the Arduino (as given in this tutorial: http://bildr.org/2011/08/thermal-printer-arduino/) does these pins are required to be used or we can just use the RX TX pins only?