During the 20th century, technicians tested typewriters and teleprinters by typing the sentence. Connecting the various sites was an enormous network of telephones, modems and teleprinters. Telex (TELegraph EXchange) was a public switched network of teleprinters. ASCII was designed in the 1960s for teleprinters and telegraphy, and some computing. These machines were superseded by personal computers (PCs) running software to emulate teleprinters. They were attached in-line to standard Lorenz teleprinters. The first standard, IPTC 7901, bridged the eras of teleprinters and computers. Computers used teleprinters for input and output from the early days of computing. If slow electromechanical teleprinters are used, one-and-one half or two stop bits are required. Teleprinters generated the same code from a full alphanumeric keyboard. TTS was also of some benefit to teleprinters as well as computers. Message centers had rows of teleprinters and large racks for paper tapes awaiting transmission. This at a time when 10-character-per-second teleprinters were the norm. Approximately 200,000 Model 15 teleprinters were built. The Baudot code was never used in teleprinters.