Can an USART peripheral works (in Synchronous mode) with only TX, RX and Clock lines. And without the other left handshake signals (CTS, RTS, DTR, DCD)?

  • \$\begingroup\$ Yes, the modem control signals are to control modems for connections (test) and flow control. If the connection is permanent and buffers are adequate the link can be made without use of hardware handshaking. \$\endgroup\$
    – KalleMP
    Aug 27, 2015 at 22:33

1 Answer 1


Yes. Neither mode actually requires the handshake signals. The handshake signals are more of a convenience feature that can be used in each mode to determine when the devices are ready to do something. But they can easily be ignored (and often are).

The asynchronous mode requires only the rx and tx lines. Transmission rates are generally slower in asynchronous mode to ensure good performance. Since the clock signal is not provided, timing conditions must be recovered from the data signals - typically in the form of start and stop bits. These extra bits reduce the amount of useful information that can be transmitted in a given time period.

The synchronous mode requires only clock, rx, and tx lines. Transmission rates are generally faster in synchronous mode. Since the clock signal is provided as a reference, start and stop bits are usually not necessary.

  • \$\begingroup\$ The synchronous modes were common in the past when line drivers and modems were often without any error correction and the bit rate was the same as the baud rate so the clock was already recovered and allowed the USART to run faster as it did not have to use a 16x clock .... \$\endgroup\$
    – KalleMP
    Aug 27, 2015 at 22:28
  • \$\begingroup\$ As times changed the terminal bit rate was decoupled from the line baud rate with the advent of block error correction and larger buffer and asych UARTs were able to handle progressively higher speeds with sampling clock speeds no longer really limited most of the reasons for synch connections fell away, some corporates have held onto them due to legacy protocol types more than anything else. Long haul circuit switching preferred synch connections though packet switching has made this redundant now as well. \$\endgroup\$
    – KalleMP
    Aug 27, 2015 at 22:31

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