Comparison With Hardware Flow Control
The principal advantage of software flow control is the reduction in the number of electrical conductors between sender and receiver. Given a common ground, only two signals are needed, one to send and the other to receive. Hardware flow control requires additional wires between the two devices. It also requires specific hardware implementation, which had more significant costs in earlier days of computing (i.e., 1960s and 70s).
However, software flow control is not without its problems. Sending XOFF requires at least one character time to transmit, and may be queued behind already-transmitted data still in buffers. Hardware signals may be asserted almost instantaneously, and out-of-order.
As the name "software flow control" implies, flow control using this method is usually implemented in software (or at least a higher level of firmware), which can cause further delays in XOFF response. Hardware flow control is typically under the direct control of the transmitting UART, which is able to cease transmission immediately, without the intervention of higher levels.
Finally, since the XOFF/XON codes are sent in-band, they cannot appear in the data being transmitted without being mistaken for flow control commands. Any data containing the XOFF/XON codes thus must be encoded in some manner for proper transmission, with corresponding overhead. This is frequently done with some kind of escape sequence. For printing devices that directly interpret ASCII codes, this is not a large problem, because the XON and XOFF codes use ASCII "device control" code numbers.
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