341:. Since the computer is faster at sending data than the printer can print it, the printer falls behind and approaches a situation where it would be overwhelmed by the data. The printer reacts to this situation by sending XOFF to the computer, which temporarily stops sending data. When the printer is again ready to receive more data, it sends XON to the computer, which starts sending data again.
47:
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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
367:
However, software flow control is not without its problems. The most important drawback is that software flow control is less reliable. 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
364:, 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).
563:
Robust XON is a technique to restart communication, just in case it was stopped by an accidentally received XOFF. The receiving unit sends periodic XON characters when it can receive data, and the line is idle. One common use is by serial printers (like
601:
170:(from "transmit off" and "transmit on", respectively). Thus, "software flow control" is sometimes called "XON/XOFF flow control". This is in contrast to flow control via dedicated
326:
When one end of a data link is unable to accept any more data (or approaching that point), it sends XOFF to the other end. The other end receives the XOFF code, and suspends
652:
523:, employ "padding" (short delays using millisecond granularity) to allow such equipment sufficient time to perform the requested actions without the need to assert XOFF.
472:
610:
64:
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II) to indicate they are online and ready to receive data. The XON is sent every 1 to 30 seconds depending on the printer's firmware design.
483:, may suffer from buffer overruns when using software flow control, although this can be somewhat mitigated by disabling the UART's FIFO.
475:, which is able to cease transmission immediately, without the intervention of higher levels. To handle the latency caused by builtin
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130:
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491:. For printing devices that directly interpret ASCII codes, this is not a large problem, because the XON and XOFF codes use
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ASR adopted two of these, DC3 and DC1, for use as XOFF and XON, respectively. This usage was copied by others, and is now a
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for use as XON/XOFF specifically. However, it does provide four generic "device control" characters (DC1 through DC4). The
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68:
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479:, more advanced UARTs, like the 16950, provide "on-chip" software flow control. UARTs that lack such support, like the
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XOFF/XON are still sometimes used manually by computer operators, to pause and restart output which otherwise would
511:, to indicate they are temporarily unable to accept more data. Typically, this is due to a combination of limited
79:
57:
549:
609:. Moxa Tech Note (1.0 ed.). Moxa Technical Writing Center (published September 30, 2009). Archived from
330:. Once the first end is ready to accept data again, it sends XON, and the other end resumes transmission.
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195:
471:. Hardware flow control, on the other hand, is typically under the direct control of the transmitting
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357:
467:), which can cause further delays in XOFF response. These delays can lead to data corruption due to
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software generally implements XOFF/XON support as a basic function. This generally includes the
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The principal advantage of software flow control is the reduction in the number of
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Software flow control is used extensively by low-speed devices, especially older
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As the name "software flow control" implies, flow control using this method is
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35:
162:, over the primary communications channel. These codes are generally called
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151:
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653:"HP LaserJet IIP and IIP Plus Printers - Control Panel Keys and Menus"
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17:
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XOFF/XON can be employed in both directions, for example, two
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34:. For the oil company which formerly traded as XON, see
30:"Xon" redirects here. For the Star Trek character, see
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character code, XOFF is generally represented using a
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being full. Some terminal control packages, such as
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The
Secrets of Flow Control in Serial Communication
71:. Unsourced material may be challenged and removed.
8:
380:
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368:almost instantaneously, and out-of-order.
202:with decimal value 19; XON with value 17.
131:Learn how and when to remove this message
636:"The Termcap Library - Describe Padding"
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287:
283:
235:
230:
226:
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205:The ASCII standard does not reserve any
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360:between sender and receiver. Given a
233:for XON, also derive from this usage.
158:. It uses special codes, transmitted
352:Comparison with hardware flow control
7:
69:adding citations to reliable sources
375:Summary of flow control tradeoffs
237:XOFF/XON representations in ASCII
25:
45:
495:"device control" code numbers.
333:For example, one may imagine a
217:. The keyboard equivalents of
56:needs additional citations for
32:Star Trek: Phase II § Cast
430:Software f.c. (FIFO disabled)
1:
530:off the display too quickly.
444:Software f.c. (FIFO enabled)
463:implemented in software (or
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29:
348:connected to each other.
487:done with some kind of
337:sending data to a slow
80:"Software flow control"
416:On-chip software f.c.
402:Hardware flow control
190:For systems using the
600:Yang, Casper (2009).
578:Ethernet flow control
548:machines, as well as
358:electrical conductors
176:hardware flow control
144:Software flow control
297:Resume transmission
65:improve this article
678:Flow control (data)
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271:Pause transmission
238:
27:Flow control method
552:emulators such as
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207:control characters
534:Terminal emulator
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319:
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215:de facto standard
211:Teletype Model 33
150:used in computer
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16:(Redirected from
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659:. Archived from
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663:on May 9, 2006.
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469:buffer overruns
433:Good, but slow
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146:is a method of
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616:on Aug 4, 2022
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538:system console
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405:Most reliable
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225:for XOFF, and
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186:Representation
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180:RS-232 RTS/CTS
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558:Win32 console
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515:rate and any
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362:common ground
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156:RS-232 serial
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82: –
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76:Find sources:
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60:
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54:This article
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43:
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37:
33:
19:
661:the original
639:
630:
618:. Retrieved
611:the original
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499:Applications
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370:
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346:teleprinters
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328:transmission
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178:" — such as
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148:flow control
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118:
108:
101:
94:
87:
75:
63:Please help
58:verification
55:
640:www.gnu.org
566:HP LaserJet
447:Unreliable
396:Out of Band
174:signals — "
172:out-of-band
584:References
540:on modern
152:data links
121:March 2009
91:newspapers
36:ExxonMobil
322:Mechanism
263:Keyboard
196:character
672:Category
572:See also
556:and the
505:printers
465:firmware
391:Low cost
335:computer
245:Meaning
521:termcap
517:buffers
461:usually
339:printer
160:in-band
105:scholar
620:Aug 4,
528:scroll
513:output
107:
100:
93:
86:
78:
614:(PDF)
607:(PDF)
554:xterm
546:Linux
493:ASCII
481:16550
477:FIFOs
422:Some
419:Good
268:XOFF
249:ASCII
242:Code
192:ASCII
112:JSTOR
98:books
622:2022
544:and
542:Unix
507:and
473:UART
450:Yes
436:Yes
411:Yes
381:Type
310:Ctrl
300:DC1
294:XON
284:Ctrl
274:DC3
227:Ctrl
219:Ctrl
200:byte
166:and
164:XOFF
84:news
550:GUI
453:No
439:No
425:No
408:No
306:11
303:17
280:13
277:19
259:Hex
254:Dec
198:or
168:XON
67:by
18:XON
674::
655:.
638:.
592:^
560:.
182:.
642:.
624:.
314:Q
312:+
288:S
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231:Q
229:+
223:S
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134:)
128:(
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119:(
109:·
102:·
95:·
88:·
61:.
38:.
20:)
Text is available under the Creative Commons Attribution-ShareAlike License. Additional terms may apply.
