Messages in this thread | | | Date | Thu, 04 Jan 2001 20:22:40 +0100 | From | (Gunther Mayer) | Subject | Re: Printing to off-line printer in 2.4.0-prerelease |
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Jesse Pollard wrote: > Originally, (wayback machine on) this was handled by a pull-up resistor > in the parallel interface, on the "off-line" signal. ANY time the printer > was powered off, set offline, or cable unplugged, the "off-line" signal > was raised by the pull-up. No data lost. > > Now the parallel interface is bidirectional, and can have multiple devices > attached - this "fix" cannot be used. The interface is now more of a > buss than a single attached interface, and signals from a missing device > (powered off or disconnected) are floating. They may float high or low, > and depending on the environment (and which end of the cable is unplugged) > any thing in between.
Not true. Electrical characteristics for parallel port implementations/cards differ wildly, nevertheless most implementations have: - data lines: bidirectional (see datasheets) - signal lines: see datasheets, never floating !
Floating signal lines are a silicon bug/bad engineering and have nothing to do with bidirectional interfaces !
Nowadays most integrated chips have internal signal line pull-ups internally, e.g. W83877TF says: -BUSY, ACK, PE, SLCT, ERR: TTL level input pin. This pin is pulled high internally. -AFD, STB, INIT, SLIN Open-drain output pin with 12 mA sink capability. Pulled up internally. -Data lines: TTL level bi-directional with 24 mA source-sink capability.
Of course I would expect add-in cards to exist, with not so sophisticated chipsets and makers that have "forgotten" external pull-ups for economical reasons (2 cents :-) We should NOT care for broken hardware !!! I haven't seen any of these yet, even.
On the other hand printer implmentations vary wildly, too. LJ1100: leave signal lines alone if powered off (0x7f) i.e. signal printer-not-ready ack-active out-of-paper DJ500: signal printer-error and off-line when powered off (0x87) !!! => Linux would dump data on this printer, if switched off.
I think the current linux lp code tries to handle exotic/weird printers gracefully and leaves mainstream printers and users alone. - Gunther - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org Please read the FAQ at http://www.tux.org/lkml/
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