You remember about 5 years ago, PC's had serial ports and parallel ports.
Right now I've got a laptop and it only has USB. I've been looking around for some sort of adapter device for converting USB to DE9 for serial ports, and also USB to DB25 for parallel ports.
All over eBay, there's USB-to-parallel-port adapters that work as follows:
There's a converter from USB to RS232 and it has a DE9 plug on the end of it. From there you have a simple adapter that goes from DE9 to DB25. Obviously, since the DB25 has 16 more pins than the DE9, I don't see how this is possible. The only thing I can guess is that 16 of the pins are "left dead". Here's an example of such a device:
If you read the description, it openly proclaims that it can be used for hooking up things like scanners and printers. Does anyone know what's going on here? Is it trully possible to get a parallel port from using this?
Also, there's a different device that claims to be a proper USB-to- parallel adapter:
Centronics style parallel ports (and the follow on IEEE 1284 ports) are nothing at all like (RS-232) serial ports. They just happen to often use DB-25 ports on the backs of PCs. Note that those are basically always DB-25 *female* port, unlike serial ports which use males connecters. At the printer end, these often use the old Centronics style connector (which were also occasionally used on the host end of the connection, but the large size was often awkward). See the second picture here:
Unlike serial ports, parallel ports transmit data in, *ahem* parallel, with some handshaking. It was designed to be a cheap to implement interface using TTL logic levels and minimal ports at both ends (eg. no expensive UART).
RS-232 is defined with the 25 pin connector, but back in the AT days, PC vendors started using the DB-9s to save space, since for async serial communications, most of the RS-232 signals are useless, and you can fit what you actually need in those nine pins.
The USB-to-serial dongles work OK (although driver issues abound for some unknowable reason), so long as the serial device is, like a modem, fairly tolerant of slow responses to the handshaking and flow control signals. A "real" serial port can often respond to a drop of CTS in a handful of character times, while the USB serial ports can have hundreds of bytes in buffers. Also, some people use the flow control signals on serial ports to control some device directly - while on a "real" serial port these can be modified very quickly, the USB serial devices may require milliseconds to propagate the change to the actual device. But within those constraints, they work passably well. There are some USB serial devices which claim to be better on some, or all, of those parameters, so if you need something like that, you may want to do some research.
The USB to parallel port adapters, OTOH, tend to not be direct replacements for "real" parallel ports, and are typically useful only for driving parallel printers. Many people who have used the parallel port as a cheap DIO port to control something have had to change with the demise of real parallel ports.
Note that "real" parallel exist on PCI, and PCIe(!!) cards, and in PCMCIA (aka CardBus) versions as well, which you can add to most PCs. You can also add "real" PCI, PCIe and PCMCIA serial ports as well.
DB25 is nothing more than a connector, it can be used with different kinds of transmission protocol
Serial ports use the RS232 protocol
Parallel ports use the IEEE 1284 protocol
Serial ports have a DE9, and parallel ports have a DB25.
Now I've been told that serial ports can use a DB25, which is a 25 pin connector. When I don't understand about this though is how you can have an adapter that goes from DE9 to DB25.. ? I mean how can 9 pins suddenly become 25 pins? Are the extra 16 pins left dead?
If the extra pins are left dead then how could this adapter possibly have a use? I mean if a device is using a DB25 for serial communication then I presume it must need the extra pins, otherwise it'd be using a DE9, right?
Still I don't see how a 25-pin plug can become a 9-pin plug?
What happens to the extra signals on the 16 dead pins?
If there's no signals on these pins to begin with, then why does the advice use a DB25 instead of a DE9 in the first place?
Why do you say that the answer is no? USB is a hell of a lot faster than even the most advanced parallel port (IEEE 1284), so it seems to me that this can be done perfectly if you have:
A microcontroller inside the adapter that does the job of converting USB signals to parallel port signals * A device driver for the PC which fools the computer into thinking it has a fully-fledged parallel port
It's a device I bought off eBay a few days ago, it's for hooking up an internal hard disk via USB (i.e. USB to IDE, or USB to SATA). I haven't received it yet but I'll let you know how well it does or does not work.
Because the original used DB25. There are many standards that used DB25. In the older days, there weren't quite so many different connectors as there are now (while there were, but most of those are long dead, like the DD50, or some of the 37-pin ISO connectors), so it got used for many more different protocols and connectors.
Some of the protocols took over many of the 25 pins available. Some, like RS-232 only used a handful of pins. Then the IBM PC/AT designers felt like they wanted to have two serial ports in one card slot rather than two in two slots, and used a non-standard DE9 with the reverse (ie. male) pins to do it. Then it took off, and the whole world had to go along with it, and the PC world expects a serial port to be a male DE9 instead of the original female DB25.
Since RS-232 only used 9 pins anyway, it fit just right on it. The other 16 pins go unused, and aren't connected to anything anyway.
Again: The RS-232 standard is for a 25 pin plug, the DB-9 is a (non- standard, although universally supported) subset of that. When you wire the two together, the other pins are left unconnected. There
*are* signals defined for all 25 pins, but most are of no use for async serial communications (which is almost all you see on PCs). Some early PC manufacturers decided to use a DB-9 instead of a DB-25 because the brackets on ISA cards were only so big, and they could put more stuff on it with the smaller connecter. The DB-9 only carries the signals useful for async. You'll notice that even now many external serial (async) modems have a DB-25, and you use a cable with a DB-9 at one end and a DB-25 at the other - that simply wires the adapter into the cable.
Note that it's possible to have RS-232 async communications with only three wires (transmit, receive and ground), although that means giving up some generally useful status information and using Xon/Xoff flow control.
Note that if you actually had a synchronous serial device attached to a synchronous serial port (*not* a normal PC async port), you would need the full connecter, although not necessarily all 25 wires).
The latency across the USB link makes it impossible for all but the slowest applications. Nominally the date/strobe/acknowledge sequence is about 10us (and visibility of the other handshake signals is closer to 1us). Full-Speed (12Mbps) USB will likely be a minimum of 1ms each way, and Hi-Speed (480Mbps) USB 100us. If you try to get cute by moving the handshake logic out to the USB device itself, you end up with something tied to a specific application - like the USB parallel port printer devices you can get today.
In the old days, long before you were born, computers were big machines, controlled using dumb terminals. These dumb terminals were connected serially, either by wire or over a modem, and it was useful to have a number of extra signals (clocks, power, handshaking, etc.). Thus DB25 connectors were used. Later, it was noticed that a lot of equipment could work fine with fewer signals, and the DB9 pinning became common. The DB25 connector was kept on PCs for backwards compatibility, but only the signals also found on the DB9 were implemented.
You think this because you don't understand what "speed" and "faster" means. The parallel port is faster than USB (even USB 2.0) in the same way that your PIC microcontroller is faster than your laptop. See if you can figure this one out before others drop too many hints. Remember, throughput for bulk transfers is only one way to describe speed.
I have also seen serial connections on RJ45, RJ11(both the 4 & 6 pin versions), and DEC used a MMJ which was an RJ11 with an offset tab, and HP (I think) once used a DB9 shell with a custom 3 pin arrangment. And wasn't it DG that used 6 or 8 pins in a line on one of their terminals.
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Not just Apple. SCSI DB-25s were common on ZIP drives and on scanners, and at least a few PC ISA SCSI adapters had DB-25s. Getting the right cable was always a challenge.
Frankly as far as parallel SCSI goes, you'd have an easier time listing connecters that *haven't* been used. (Yes, I'm exaggerating, but only a little).
Since we've mentioned Apple, they did serial on DIN-8s (similar to the old "round" PC keyboard and mouse connectors) for a while. TRS plugs (AKA audio jack plugs) are also used by some small devices. Lots of old comm gear used screw terminal blocks rather than plugs. Yes, it's a mess...