PC to printer interface

[jjimenez101]

Hello all,

As I read about PC to printer interfacing, I learned that each data pin ( 8 in total) in the printer cable has its own ground for that data signal. Meaning 8 data signals and 8 grounds. Apparently, this is to reduce electrical noise.

I would assume that there should only be one ground (common ground) for this connector. Theoretically, how does this scheme reduce noise?? And why is this method not used in serial communications?

Thanks

 

[Sceadwian]

Actually, it is used in serial communications, because there's only one data line there only needs to be one ground. The rest of the lines are flow control or status lines and not meant to be operated at high speed.
A a printer port in order to minimize noise as much as possible the individual data line ground would be attached to the shielding that goes around the data lines grounding any RF or inteference signals that strike the shielding. Not sure if this is even done in modern connectors, they're not used at high enough speed or over long enough distances to really matter. Virtually all modern high speed communications bus's are now done using low voltage differential signalling, which doesn't require invidual grounds for noise reduction.

 

[ericgibbs]

hi JJ

Some of the earlier Prn cables had twisted pairs, ie: 1 data twisted with, 1 ground, for the 'data' lines,
this does reduce interference quite well.

The later cables I have seen dont seem to bother with the twisted pairs and rely on the overall outer screen.

On my external Prn port projects I just link pins 18 thru 25 together as ground.
Also on some PC's especially Laptops the ground is not always connected on all 18 thru 25 pins.

Never experienced additional noise problems.

Is this what you mean ?

Eric

 

[Hero999]

I thought most modern printers used USB anyway.

 

[jjimenez101]

Sceadwian, being that printer interfaces are not high speed, why would they need low voltage differential signaling?

ericgibbs, If you connect pins 18 through 25 to ground, that would mean ONE common ground. But, apparently they are supposed to be connected to different grounds. From what you replied, if I understand correctly, you connected these 8 lines to one common ground and DID NOT experience any noise problems?

 

[ericgibbs]

hi JJ,

Correct, I have never experienced noise problems with the Parallel port using 18 to 25 linked to 0V on a project board.

What are you planning to use the port for.??, maybe I can help.

Eric

 

[Sceadwian]

jjim, when they came out and for MANY years afterwards, parallel ports were considered high speed, up untill a couple handful of years ago you could still buy external hard drives that used a parallel port, I even have a couple of Xircom ethernet adapters floating around that work on a parallel port. Though very little used most printer ports on PCs are capable of using ECP mode which allows data transfer rates in the neighboorhood of 1 or 2 megabytes per second. According to specs at least ECP maxes out at 2.5 megabytes per second (ideal conditions and hardware) that's twice as fast as standard 10mbit ethernet.

 

[Leftyretro]

jjim, when they came out and for MANY years afterwards, parallel ports were considered high speed, up untill a couple handful of years ago you could still buy external hard drives that used a parallel port, I even have a couple of Xircom ethernet adapters floating around that work on a parallel port. Though very little used most printer ports on PCs are capable of using ECP mode which allows data transfer rates in the neighboorhood of 1 or 2 megabytes per second. According to specs at least ECP maxes out at 2.5 megabytes per second (ideal conditions and hardware) that's twice as fast as standard 10mbit ethernet.

And it's applications like this that requires using all the best cabling techniques that the 'centronics connector' has available including using twisted pair for each signal, a good overall cable shield and proper signal termination at both ends and overall length only as long as needed. The IBM parallel interface port is capable of a wide range of performance speed and has always been a favorite port for DIY and hobbyist. As well as the 8 data channels there are control bits in both directions and strobe lines for good timing practices.

Lefty


Lefty

 

[jjimenez101]

Ok, now other than PC to printer interfaces, what other applications need the individual signal lines/channels to require their own ground line? Or is this type of scheme been replaced or overcome by different methods, for example as Sceadwian mentioned, the differential signaling scheme.

 

[ericgibbs]

hi
There are so many factors that would influence the decision to use twisted pairs, screened pairs, screen overall etc,

eg: cable lengths, signal switching rates, 'noisy' environments, EMI regulations, life/critical dependancy.....on...on....

These noise reduction/screening techniques don't only apply to a PC's printer port.

Tell us what are you planning to use the port for and we can advise you.

So far you have asked general questions whose answers are to be found in reading a good electronics reference book.

I believe Sceadwian was referring a different form of data transmission, USB etc

Eric

 

[jjimenez101]

I am not working on any project using PC/parallel port interfacing, the questions that I asked are just as you stated "general" so that I can start a discussion about this specific topic of which interested me.

Of course, any question that is posted in this site can be answered by reading an electronics book. The point here is to talk to fellow hobbyists, students and professionals rather than reading a plain text book that can't interact with the reader.

Thanks

 

[Sceadwian]

Twisted pair differential signals reduce common mode noise which is the biggest cause of noise in a transmission line. The increase in the precision, accuracy, and speed of semiconductor electronics over the years has allowed lower voltages to be used which reduces the effects the RLC characteristics of the transmission line have on a signal, greatly increasing the possible bandwidth and making transmission line length less critical.

 

[mneary]

Twisted pairs also reduce radiated noise, and can sometimes make the difference of passing FCC limits without a shield layer around the cable.

 

[ericgibbs]

hi JJ,
The reason why I asked re-project, is that often the OP, changes the question as the thread develops and
then finally says BUT what I really want to know is..........?????

The last two threads Sceadwian and Mneary, clearly define the usage of twisted pairs!.

Regulations regarding EMI/RFI are constantly being reviewed, copies of EMI regs can be downloaded.

Eric

 

[jjimenez101]

Great discussion.

Thanks to all.