Why do we have cross-over cables.

You missed something, and there's a good reason why this condition persists.

In a minimalist DB25 connection between DTE and DCE we have this:

DTE DCE DB25M CABLE DB25F +---------+ +---------+ | TXD 3>>3--------------------3>>3 RXD | | RXD 2>>2--------------------2>>2 TXD DCE | | SIG GND 7>>7--------------------7>>7 SIG GND | +---------+ \\ / +---------+ DB25F DB25M

Notice that the cable conductors do not cross and are wired 1:1 from end to end.

But now assume someone wants two computers to talk to each other through their serial posts, and we wind up with this:

DTE DTE DB25M DB25M +---------+ +---------+ | TXD 3>

The way the Yost wiring system works is one way around this.

It uses an RJ-45 jack. Every device (DCE or DTE) is wired up to put the "data I am sending to you" on pin 6 (yellow wire), and to look for "data from you" on pin 3. In the case of DCE, "data I am sending" is RD and "data I am getting from you" is TD; in the case of DTE, the opposite is true.

The 8-pin cable used for Yost is wired up with a "flip over", so it connects pin 3 at each end to pin 6 at the opposite end.

There's a similar pairing-and-cable-flipping between "terminal ready" signals (DTR from the DTE and Carrier Detect from the DCE) and "flow control" signals (RTS from DTE, CTS from DCE).

So, using the same cable, you can connect together a DCE and DTE, or two DCEs, or two DTEs. Every device has the same sort of connection port, and any two ports can be connected successfully using an RF-45

The magic here depends on two things: you're always using a flipover cable, and the devices at either end (DCE or DTE) are responsible for connecting the RJ-45 goezintas and goesoutas to their correct internal signal points. [In practice, the latter is usually done by wiring up DTE- and DCE-specific "DB-9/25-to-RJ-45 adapters").

I've used the Yost methodology at home for several years and quite like it.

You still do not understand. When these standards were made, and when such production began "mass production" was individual hand assembly, and regardless of who does which end (passing it off to another wastes yet more time), other than pin for pin wiring can and does increase prime pass yield failure rates.

If the wires change, the error rate goes up. If ALL assemblers wire ALL cables the same on ALL ends, the likelihood for error decreases exponentially, because they all learn, and inspect the same pin-out every time. Errors get made less often, and inspection error get made less often. With the advent of cable test fixtures, the only place left for the errors to occur is at the individual cable builder's bench. That builder's learned mindset, if hard wired to one spec, becomes less prone to error.

No luck about it. Make-and-break dialing STILL works.

Wire coloured x goes to pin y. You just need two groups of people, who've learned different schemes.

Sylvia.

Best to actually use a known good, proper cable, and not rely on a nic or router to be your cable tester, or repair facility.

Now tell me that one was wrong too.

And due to your quick scan of the ONE post it appears you read, you completely missed the point of the entire thread, much less the one post you responded to.

The port would have to have an expensive multi pin switch incorporated into the panel next to it. Very bad.

Yes, clearly with the endpoints wired as they are we need different cables depending on the kind of endpoints we're connecting. But that wasn't my quesion.

My question related to why this situation was created in the first place, since it is not a logical necessity.

Sylvia.

You are. If you want to test cables, get a cable tester. If you just want to hook things up, try it. You might get lucky. You'll always get it wrong, in any case.

Is there a standard specifying how this works?

I'd have to wonder how two such interfaces manage when they are connected together.

Sylvia.

You have no concept of hand assembly processes.

One person wire both ends. That ONE person only needs to examine the wiring of that one cable, one cable at a time.

Gathering up a bunch of cables to pass them off to yet another assembler has an attached cost that reduces profit, and cable fab shops pinch every penny. That's why they hire unskilled workers.

Then there is the error rate thing, which invariably also goes up.

When a single lost cable occurs, it costs the company more than twice the value of the lost labor. Usually about 5 times when the error is caught in-house. It can be 16 times the cost if caught in the field.

Rework is a very costly aspect of lean electronics manufacture. If error rates are high, it quickly becomes the most costly expenditure.

Yes, cables are simple. But that just means that the assemblers *think* they can be more casual. Otherwise there would be ZERO failures for a simple 9 pin interconnect. That is only 18 connections, after all, and that is if all pins were used. Yet still, we have much error.

The cables that have historically shown to be the least error prone assemblies are, in fact, pin-for-pin matings.

He already showed how it WAS and still IS a necessity.

Because the modem is "Data Communications Equipment", or DCE, and the computer/teletype is "Data Terminal Equipment", or DTE. When you use a computer to simulate a modem (or a server on the other end of the line) you need to turn the sense of the connector around. This is why reversing cables used to be called "null modem" cables.

Hope This Helps! Rich

No it doesn't. It entirely misses the point, which is why there is a distinction betwen DCE and DTE.

Sylvia.

situation.

This krw idiot, folks... is a troll. In this case, however, you were baited into it.

I was, in fact, correct. Note how it was me that stated that a nic is NOT a cable tester. You are just too retarded to have seen it in your glaring need to troll me and the group. F*ck off, KeithTard.

If a handshake fails, a different connection schema is used (read switched in) until success is achieved. Pretty simple.

And the other end is doing the same. Might go on forever.

Sylvia.

Actually, it is YOU that is missing the entire point.