Making the adapter or cable you need

I've got shelves, covered with tupperware tubs and drawers of adapters, sorted by wiring type and DCE/DTE, and labeled (both the adapters, and the tubs and drawers). I also have a variety of test gear handy to help me test serial ports and devices. But I know that I'm a bit unusual in this regard.

Most shops I've visited have one box or drawer where they collect 'stuff' that isn't 'Ethernet'. The problem is, many vendors do little to label their parts, other than to add a part number. (Sometimes, though, the part number is on a baggie around an unlabeled part. Once the baggie is lost, the part becomes useless as soon as you seperate it from its associated equipment.)

You can spend a lot of time trying various combinations of the parts at your disposal, testing at each step whether it shows signs of working, or you can set out to build the cable or adapter that you need.

To build the cable or adapter, you need to know:
1) are both devices using the same Electrical Signalling (i.e. RS232, TTL, RS488, etc.)
2) which pin/contact numbers are which on the connectors you need to use
3) which signals are on which pins, and which are inputs vs outputs

Your search for information should start with the equipment you need to connect. Some equipment will include pinouts and signal info on a label on the chassis itself. Next comes reading the manual, which should be on a bookshelf in your office if the equipment is still in use. Sometimes you can find the documentation on a CD-ROM that came with the equipment (you keep those, too, right?), or the vendors website, in their support section. Start with these if you can, since configuration information for the port may be needed later, and these documents will be specific to the software revision loaded on your equipment.

If you can't find the information resources from your vendor, your favorite search engine may be useful. I prefer Google, but they have their good and bad weeks when it comes to finding useful results. (There are occassional bouts with bogus search portals saturating the Google results. It's like a tide, really...when it gets really bad, they manage to clean it up, and results get really good again. No, I'm not trying to bite the hand that holds my new blog. ;-) I've had poor results with MSN, and Yahoo is hit-and-miss but they have too many ads for my tastes. The lesson here is to pick your favorite, but to try others if you are not successful in your searches.

You want to be sure that you only connect two devices if they have the same electrical signalling specifications. TTL signals are from 0v to +5v, while RS-232 signals are -12v to +12v. If you connect a TTL device to an RS232 device, you may not damage the RS232 port, but you will most likely 'let the smoke out' of the TTL interface chip (e.g. permanently damage it).

Now, it IS possible to connect a TTL device to an RS232 device, using an interface (electrical) level converter between them, but you cannot do it with a simple cable or adapter.

Once you know that your two interfaces are electrically compatible, just make a wiring diagram for your cable or adapter. List one connector on the left side of the page, and the other on the right side. Include which devices are being connected, as well as the connector type and gender, so you can identify this wiring diagram easily if you need it again in the future.

DCE and DTE were defined 'in the old days', but it was a 'draft specification'. It was an attempt to make things interchangeable. But it wasn't something which culd be enforced, so there are manufacturers that picked the 'less intuitive' option when designing their devices. As a result, they stack the odds against us if we are just guessing. So, when you find the pinout descriptions for the signals, you could 'bet' that TXD (Transmit Data) is an output, and RXD is an input. But, if you're using a less-popular/smaller manufacturers device, you may be disappointed. That's why I test the pinouts after I know where the Signal ('Reference') Ground pin is located. You should try to test your ports as well, whether you use a voltmeter, or a signal tester.

I usually list the pin numbers sequentially (with 1 at the top), and I also add the signal name, and the 'direction' of the signal. If the signal is an output, I use an arrow that points away from the number. An input will point at the number. The reference ground signal will not have an arrow. (You can see an example of this on my Signals page.) You can even use this format in simple text files or email, like so...
Cisco AUX         Cyclades ACS
RJ45-M RJ45-M
1 RTS --> <-- RTS 1 2 DTR --> <-- DTR 2 3 TD --> <-- TD 3 4 GND --- --- GND 4
5 gnd* <- --> CTS 5
6 RD <-- --> RD 6
7 DSR <-- --> DCD 7
8 CTS <-- --> DSR 8
You can now start drawing the lines that connect 'complimentary' signals, but I find that all of the crossing lines are difficult to read. Instead, I draw a second version, where I draw the signal straight across from the left side to the right side, and I put the appropriate number in the 'right-hand column'. It makes it easier for my eyes to follow. It would look like this...
Cisco AUX         Cyclades ACS
RJ45-M RJ45-M
1 RTS --> --> CTS 5
2 DTR --> --> DSR 8
3 TD --> --> RD 6
4 GND ---+-- --- GND 4
5 gnd* <-' nc -> DCD 7
6 RD <-- <-- TD 3 7 DSR <-- <-- DTR 2
8 CTS <-- <-- RTS 1
This gives you a chance to count all of the pins on the right side. Is there one of each number? It also lets you see the 'odd' connections. In this case, the Cisco side needs to loop pin 4 to pin 5 (it's a switch in legacy products that tells the interface to be RS232. Without that input, the hardware doesn't use RS232 signalling.) Since the Cisco doesn't have a DCD output, the input on the Cyclades would not be connected. You can program the Cyclades to ignore the state of the DCD input. By diagraming the cable or adapter like this, it's a visual reminder that you need to check whether the device cares about an input which will NOT be connected.

I also find it's helpful to have a reference illustration for the pinouts of my connectors handy, when it's finally time to make the adapter or cable. You can find illustrations on the web, but you can find the cmmon connectors used for RS232 connections on my Signals page. Scroll to the buttom of the page for the DB and DE connectors. You can find a more verbose collection of basic RS232 clues and signal information on my Minor Scroll of Console Knowledge.


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