RJ45 used as RS485 - why not properly wire-compatible with cat5 cable

[Grossel]

Hi - sorry if posted on wrong forum, but I didn't find any that fit.

Where I work, we have some master modules capable of comunicating with modbus/rs485 sensors. So far so good. That is made by a well-known manufacturer.

However - the master module have a RS485 port physically compatible with any ethernet cable around. This is also pretty useful.

However, there is something really stupid by design (in my mind at least) and I have questioned this but not got any good arguments from the manufacturer on why this is.

So, any cat5e cable have this standard layout of four pairs. Therefore, when designing the RS485 connector and pinouts - one should assume the plus and minus of RS485 is arranged in such a way that it takes advantages of a standard cat5 cable to avoid cross talk, by ensuring the plus and minus is assigned to the same pair of wires.

However, this is not the case - the plus and minus in RS485 on these devices are assigned to a different wire pair when using a standard cat5e cable, and thus not being as effective as possible for long distance + high baud rate ; that is without me have to replace the RJ45 connector in order to get the plus and minus on the same wire pair.

My question is - What reasons would a manufacturer of signal processing devices have to make the pinout of RS232/RJ45 socket that is not compatible with the standard of what to be assumed being the most used network cable on the marked ?

 

[Pommie]

Could it be so they could sell compatible cables as well. I.E "Special" RS485 cables instead of "Crappy" ethernet cables.

Mike.

 

[crutschow]

What reasons would a manufacturer of signal processing devices have to make the pinout of RS232/RJ45 socket that is not compatible with the standard of what to be assumed being the most used network cable on the marked ?

Sounds like the junior engineer that assigned the cable connections didn't know what he was doing.

 

[Nigel Goodwin]

Sounds like the junior engineer that assigned the cable connections didn't know what he was doing.

I would suggest Pommie's suggestion is more likely, although if you're not planning making it compatible with existing wiring, it makes no difference.

Interestingly some DMX units use Ethernet connectors rather than XLR ones.

 

[Grossel]

Could it be so they could sell compatible cables as well. I.E "Special" RS485 cables instead of "Crappy" ethernet cables.

Mike.

Well, I haven't seen that particular item in their product list.

However, they have another product that have a RS232 port in order to configure using e.g. Putty, and for that particular device I have seen they sell a "special rs232 cable" - If remembering correct that is a connector similar to RJ45, but having only 6 pins. However because of low baud rate I found this to be not very difficult to create (and it's very rare anyway so spending some minutes creating that single cable once is no big deal).

The RS485 connector however is way more used and must spend more time (wasted) making weird rare network plugs in order to functioning.