Profibus DP

Hi,

I have a couple of RS485 devices which i want to connect with each other with a RS485 bus. Now is my question can I use a Profibus DP cable as RS485 bus or not?

Thx

Yes, if the devices operate on a two-wire ungrounded connection.

No, if the devices operate on a four-wire (two pair, full duplex) connection.

Maybe, if the devices operate on a two-wire (one pair, half duplex) one-side grounded connection.

In any case, the standard profibus DB9 connectors may be unsuited to the application.

Good luck.

two-wire one side grounded connection

Hi,

Thx for the reply,

I use 2-wire RS485.

I have 21 devices connected on the bus and one USB to RS485 Hub.
In the past I used a RS232/RS485 converter and their were now problems. Now I use the USB hub and have problems. Is it possible that the hub haves some difficulties with the profibus DP cable and the converter not.

On the hub I have connected wire D+(B), D-(A) and the ground on the other deviced I use the profibus connector. Now the problem is that I find only 15 devices and that their are 6 devices missing, when I replace the hub by the converter everything is OK again,

Is it possible that you can't use the profibus cable with the USB hub?

Thx



Maybe, if the devices operate on a two-wire (one pair, half duplex) one-side grounded connection.

two-wire one side grounded connection

Hi,

Thx for the reply,

I use 2-wire RS485.

I have 21 devices connected on the bus and one USB to RS485 Hub.
In the past I used a RS232/RS485 converter and their were now problems. Now I use the USB hub and have problems. Is it possible that the hub haves some difficulties with the profibus DP cable and the converter not.

On the hub I have connected wire D+(B), D-(A) and the ground on the other deviced I use the profibus connector. Now the problem is that I find only 15 devices and that their are 6 devices missing, when I replace the hub by the converter everything is OK again,

Is it possible that you can't use the profibus cable with the USB hub?

Thx

RS485 can be a bit picky.

First thing to check is that the bus is daisy chained point to point, and not using any kind of a "Star" configuration...


DevA --- DevB --- DevC --- DevD --- etc.

Second thing to look for, is that there are proper termination resistors on both "dead ends" of the bus.

Third thing, is that for some devices/drivers, you need to actively pull the (+) line high and the (-) line low, on at least one end, if not both. The pull-up/pull-down lines are generally connected to the powersupply rails of the 485 line, through resistors. Check your device documentation regarding this.

Other than that, using standard ProfiDP cable should be no problem at all on a normal single pair 485 system.

grounded RS485

Hi,

Thx, i will check it,

I have one question, I use grounded RS485 and you said in your previous post that it maybe works with this kind of connection,

Is it possible that this the reason is of the strange behaviour of the bus,

Thx

Re: Grounded connection...

Yes, that can be a problem, as some devices don't like either side of the 485 bus to be hard-grounded. Most implementations of RS485 have a 'soft' ground, or in other words, one signal lead tied to system common through a loading resistor.

If you are mixing devices that have hard grounding and devices that don't, you may be in for trouble.

RS485 in it's pure form is a paired-line-driver communications standard; essentially, it transmits by switching the polarity of the pairs, rather than just toggling a single signal lead high and low (ala RS232). With devices that have a grounded lead, the polarity inversion is not possible. Devices that look for the inversion will reject a grounded transmission as invalid, while devices that expect it to be grounded operate more like RS232.

Long distance comms are only truly possible with full differential signals.