How to test RS485 in loopback?

Hi. When I want to test RS232, I simply cross
Tx and Rx pins (2 and 3 in DB9).
In RS485, on the other hand, there are four
pins used for transmission, ie Tx+, Tx-,
Rx+, Rx-. Is it enough to connect Rx+ to Tx+
and Rx- to Tx-, or is there any connection
that should also be done?
What are the possible reasons for the lack of
transmission?

"kaczfan" <kaczfan@poczta.fm> wrote in message 
news:slrnecf3tf.1if.kaczfan@psiloziben.chello.pl...
> Hi. When I want to test RS232, I simply cross
> Tx and Rx pins (2 and 3 in DB9).
> In RS485, on the other hand, there are four
> pins used for transmission, ie Tx+, Tx-,
> Rx+, Rx-. Is it enough to connect Rx+ to Tx+
> and Rx- to Tx-, or is there any connection
> that should also be done?

Strictly speaking, RS-485 is a 3-wire half-duplex bus, while RS-422 is a 
5-wire full-duplex bus. However, the newer RS-485 drivers/receivers are 
superior to the older RS-422 versions - the specs are electrically 
different - and tend to get used in RS-422 5-wire applications perfectly 
happily.

From your description, you're using a 5-wire bus. The loopback you describe 
should do the trick. You may or may not need terminators, depending on the 
baud rate you're using.

> What are the possible reasons for the lack of
> transmission?

I assume you've tried the above above and are seeing nothing. Be aware that 
RS-485 transceivers tend to have both driver enables and receiver enables, 
so you would need to enable both for the loopback to work. (RS-485 is a 
tristate-able bus, unlike RS-232.)

Have you tried looking at the signals with a 'scope?

Steve
http://www.fivetrees.com 

On 2006-07-26, Steve at fivetrees <steve@NOSPAMTAfivetrees.com> wrote:
> "kaczfan" <kaczfan@poczta.fm> wrote in message 
> news:slrnecf3tf.1if.kaczfan@psiloziben.chello.pl...

> Strictly speaking, RS-485 is a 3-wire half-duplex bus, while RS-422 is a 
> 5-wire full-duplex bus. However, the newer RS-485 drivers/receivers are 
> superior to the older RS-422 versions - the specs are electrically 
> different - and tend to get used in RS-422 5-wire applications perfectly 
> happily.
> From your description, you're using a 5-wire bus. The loopback you describe 
> should do the trick. You may or may not need terminators, depending on the 
> baud rate you're using.

I would have no idea where to plug them :/ [I'm working with 9600 bauds]
On the board there are four pins, described as Rx+, Rx-, Tx+ and Tx- for
rs485/rs422. There is also an rs232 port that is used at the same address,
depending on the device's settings, so the ground pin can be shared

>> What are the possible reasons for the lack of
>> transmission?
>
> I assume you've tried the above above and are seeing nothing. Be aware that 
> RS-485 transceivers tend to have both driver enables and receiver enables, 
> so you would need to enable both for the loopback to work. (RS-485 is a 
> tristate-able bus, unlike RS-232.)

To tell the truth, I have no idea how to do it. The device is 
an almost regular PC. You think this can be set in the UART?

> Have you tried looking at the signals with a 'scope?

Yes, the output is visible (I can also read it from
another device and it works correctly), the only problem
is that I can't read the input

Cheers

"kaczfan" <kaczfan@poczta.fm> wrote in message 
news:slrnecmrk0.1k3.kaczfan@psiloziben.chello.pl...
> On 2006-07-26, Steve at fivetrees <steve@NOSPAMTAfivetrees.com> wrote:
>> "kaczfan" <kaczfan@poczta.fm> wrote in message
>> news:slrnecf3tf.1if.kaczfan@psiloziben.chello.pl...
>
>> Strictly speaking, RS-485 is a 3-wire half-duplex bus, while RS-422 is a
>> 5-wire full-duplex bus. However, the newer RS-485 drivers/receivers are
>> superior to the older RS-422 versions - the specs are electrically
>> different - and tend to get used in RS-422 5-wire applications perfectly
>> happily.
>> From your description, you're using a 5-wire bus. The loopback you 
>> describe
>> should do the trick. You may or may not need terminators, depending on 
>> the
>> baud rate you're using.
>
> I would have no idea where to plug them :/ [I'm working with 9600 bauds]
> On the board there are four pins, described as Rx+, Rx-, Tx+ and Tx- for
> rs485/rs422. There is also an rs232 port that is used at the same address,
> depending on the device's settings, so the ground pin can be shared

Ok. 9600 baud is fairly low; with short cables you can probably get away 
without terminators. Since you've looked with a 'scope, maybe you can tell 
whether the edges are clean. If you do need terminators, this would show up 
as excessive ringing on the edges. Anything longer than about 1/10th of a 
bit time is too much.

Should you need terminators, or want to try eliminating that as a 
possibility, grab a pair of 120R resistors. Wire one between RX+ and RX-, 
and the other between TX+ and TX-.

Beware of a potential gotcha, though: with RS-485 and RS-422, when the bus 
is tristated the receiver can see either a mark or a space, since the normal 
idle state is no longer enforced. Some devices get around this with a small 
bleed current, but nowhere near enough to ensure a known state with 
terminators fitted - i.e. adding terminators can sometimes cause problems. A 
classic way round this is to pull the RX lines up and down, as using the 
pullups as part of the terminators (effectively three resistors in series 
from 5V to 0V, with the RX lines across the middle resistor). If memory 
serves, you need around > 0.3V across the terminator to ensure a true idle 
state.

>>> What are the possible reasons for the lack of
>>> transmission?
>>
>> I assume you've tried the above above and are seeing nothing. Be aware 
>> that
>> RS-485 transceivers tend to have both driver enables and receiver 
>> enables,
>> so you would need to enable both for the loopback to work. (RS-485 is a
>> tristate-able bus, unlike RS-232.)
>
> To tell the truth, I have no idea how to do it. The device is
> an almost regular PC. You think this can be set in the UART?

On a PC or a standalone RS-232 to RS-422/485 converter, the modem handshake 
outputs (RTS, and/or DTR) are typically used to control the enabling of the 
transmitter and receiver.

>> Have you tried looking at the signals with a 'scope?
>
> Yes, the output is visible (I can also read it from
> another device and it works correctly), the only problem
> is that I can't read the input

Ok, that helps. Perhaps the receiver isn't enabled: if it's configured as a 
half-duplex RS-485 system, you might find that enabling the TX disables the 
RX. More usually, each is controlled individually - it's sometimes 
convenient to be able to receive one's own transmission in order to know 
when it completes...

HTH,

Steve
http://www.fivetrees.com 

"Steve at fivetrees" <steve@NOSPAMTAfivetrees.com> wrote in message 
news:LqydndZatP1mCFbZRVnyjg@pipex.net...
>
> Should you need terminators, or want to try eliminating that as a 
> possibility, grab a pair of 120R resistors. Wire one between RX+ and RX-, 
> and the other between TX+ and TX-.

Hmmm. To be clearer: for a loopback test, you only need one resistor (since 
RX=TX). And, as I said, for short cables it's probably not a problem. I'd be 
inclined to look into whether the receiver is enabled. All of this should be 
in the manual for the converter/board - do you not have one? Where did the 
board come from?

Steve
http://www.fivetrees.com