RS422 interface
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From: svyamuna on 7 Jul 2008 13:15 I am trying to interface two systems through RS422 link. There are only four lines connected between them (tx+/- & Rx+/-) and no ground. Independently both systems work fine but when interconnected and i try to communicate with each other there are some problems. initially master units Tx lines have been set to a known state (3.6v app). if i transmit 4 bytes from master unit only 3 bytes are received at the slave unit, though when captured the waveform in CRO shows all the 4 bytes.iam glowing separate LED's for each byte received.the 4th byte LED doesnt glow at all but when the master is powered off, then the fourth byte LED glows. what could be the problem.
From: Grant Edwards on 7 Jul 2008 13:24 On 2008-07-07, svyamuna <svyamuna(a)yahoo.co.in> wrote: > I am trying to interface two systems through RS422 link. There > are only four lines connected between them (tx+/- & Rx+/-) and > no ground. That's probably what's wrong: you need to connect the two system's grounds together. > Independently both systems work fine but when interconnected > and i try to communicate with each other there are some > problems. initially master units Tx lines have been set to a > known state (3.6v app). if i transmit 4 bytes from master unit > only 3 bytes are received at the slave unit, though when > captured the waveform in CRO shows all the 4 bytes.iam glowing > separate LED's for each byte received.the 4th byte LED doesnt > glow at all but when the master is powered off, then the > fourth byte LED glows. what could be the problem. -- Grant Edwards grante Yow! ... bleakness at ... desolation ... plastic visi.com forks ...
From: Paul Keinanen on 7 Jul 2008 14:43 On Mon, 07 Jul 2008 12:15:17 -0500, "svyamuna" <svyamuna(a)yahoo.co.in> wrote: >I am trying to interface two systems through RS422 link. There are only >four lines connected between them (tx+/- & Rx+/-) and no ground. This is quite OK as long as there is a termination resistor (typically 100..120 ohms) between the Tx+ and Tx- lines and also between the Rx+ and Rx- lines. The ground connection is not needed, as the RS-422 works as a balanced current loop. >Independently both systems work fine but when interconnected and i try to >communicate with each other there are some problems. initially master >units Tx lines have been set to a known state (3.6v app). if i transmit 4 >bytes from master unit only 3 bytes are received at the slave unit, Sounds like an Rx ISR problem. Paul
From: Grant Edwards on 7 Jul 2008 14:53 On 2008-07-07, Paul Keinanen <keinanen(a)sci.fi> wrote: > On Mon, 07 Jul 2008 12:15:17 -0500, "svyamuna" <svyamuna(a)yahoo.co.in> > wrote: > >>I am trying to interface two systems through RS422 link. There are only >>four lines connected between them (tx+/- & Rx+/-) and no ground. > > This is quite OK as long as there is a termination resistor (typically > 100..120 ohms) between the Tx+ and Tx- lines and also between the Rx+ > and Rx- lines. The ground connection is not needed, Wrong -- take a look at the receiver's spec for max common-mode voltage. It's usually only 8-12V. You've got to have a common ground to make sure you don't exceed the receiver common-mode voltage spec. > as the RS-422 works as a balanced current loop. Wrong -- it's a differential-mode voltage signal. It's not a current loop signal. -- Grant Edwards grante Yow! I want EARS! I want at two ROUND BLACK EARS visi.com to make me feel warm 'n secure!!
From: Vladimir Vassilevsky on 7 Jul 2008 14:58
Paul Keinanen wrote: >>I am trying to interface two systems through RS422 link. There are only >>four lines connected between them (tx+/- & Rx+/-) and no ground. > > > This is quite OK as long as there is a termination resistor (typically > 100..120 ohms) between the Tx+ and Tx- lines and also between the Rx+ > and Rx- lines. The ground connection is not needed, Ground connection is required unless the transceivers are floating. > as the RS-422 > works as a balanced current loop. ??? RS-422 has voltage output. Vladimir Vassilevsky DSP and Mixed Signal Design Consultant http://www.abvolt.com |