Maker Pro
Maker Pro

RS485 tranceivers?

A

Anders F

Jan 1, 1970
0
I can't quite find out why the RS485 tranceivers usually's only spec'ed for
a very limited range. I'd like a 3-5.5V type (1/8 load and slewrate
limited). Why would that be so hard to design/manufacture?

Cheers,
Anders
 
A

Anders F

Jan 1, 1970
0
valentin tihomirov said:
most logic has this range.

Yep - then why wouldn't they make tranceivers this range too? I mean the
differential drivers can be made as 3.3V and 5V...

/A
 
V

valentin tihomirov

Jan 1, 1970
0
Yep - then why wouldn't they make tranceivers this range too? I mean the
differential drivers can be made as 3.3V and 5V...

Actually, most rs422/485 drivers use these logic levels.
 
A

Anders F

Jan 1, 1970
0
valentin tihomirov said:
Actually, most rs422/485 drivers use these logic levels.

Yeah - but they're either specified for 3.3V OR 5V (it's the supply I'm
referring to). I'm wondering why the device I need (3-5V supply range) does
not exsist....

/A
 
M

Mac

Jan 1, 1970
0
Yeah - but they're either specified for 3.3V OR 5V (it's the supply I'm
referring to). I'm wondering why the device I need (3-5V supply range) does
not exsist....

/A

Well, I think the rs (or actually EIA) 422 specification specifies what
the complementary (they're usually NOT differential) voltage levels are
supposed to be.

As far as 3.3 and 5 V levels go, almost any device designed to work
at a nominal 3.3V will be specified to work down to 3V (usually the fine
print will say 3.3V +/- 10%). And any CMOS device which works at 3.3V AND
5V will work everywhere in between, as long as it is interfaced to devices
connected to the same rail.

Maybe you could post a specific part number and a little more
information about your application. Then you might get some meaningful
feedback about whether your application is likely to work.

You could also try asking the vendor. Some of them actually have good
applications engineers who get paid to answer your questions.

--Mac
 
A

Anders F

Jan 1, 1970
0
Mac said:
Well, I think the rs (or actually EIA) 422 specification specifies what
the complementary (they're usually NOT differential) voltage levels are
supposed to be.

As far as 3.3 and 5 V levels go, almost any device designed to work
at a nominal 3.3V will be specified to work down to 3V (usually the fine
print will say 3.3V +/- 10%). And any CMOS device which works at 3.3V AND
5V will work everywhere in between, as long as it is interfaced to devices
connected to the same rail.

The problem is they are EITHER 3.3V or 5V types.
Maybe you could post a specific part number and a little more
information about your application.

Well, at the moment the MAX3072E is the more promising.
Similar types from TI and a lot of pincompatible 5V types have also been
investigated
Then you might get some meaningful
feedback about whether your application is likely to work.

The application involves a lot of units supplied through a ribbon cable.
Thus, I'd like to save the power, cost and complexity of having to output
excess power and dissipating this in a LDO. If I had 3-5.5V types the say
0-1V drop over the cable could easily be tolerated....

Cheers,
Anders
 
R

Ross Herbert

Jan 1, 1970
0
The problem is they are EITHER 3.3V or 5V types.


Well, at the moment the MAX3072E is the more promising.
Similar types from TI and a lot of pincompatible 5V types have also been
investigated


The application involves a lot of units supplied through a ribbon cable.
Thus, I'd like to save the power, cost and complexity of having to output
excess power and dissipating this in a LDO. If I had 3-5.5V types the say
0-1V drop over the cable could easily be tolerated....

So what you are really saying is that you want a chip manufacturer to
design a device capable of operating over a 3.0 - 5.0 volt dc supply
range without compromising your slew rate or other operating
specifications, just so that you can have the luxury of a low system
cost design?

Supplying the receiver dc operating voltage over a long buss is not
good design practice, UNLESS you provide local
regulation/filtering/decoupling at every receiver drop-off point along
the buss. This is just good design practice and it should not be
compromised. You are trying to do it on the cheap it seems, and IC
manufacturers don't usually go to such lengths to cover the
inadequacies inherent in low cost designs. It would mean they would
have to include on-chip regulation thus pushing the design complexity
and production cost above alternative devices from other
manufacturers, thus making their devices unviable for most oem
equipment manufacturers.
 
M

Mac

Jan 1, 1970
0
The problem is they are EITHER 3.3V or 5V types.


Well, at the moment the MAX3072E is the more promising.
Similar types from TI and a lot of pincompatible 5V types have also been
investigated


The application involves a lot of units supplied through a ribbon cable.
Thus, I'd like to save the power, cost and complexity of having to output
excess power and dissipating this in a LDO. If I had 3-5.5V types the say
0-1V drop over the cable could easily be tolerated....

Hmm. I wonder if you could just use a 5V type at 3.3 V? What baud rate are
you running at? If it is slow I bet it would work, particularly if you are
not going over long cables, and are only interacting with devices of your
own design. I would definitely try asking an applications engineer from
the relevant company.
Cheers,
Anders

Good luck.

--Mac
 
Top