RTS format

[DaveChappers]

Hello experts,

I am trying to develop a better test for our embedded serial ports.

I have copied some code from another project that works great at all temperatures for RS232 and RS422 but at cold (-45°C) the RS485 test fails. I believe this is because the RTS is not being driven either hard/strong enough or for long enough for the CTS to see it correctly on the looop back cable.

I don't seem to see anywhere that defines that RTS must be driven to a minimum/max of x/y volts and for x/y seconds. Is this even defined? Or are there just generally accepted limits?

Any help/advice/best practice greatly appreciated.

Regards,

Dave

[Jan Axelson]

RTS and CTS are RS-232 control signals. How are you using them with RS-485? What are the chips and connections?

Jan

[mdlayt]

I, too, have never found anything like an actual spec.

From my experience, RS232 etc are *current* signals.  Meaning the driver should provide a plus or minus current.  At least 5mA, preferably several times that, and hopefully limited at some level that doesn't start a fire (10-20mA?).  And ideally the current is sourced/sunk up to +/- 12 volts.  (Although 0-5V communication is possible, just not recommended.)

RTS is a level signal, so pulse widths don't apply.  That is, the driver should reach full current quickly and stay there until the signal is changed.  "Quickly" is certainly less than a millisecond and might need to be only microseconds.

[Guido Koerber]

No, the RS232 document specifies a voltage range not a current for the signals. But in the last version of the spec the voltage can go down to something like +/-3 V is I remember it correctly.

[Jan Axelson]

Yes, in a true RS-232 link, there are specs for RTS. But RS-485 doesn't use RS-232 voltages for data or anything else, though it may use the UART's RTS line to control the transceiver.

Jan

[mdlayt]

No, the RS232 document specifies a voltage range not a current for the signals. But in the last version of the spec the voltage can go down to something like +/-3 V is I remember it correctly.

Exactly which "document" would that be?

[Jan Axelson]

TIA-232

Available from:

http://global.ihs.com/

Jan

[mdlayt]

I'm not exactly thrilled to spend $142 just to find this why this spec is better than some other.

I'm guessing Telecommunications Industry Association:  "TIA is accredited by the American National Standards Institute (ANSI) to develop voluntary industry standards for a wide variety of telecommunications products."

The earliest reference I see to RS232 on their site is about 1997.  So I'm thinking it would be kind of hard for all those RS232 device made before, say 1975, to conform to TIA-232.

I think I'd be more interested in
The Electronic Industries Association (EIA) standard RS-232-C[1] as of 1969
mentioned here
http://en.wikipedia.org/wiki/Rs232

In the end, I think "driving a voltage with a current limit" or "driving a current with a voltage limit" are really just a matter of point of view.  The thing to keep in mind is, there is (hopefully) a current limit, so if you exceed it, you don't get the voltage you expect.  And it probably is NOT going to act like, say, CMOS, where the farther you get from the voltage, the more current you get--it will probably top out the current fairly soon and stay there.

[Jan Axelson]

There are many data sheets, books, and articles that explain the RS-232 specs without having to buy the document.

Jan