UART Signal Polarity

[PTI]

Both the SNAP Reference Manual p. 42 and the SNAP Hardware Technical Manual p. 6 show that UART1 CTS pin 11 and UART1 RTS pin 12 are positive logic (high when asserted). This is contrary to the RS232 standard, which requires positive logic AFTER the signal is inverted by the RS232 transceiver.

The forum post at Serial Port Handshaking/Flow Control; 06-18-2008, 06:36 AM -- states that the CTS and RTS pins are negative logic when asserted.

Which is correct? Do I or don't I need to place an inverter between pins 11-12 and the RS232 transceiver?

Which other signals are inverted, but not disclosed in the REFERENCE manual and HARDWARE manual?

It would be nice if your bulletin board allowed posts from Opera browser. Instead, it loops to a re-login.

[kbanks]

Which is correct? Do I or don't I need to place an inverter between pins 11-12 and the RS232 transceiver?

Wouldn't that depend on the particular transceiver you use?

Anyway, sorry for the confusion. When the RFE uses flow control, it (like most UART chips) takes the pins "logic low" to indicate "true", and "logic high" to indicate "false".

You might look at the Proto Board schematic (elsewhere on this forum) as a reference - there is a 232 driver on that board.

[kbanks]

Both the SNAP Reference Manual p. 42...

In my copy of the SNAP Reference Manual, page 42 is about the getMs(), getNetId(), getStat(), and imageName() functions.

What section number are you looking at, and what is before and after it? I want to be sure this portion of the manual gets updated in the coming updates.

[PTI]

1. A search for the word "schematic" on the synapse website returns "job opportunities," not any link to one of the Synapse proto boards. Perhaps Synapse would be so kind as to organize its support section of the website so that support documents are reasonable to find, for those of us consultants who bill by the hour? Perhaps in support of its products, tech support might kindly provide a URL for the documents to which it refers its customers?

The support page provides links to: (1) About Support; (2) Synapse Customer Forum; (3) Articles & White Papers; (4) Application Notes; (5) Product Datasheets;(6) Case Studies. None of these links identifies a schematic or download page.

2. Until just now, I used the following documents which you posted on 03-31-2008:
Document Number 600-101.01A
Document Number 600-0007A
Perhaps Synapse could be so kind to label old documents with a label of obsolete or superceded?

3. In v1.2 of the SNAP HARDWARE TECHNICAL MANUAL p. 9, and v1.2 of the SNAP REFERENCE MANUAL p. 127, pins 11-12 RTS and CTS, are identified as positive logic. This is in contrast to Pin 23, which is identified as negative logic ("active low"). As you have answered to my question, that is erroneous.

Additionally, none of the Synapse documentation properly identifies whether the signal names reflect origin as DCE or DTE.

4. A reasonable assumption would be that an "industry standard" RS232 transceiver would be used, such as the MAX3223 on the Synapse proto board.

5. SNAP REFERENCE MANUAL v1.2 gives a very poor explanation of CTS and RTS at p. 39. The reason that the signals at \CTS and \RTS are active low is because when they are inverted in an industry standard transceiver, they then conform to the RS232 standard requiring active high (positive logic enable). Both Maxim and TI refer to "standard inverting output" for RS232 transceivers, while noninverting outputs are not standard.

For positive logic, enable is high, and disable is low. For negative logic, enable is low, and disable is high. The terms "logic low" and "logic high" are ambiguous and atypical. They are easily confused with positive logic and negative logic. I recommend reviewing DeMorgan's Theorem, and using a product like FrameMaker to write your literature, which allows an overbar to reflect negative logic hardware labels like the "big boys" do it.

In short -- When flow control is enabled in software, the hardware is \CTS and \RTS (or, negtive logic CTS and RTS). This must be inverted in an industry standard RS232 transceiver, to conform to the RS232 standard. The UART signals are named in accordance with DCE (Data Communication Equipment) origin, not DTE (Date Terminal Equipment) origin.

No further response required. Thank you.

[kbanks]

1. A search for the word "schematic" on the synapse website returns "job opportunities," not any link to one of the Synapse proto boards. Perhaps Synapse would be so kind as to organize its support section of the website so that support documents are reasonable to find, for those of us consultants who bill by the hour? Perhaps in support of its products, tech support might kindly provide a URL for the documents to which it refers its customers?

The support page provides links to: (1) About Support; (2) Synapse Customer Forum; (3) Articles & White Papers; (4) Application Notes; (5) Product Datasheets;(6) Case Studies. None of these links identifies a schematic or download page.

Go back and re-read my post. I said "You might look at the Proto Board schematic (elsewhere on this forum) as a reference". I did not say "go search on the separate Synapse corporate website".

There is a "Search" tool in the center of the bar near the top of this page.
You should have been able to find the schematic over in the following thread.

http://forums.synapse-wireless.com/showthread.php?t=9

2. Until just now, I used the following documents which you posted on 03-31-2008:
Document Number 600-101.01A
Document Number 600-0007A
Perhaps Synapse could be so kind to label old documents with a label of obsolete or superceded?

We thought it would be obvious that 2.1 documentation superceded the 2.0 versions (just as the upcoming 2.2 docs will replace the 2.1 versions).

http://forums.synapse-wireless.com/forumdisplay.php?f=5

On this same forum, we maintain a thread where the latest files are kept at the top.

http://forums.synapse-wireless.com/showthread.php?t=9

You definitely should upgrade to the latest docs, we put a lot of effort into the 2.1 updates.

3. In v1.2 of the SNAP HARDWARE TECHNICAL MANUAL p. 9, and v1.2 of the SNAP REFERENCE MANUAL p. 127, pins 11-12 RTS and CTS, are identified as positive logic. This is in contrast to Pin 23, which is identified as negative logic ("active low"). As you have answered to my question, that is erroneous.

Thank you for bringing this to our attention.

Additionally, none of the Synapse documentation properly identifies whether the signal names reflect origin as DCE or DTE.

The SNAP nodes lean more towards "DCE". Since they do not implement the full complement of handshake signals (no DSR or DCD for example), it would be somewhat incorrect to refer to them as either DTE or DCE. Bottom line, a "straight cable" can be used to connect a Synapse SNAP node to a (DTE) PC.

4. A reasonable assumption would be that an "industry standard" RS232 transceiver would be used, such as the MAX3223 on the Synapse proto board.

Yes.

5. SNAP REFERENCE MANUAL v1.2 gives a very poor explanation of CTS and RTS at p. 39. The reason that the signals at \CTS and \RTS are active low is because when they are inverted in an industry standard transceiver, they then conform to the RS232 standard requiring active high (positive logic enable). Both Maxim and TI refer to "standard inverting output" for RS232 transceivers, while noninverting outputs are not standard.

We will try to make this clearer in the next revision of that manual. However, please be sure to at least be using the manuals from the 2.1 release of the product, as significant content was added between 2.0 and 2.1.

For positive logic, enable is high, and disable is low. For negative logic, enable is low, and disable is high. The terms "logic low" and "logic high" are ambiguous and atypical. They are easily confused with positive logic and negative logic.

That would be "my bad". I was trying to get the "low voltage"/"high voltage" point across.

I recommend reviewing DeMorgan's Theorem, and using a product like FrameMaker to write your literature, which allows an overbar to reflect negative logic hardware labels like the "big boys" do it.

We'll try and take a look at "FrameMaker" some time. Currently our manuals are crafted in plain old Microsoft WORD.

In short -- When flow control is enabled in software, the hardware is \CTS and \RTS (or, negtive logic CTS and RTS). This must be inverted in an industry standard RS232 transceiver, to conform to the RS232 standard. The UART signals are named in accordance with DCE (Data Communication Equipment) origin, not DTE (Date Terminal Equipment) origin.

No further response required. Thank you.

I have replied anyway, in the hopes that some of the material covered may be helpful to other forum users.