Serial Port connector

[Tadeusz Pycio]

I'm thinking of changing the standard serial port connector a bit by adding the DTR signal, which is in every USB-Serial converter, as a Reset signal. By default DTR is in the low state, so it would need to be negated (via a transistor?). The Tera Term terminal software allows this signal to be controlled via a macro. This minor change would make it possible to reset the computer without physically accessing it. You would have to think about the physical implementation of the negation on the module, as using a logic gate in this case would not be a good idea.  What do you guys think of such an idea?

[Brad Hines]

That's clever; it's how Arduino boards reset after programming, I believe.  If you were to mimic the Arduino implementation, you'd have a nice technology base to draw from for both hardware and software approaches.

On Fri, May 17, 2024, 1:20 AM Tadeusz Pycio <ta...@wp.pl> wrote:

I'm thinking of changing the standard serial port connector a bit by adding the DTR signal, which is in every USB-Serial converter, as a Reset signal. By default DTR is in the low state, so it would need to be negated (via a transistor?). The Tera Term terminal software allows this signal to be controlled via a macro. This minor change would make it possible to reset the computer without physically accessing it. You would have to think about the physical implementation of the negation on the module, as using a logic gate in this case would not be a good idea.  What do you guys think of such an idea?

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[Tadeusz Pycio]

Yes, this solution is commonly used in microcontrollers. Returning to the RCBus, I think the appropriate place would be the existing pin 6 which will be selected with a soldered jumper between the existing, rarely used CTS function and the DTR signal used for resetting. This solution will enable a direct pin-to-pin connection to the popular FTDI USB-Serial converter, retaining the existing functionality if one wishes to use this connector for a classic RS232 connection via MAX232 with RTS/CTS flow control.

[Andrew Kendall]

 

“…thinking of changing the standard serial port connector…”

 

Please don’t.

 

You’ve referred to the term in your message. “ standard”

 

In my 50 years as a Comms and Network Engineer, I’ve come across, and had to work around, more variations of the Serial Comms Standard RS232 than I care to remember. The world doesn’t need another. I used to have to carry around a bucket load of 9 pin, 15 pin, 25 pin, male, female, straight through, crossover adapters and tone of these http://www.new-forest-electronics.co.uk/Comtest%20100C%20close%20up.htm just to enable me to hack my way around whatever permutation the vendor had seen fit to implement on their hardware.

 

The reason for my objection is, that twenty years from now, someone will be struggling to “talk” to one of these interfaces and will have problems. Look at what we are all doing now, creating and recreating 40+ year-old computers, and look at how little information is still available for them.

 

Of course if we can get the IEEE to accept your amended interface and register the variant then I’m all for that.

 

Having said that, we did actually do exactly what you are proposing on one particularly troublesome piece of equipment. We made up an adapter and picked off the DTR signal to trigger the RESET signal. But we did it with an adapter. We left the “Standard” connector alone.

 

You did ask.

 

Best wishes.

 

Andy.

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[Alan Cox]

On Fri, 17 May 2024 at 09:20, Tadeusz Pycio <ta...@wp.pl> wrote:
>
> I'm thinking of changing the standard serial port connector a bit by adding the DTR signal, which is in every USB-Serial converter, as a Reset signal. By default DTR is in the low state, so it would need to be negated (via a transistor?). The Tera Term terminal software allows this signal to be controlled via a macro. This minor change would make it possible to reset the computer without physically accessing it. You would have to think about the physical implementation of the negation on the module, as using a logic gate in this case would not be a good idea. What do you guys think of such an idea?

For Fuzix at least DTR is used on supported device as the goddess
intended, but a jumper for this mode of operation would be quite
useful for some people so it sounds like an excellent plan. A lot of
USB serial converters don't have DTR but it would be easy to pick one
that did if you cared. You'd also need a resistor to pull the signal
the right way when not connected which is easy enough.

Not sure it's much use with a CP/M style system but for monitor type
things definitely, and it's a lot cheaper than the bus raider!

[Tadeusz Pycio]

Hi Andy,

I think we misunderstood each other, which is probably due to my Tarzan-English. On serial transmission modules RCBus has conventionally adopted a single "standard" connector that allows signal converters to connect to USB or "real" RS232. This consists of the following signals with 5V levels: 1-GND, 2-RTS, 3-VCC, 4-RXD, 5-TXD, 6-CTS. My suggestion is to add the DTR signal, which is not used in RCBus, to implement RESET. The CTS signal is rarely used in RCBus computers, so this 6th pin could be used for another function (DTR as RESET) using a jumper. RCBus systems either do not use flow control or use RTS/CTS. The connectors used to connect to the outside world via converters correspond to standards known for years, limited by the availability of signals on the serial port modules.

[Tadeusz Pycio]

Hi Alan,

I had in mind such a USB-Serial converter, it is very popular.

The resistor alone is not enough, the default state of the DTR is low, so negation must be done, for example like this:

During typical work with CP/M, such a system is not needed, but it would make my life much easier because I could work remotely via a PC with the Z80 computer connected to it without fear of its failure. I wish I had thought of it sooner.

[Wayne Warthen]

The concept sounds fine to me (and a useful way to implement a remote reset).  The official RC2014 SIO module does not connect CTS to the adapter, but it does make sure to pull down the /CTS signal at the SIO chip (/CTS always asserted), so no issues.  However, I have seen some serial adapters that do not do this and connect CTS to the adapter.  That is also fine and also works well.  The problem will occur if CTS is left high or floating.  Software will not be able to deal with this well.  For example, for a 16C550 serial port using automatic flow control, flow control will need to be disabled entirely if CTS is high or floating.  This is because the 16C550 automatic flow control cannot disable only /CTS.  It will disable /CTS and /RTS and leave you with no flow control at all.

The point is that if you disconnect /CTS from the serial chip in the process of providing the /DTR reset, please make sure that /CTS into the serial chip is pulled low in some way.

Thanks,

Wayne

[Chris Odorjan]

Here's the USB-serial portion of the I/O expander for an SBC I've been working on:

I used the Pololu adapter because I already had a bunch of them and a 74LVC1G06 mostly as an excuse to practice soldering an SMT component. (I made it "optional" in case it didn't work out. :-) )

/RESET is already pulled high on the SBC.

Linux (and probably most other OSes) deassert DTR when the port is closed which means the board will come out of reset every time the port is opened. This could be seen as either an issue or a feature, hence jumper JP5 to disable it.

(picocom has key sequences to both toggle and pulse DTR. Maybe minicom does too, I'm not sure.)

Unfortunately the reset portion is the only part I haven't tested yet. (Not due to SMT soldering like you'd expect but mostly real life getting in the way.)

[Ronny Ribeiro]

Hi everyone,

I'm following this thread and I'm a bit confused with the DTR pin on the serial adapter: the picture below shows a SC521 from Stephen Cousins where he assign it as CTS...

Any thoughts?

Thanks,

Ronny 

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[Tadeusz Pycio]

Hi Ronny,

As I mentioned, the DTR signal is not used and when controlling the CTS/RTS flow using a USB-Serial converter, the CTS signal also has no role. A PC with a terminal program is on a much faster device and practically never will be, so such a connection is acceptable. The CTS signal may have a role with slow terminals connected via RS232, in which case the pictured converter will not be used. This connection (DTR-CTS) has no effect on transmission and was probably only used for the convenience of using this converter while retaining the connector for its eventual functionality when working with free devices.

[horus falcon]

Good Afternoon, Group,

Folks, any discussion of this should rightfully include a review of the latest EIA/TIA RS-232-F standard.  TI has a technical paper on this that is reasonably credible, and may be had for free from:
https://www.ti.com/lit/an/slla037a/slla037a.pdf

DTR is already implemented on pin 4 of the DB-9S connector, and on pin 29 of the DB-25S  (Referred to as "DTE Ready" in some EIA documents...)

Later On,
David

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[Phillip Stevens]

On Friday 17 May 2024 at 18:20:04 UTC+10 Tadeusz Pycio wrote:

I'm thinking of changing the standard serial port connector a bit by adding the DTR signal, which is in every USB-Serial converter, as a Reset signal. By default DTR is in the low state, so it would need to be negated (via a transistor?). The Tera Term terminal software allows this signal to be controlled via a macro. This minor change would make it possible to reset the computer without physically accessing it. You would have to think about the physical implementation of the negation on the module, as using a logic gate in this case would not be a good idea.  What do you guys think of such an idea?

It is useful but very dangerous tool. Many modern machines cannot reconnect a USB port without toggling the DTR line. This means you cannot operate the MCU through a serial port disconnect!

A quick search for of Arduino or Parallax forums will clarify. Or see the (non) solutions I tried here.

Cheers Phillip

[Tadeusz Pycio]

Hi Phillip,

This is bad news, I have to look for information about this problem. Although it is surprising to me, as I did not expect that during a particular session this could happen. Yes, during a USB reconnection or when it is first connected, it can happen, but during a session a change in the DTR can lead to such a situation, I did not foresee this. I need to do some reading on the subject and eventually create a a real model to test instead of just talking about it. Thank you for this information.