For the TIU Gurus - TMCC command output?

John,

When you set up a TMCC engine to operate from the DCS system, does the TIU simply output standard TMCC commands over the serial port, which are then read by the TMCC base as they would be from a computer or other device?  The simple of it is, can the TIU be used to just output the 3 byte commands that TMCC uses?  I'm of the understanding that only ENGine commands are available, not track/train, switch or accessory.  Is that correct? 

All of the above is correct.

The one thing for engines that's missing is separate control of the SET key as well.  The DCS outputs the SET key sequence when you change ID, but you can't use the SET at other times, say to set the stall of a non-cruise locomotive.  You may also have issues programming ERR stuff, there are key sequences that use the SET key.  This was discussed some time ago in the TMCC forum.

Barry and GRJ, thank you for the information.  

JGL

Figure I'll throw a couple more technical questions about the TIU out here.  

Does anyone know if the TIU uses a 'true' RS232 standard output with positive and negative voltage output in the 8-15 volt range, or does it use the 0/+5 TTL level version of the standard?

Second, is there anything special about the cable that is used to connect the TIU to a TMCC base, or is it just a null modem cable?

JGL

John,

Second, is there anything special about the cable that is used to connect the TIU to a TMCC base, or is it just a null modem cable?

Yes, it is "special" and is available only from MTH. The latest version of the cable is MTH product #50-1032 and it works with all of the Lionel command bases.

This and a whole lot more is all in “The DCS Companion 3rd Edition!" This book is available from many fine OGR advertisers and forum sponsors, or as an eBook or a printed book at OGR’s web store!

Its been a while, but I was under the impression that to communicate between a PC and a Command Base, a straight thru DB9 cable is used. A null modem cable is not a straight thru, it swaps 2/3 to 3/2, and at least two other signal.

A couple of weeks ago I fired up Visual Studio 2015 and opened a program I wrote in VB 2005 about TMCC, and after some Microsoft mischief, it still worked; but 2015 overwrote some of the files.

Michael,

I was under the impression that to communicate between a PC and a Command Base, a straight thru DB9 cable is used. A null modem cable is not a straight thru, it swaps 2/3 to 3/2, and at least two other signal.

That is incorrect. Neither a straight-through cable nor a null cable will work. You must use the MTH cable.

I have to admit confusion at this point, Barry.  Earlier you stated that I was correct in stating the TIU outputs standard TMCC commands over RS232.  If that is the case, how can a null cable not work?  Does the TIU not use a RS232 compatible port, and instead output the TMCC commands on some other set of pins besides RX-pin 2, and TX-pin 3, that requires a special cross over cable?  Or is the output of the TIU in some format that must be decoded by some circuit in the cable?  

JGL

I believe there's some sort of buffer in the cable, I should pry one apart and see why it's necessary.

JGL,

If that is the case, how can a null cable not work?

Because the connection is more than you know. Read on.

Does the TIU not use a RS232 compatible port, and instead output the TMCC commands on some other set of pins besides RX-pin 2, and TX-pin 3, that requires a special cross over cable?  Or is the output of the TIU in some format that must be decoded by some circuit in the cable? 

Try this: DCS Pin 5 to TMCC Pin 5, and DCS Pin 9 to TMCC Pin 3.

This should work for Legacy (and probably Base 1L), however, results with TMCC may be spotty. TMCC actually needs something with a resister, diode or, preferably, an opto-isolator in the circuit.

It's been double-digit years since I made one of these. I much prefer the current MTH cable, which just plain works very well with all flavors of Lionel command bases.

I have confused the issue by bringing up communication between a PC and a Command Base, trying to point out what the difference between a null modem and a straight thru cable.

Barry is correct about communication between MTH TIU and a Lionel Command Base, and incorrect about the correction to my previous statement.

As I recall, there were issues related to earth ground. The answer is somewhere in some old forum OGR post. Bottom line, buy the MTH cable and enjoy your trains.

Michael,

The TMCC command base has a nonstandard voltage swing on the serial port, it's not really RS-232 compatible.  I suspect some RS-232 receivers may have a problem with recognizing the data.  The schematic below illustrates what I'm saying, this is the serial port on the BASE1.  Note that the output data swings between 0V and +5V, and the input is sensed between 0V and +5V.  The RS-232 specification calls for bi-polar data, both positive and negative.

So the answer then is that the TIU does not, in fact, send TMCC data out of the standard pin 3 TX line, but instead sends TMCC data out on the ring indicator - pin 9?  

Without taking a cable apart, I can only guess that the "needed" special part in the cable would be a diode, to protect the TIU from any signal sent out on the ring indicator line from other devices.  Some form of buffer might also be included, but I don't think is needed since the data is moving one way only.  (edit, this assumes a TTL level, some sort of isolator would be needed to accomplish the same effect with proper RS232 signal levels)

That said, does anyone know, again, if this data is sent at nominal +/- 12 volt levels, or if it uses the psudo-rs232 standard of 0/+5 TTL level like the original TMCC base?

As far as the TMCC base, I can see no reason why it would not receive signals properly that work on legacy.  While it is true that the base only outputs 0/+5 volt levels, it is capable of receiving proper RS232 levels and should respond to them equally well.  It just wouldn't send out meaningful signals to equipment that can't read the TTL levels.  On the other hand, pretty much every RS232 device made in the last quarter of a century or more will read TTL levels just fine.  The primary down side of this is instead that transmission distance is reduced substantially from the 500 feet or so that a 9600 baud true RS232 signal is capable of.  

JGL

Isn't the issue that the TMCC/Legacy bases reference the serial signal to earth ground, and the TIU does not?

I seem to recall some claim that the MTH cable had some isolation to prevent shorting the TMCC earth ground to layout common, but I know from experience the isolation is not complete.

JohnGaltLine posted:

That said, does anyone know, again, if this data is sent at nominal +/- 12 volt levels, or if it uses the psudo-rs232 standard of 0/+5 TTL level like the original TMCC base?

pin 9 is +5V, not +/-12.

JGL,

As far as the TMCC base, I can see no reason why it would not receive signals properly that work on legacy.  

You may not see any reason, however, the cables have slightly different requirements for use with Legacy or TMCC. MTH has made 3 different cables over the years and each works as follows. From page 124 of The DCS Companion 3rd Edition:

At the present time, there are three MTH cables that may be used to connect to a Lionel command base. The three cables are: 

• #50-1018, TIU/TMCC 6' Connector Cable may be used only to connect the TIU to a Lionel TMCC Command Base. 
• #50-1007, TIU/TMCC 6' Connector Cable may be used only to connect the TIU to a Lionel Legacy Command Base or a Lionel Cab-1L Command Base. 
• #50-1032 TIU/TMCC-Legacy 6' Connector Cable may be used to connect the TIU to a Lionel TMCC Command Base, a Lionel Cab-1L Command Base or a Lionel Legacy Command Base. 

Also, I misspoke previously. I should have said:

TMCC actually needs something with a resister, diode (not a capacitor) or, preferably, an opto-isolator in the circuit.

 In years past, I've built all 3 versions, based on Ernie Emrich's (Lionel2341) specs.