Ideas for switching serial input to 634 LCD between serial output of three computers?


New member
I own and administer an light-industrial design shop (CleanBoot Systems) where each of about 30 employees is simultaneously using two or (most commonly) three computers and video monitors at his/her "user-station."

I would like to purchase and hook-up (in an external enclosure independent of the separate computers themselves), one of your 634 LCD modules for each user-station, to be switched between the two or three computers at each user-station, so that I can easily keep watch of various hardware parameters of the user-station computers without having to take over control of the user's keyboards and display monitors.

My question is this:
Because I have (at minimum) 25 - 30 user stations with two or three computers and monitors each, I am hoping that I can use a simple unbuffered, three-way DB25 switchbox to switch the input to a single 634 LCD module between the serial output of the two or three computers --in other words, a serial cable from each computer at a user-station routed to a three-way DB25 switchbox, and the single serial cable output of the switchbox attached to the 634 module input.

Does this sound feasible? If not, is there some other way in which the switching could be accomplished?
I would plan to use a switchbox with "break-before-make" type contacts, but otherwise unbuffered, although I could arrange to have power to the 634 cut off during switching --would that be necessary or wise?).

As I stated above, I would not plan to mount the 634 LCD panel in any of the computers, but rather in a external enclosure, or perhaps even in the switchbox enclosure itself, if mechanically feasible.

My reason for this scheme is that although I can justify purchasing 25 -30 of your 634 LCD panels (with bezels, cables, etc), I would not find it feasible to purchase three times that number of units (in other words, one for each computer at each user-station).

I would appreciate any advice you can offer, as I would like to purchase the CrystalFontz 634 modules very soon, and build and install the configurations I described above during a scheduled "down-time" between jobs that starts the first week of June.

Thank you for your help.

J. Gordon L.
CleanBoot Systems
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CF Tech

Yes, that would work fine as long as the software writing to the LCD is "aware" of what your doing...

The problem may be that the software doing the writing to the LCD may only update parts of the LCD that change (like CrystalControl does). This means on changing which PC your monitoring only part of the LCD may be updated.
I dont know if other LCD software would have this problem.

I could fix this problem for you in CrystalControl if you go ahead with installing the displays and do wish to use CrystalControl.
If you do, email me at

You would also have to do a small modification to the power pins on the LCD so that is always turned on (not turned on by the com port). This is a very easy thing to do though.

CF Tech

One little "gotcha" of using a mechanical switch box is that the the user can switch it at any time. So there is the (perhaps slim) possibility that a single byte will get "split". The LCD may see this as some random character--which may display on the screen or be interpreted as the start of a command.

So the host software should redraw the whole screen, but in order to recover from any possible garbage characters, it should first send over 9 nulls or spaces, then relocate to 0,0 and then send over the refreshed screen contents.

This is not a big deal, and maybe you would only see the potential problem once in a hundred switches, but you might as well code for it from the start :) .

CF Tech

Here is a mini tutorial on how to connect the "LCD" power section of the 632 or 634 to the "LED" (backlight) power section.

By default the 632 or 634 power the "LCD" section (microcontroller + LCD controller) by "stealing" power from the PC's DTR and RTS lines. This means that a "reflective" version of the LCD (which has no backlight) does not need any specific power connection--as long as a fully wired serial cable is used, you are ready to go.

For the backlight models, you have to supply a regulated +5v between the "+5v(LED)" and "GROUND" pins of J2:

If you order a WR232Y02 along with a 634 or a 632, we will solder the mating connector to the LCD:

Here is what it looks like from the opposite side:

To make it so the LCD ignores the state of DTR and RTS, and instead is always powered from the +5v, just make a connection between "+5v(LED)" and "+5v(LCD)" as shown here:

In this case we just used a "solder bridge" to make the connection, in fact that is why the two pins pere placed adjacent to each other.


New member
Thanks for the interest that you have given to my plans, and all the help you have provided and offered.

Sorry I've taken so long in getting back to this project --we were finishinhg up a job a barely met our deadline last week.

I've had a change of plans --I've given up on the idea of switching computers between one LCD module, and will be planning upon aquiring a seperate LCD for each machine.

Talking to my crew, we came to the conclusion that to really benefit from the monitoring capability the LCD modules will allow, we need to provide a seperate display for each of the three computers at a workstation, so that the monitored info can be compared "at a glance," without having to stop work to switch (and remember) each computers readings.

However, I still need to mount the LCD modules (we will use CFA634-YMC-KS displays) in an enclosure OUTSIDE the computer, since the machines are enclosed in "cooling chambers" below each workstation, and opening the door and bending over to read the LCDs would be too much of a hassle.

I would appreciate it if you see any problem with my new plans, as follow:

For each computer we will order your "CFA634YMCKS + Black PC Bracket Kit," consisting of the CFA634-YMC-KS, the DBB634LGB bracket, the WR232Y01 serial cable, and the WR232Y02 backlight cable.
I will use the DBB634LGB bracket to mount the LCD in a plastic enclosure of some type, and will run the WR232Y01 serial cable and the WR232Y02 backlight cable from the computer to the enclosure. (I anticipate that I will need to lengthen the WR232Y02 backlight cable, which should be no problem).
If I'm reading your your post of 5/21/02 --which you called a "mini tutorial on how to connect the "LCD" power section of the 632 or 634 to the "LED" (backlight) power section." --correctly, I understand it to mean that by adding a solder bridge between between "+5v(LED)" and "+5v(LCD)," I can power both the LCD and the LED backlight from the WR232Y02 cable, so that the 634 is always on, and does not reqire power from the serial cables DTR and RTS lines.

Is this correct?
If not, could you please breifly straighten me out as to what I must do to make the LCD module I refered to above function in a case seperate from the computer?

I should add that for some of the worstations, we have no open serial ports and so will have to rely upon USB-to-Serial conversion cables (for these I will most probably purchase your Aten USB-to-Serial product, since we have about 15 of those units presently in use).
If the USB-to-Serial configuration changes what I must do to accomplish mounting the 634 in a seperate enclosure, I'd appreciate it if you include the details in your response.

I really am grateful for the excellent assistance you've given me so far, and hope these requests are simple enough to answer.

I'm very eager to purchase the first module package and get a prototype built, so I can then put the my crew to work building the others during a lull in our schedule during the next month and a half.

Thanks again - look forward to hearing from you.

Jeffrey Lopez
CleanBoot Systems

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