Overthinking ELO Serial over Cat5?

S

Sacarino

Member
Hi guys,

I picked up a serial 15" Elo and I don't have the mfg whip to convert from the board pinout to db9... so I figured I'll just pick up a 5x2 IDC and and DB9 shell, then just wire it myself. I think I'm over-analyzing the problem though.

I found a good site describing the RS323 pinout for DB9 at zytrax.com. If I use that along with the Elo serial controller pinout, I should be able to ignore the N/C and Key pin positions, giving me a nice 8 pin setup that I can run over cat5. I *think* all I have to do is invert the Tx and Rx on one end of my cable - and possible the CTS/RTS if it needs flow control (which seems unlikely) - and then it would work.

One of these days I am going to have to sit down with a dummies book for rs232 comms.

Am I on the right track or did I complicate the situation?

Thanks,
Shane
 
I don't think the OP is concerned about a methodology to get the 9-Pin to Cat5e, rather he needs to know if the serial communication for his application uses more pins than the typical (on the 9-pin serial connector) 2, 3, and 5 (transmit, receive, and ground). Also some applications require that pins 2 and 3 be "crossed" while others do this crossing internally inside their hardware.

If this were me, I would try just using pins 2, 3, and 5 (9-Pin D connector) and do not cross 2 and 3 and see if this works. I'm sure others will jump in that have experience with this type of touch screen communication.

In places where I do want to go over Cat5e (via RJ-45 connectors) I use adapters such as THESE.
 
BraveSirRobbin said:
I don't think the OP is concerned about a methodology to get the 9-Pin to Cat5e, rather he needs to know if the serial communication for his application uses more pins than the typical (on the 9-pin serial connector) 2, 3, and 5 (transmit, receive, and ground). Also some applications require that pins 2 and 3 be "crossed" while others do this crossing internally inside their hardware.

If this were me, I would try just using pins 2, 3, and 5 (9-Pin D connector) and do not cross 2 and 3 and see if this works. I'm sure others will jump in that have experience with this type of touch screen communication.
Click to expand...

BSR, that is exactly what I was mulling over... I hadn't considered just the TX, RX, and GND. I guess it can't hurt anything to start there and work my way out, eh?

Love those Deep Surplus guys - I always get Skittles!

edit: weird, exactly 12 hours later.
 
RS232 predates PC's by at least a decade. Ever use an IBM terminal (that was a modified Selectric typewriter - no screen) connected to a S/360 with a 134.5 baud modem? Those were the days.

Whether or not you have to swap TD,RD depends on the devices. Devices are either DCE (data comm equipment) like modems, or DTE (data terminal equipment) like computers, printers, and dumb terminals.

Way back when, most everything used DB25 connectors. If you connected a DTE to a DCE (computer to modem) with a standard DB25 cable, you'd use a straight though cable. The DTE transmits on pin 2 and the DCE receives on pin 2. If you connected a DTE to a DTE (computer to serial printer), you'd swap pins 2 and 3. Depending on the device and h/w flow control requirements, you'd need to swap or backfeed some other pins for RTS/CTS, DTR/DSR, etc.

It got more confusing as things progressed. The PC's serial DB9 (technically a DE9) receives on 2 and transmits on 3. Some older mini computers even though they are supposed to be DTE (terminal equipment) were wired as DCE (so you could use straight through cabling to the terminals). The Lantronix UDS10 is DCE, but the pinout diagram in the user guide indicates that pin 2 is "TX in" and pin 3 is "RX out" instead of RD/TD. geez.

Connecting just TD/RD/SG may or may not work. Signaling is there for a reason. Signaling can let one device know the other one is there and ready before sending data. If you're sending data to a device that can only process so fast (like a printer), the device needs to tell the sender to pause until it catches up. You'd need signal lines for h/w flow control. Or you can use software flow control. Or you can lower the baud rate.

The easiest way to figure out how to connect two serial devices is to draw a picture of the pins and signals of each device. Connect TD to RD, RD to TD, etc. That's your cable schematic.

You can do just about anything you want to physically connect the devices. For compatibility, I'd make the ELO look just like a PC, a DB9M with a DTE pinout. If you're connecting to a PC, I'd use a DB9F to DB9F cable with the RJ45 connector shells. Standard straight through cat5 and swap the pins in one of the shells.
 
a trick you can use in figuring out the RX/TX cross over is if you have a multitester, set it to DC voltage and test the wires (with the equipment on). A tx pin will have a negative voltage present (with respect to ground, pin 5), usually about -9 to -12 volts. If it is an input, there will not be any voltage present. Once you know which pin is the output and which is the input on each side, you can connect them accordingly.
Note, this assumes a simple pin 2,3 connection. As sda points out, it gets ugly after that and usually an RS232 breakout box becomes invaluable but for an ELO touch screen, I would be willing to bet you can get away with a simple TX,RX,Gnd connection
 
Brightan said:
a trick you can use in figuring out the RX/TX cross over is if you have a multitester, set it to DC voltage and test the wires (with the equipment on). A tx pin will have a negative voltage present (with respect to ground, pin 5), usually about -9 to -12 volts. If it is an input, there will not be any voltage present. Once you know which pin is the output and which is the input on each side, you can connect them accordingly.
Note, this assumes a simple pin 2,3 connection. As sda points out, it gets ugly after that and usually an RS232 breakout box becomes invaluable but for an ELO touch screen, I would be willing to bet you can get away with a simple TX,RX,Gnd connection
Click to expand...

Hey! You own Brightan Microcontrollers!

I was checking out your website last week, and read through some of your older posts on CQC!

OT, but this is Wiring Closet anyway, I'm looking to control some DC motors. Arduino keep coming up... :throwup: I'll catch you at CQC.
 
This is why I love this place - even the OT stuff is interesting... high SNR is good times. :)

I hope to have the opportunity to play with the pinout stuff either tomorrow or Sunday but life seems to be getting in the way of my hobbies a bit too often. Maybe I should write my senator for bailout money?

Thank you all for your input, by the way. I'm very grateful!
 
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