strange RS232 GND problem

[large_ghostman]

hi ive finaly got rs232 working YAY!!!!!! but now have a problem

my set up is a pic connected to laptop A in mplab debug talking via a max232 to another laptop (B) via seria cable all was working well i then switched it all off and went to do something else when i came back and switched on hyper terminal didnt work the screen was blank as i was trying to sort it i broke the common gnd cable from laptop B to the max232 GND and hyper terminal started spewing rubish so the pic RX is working and the laptop B is seeing it but when i connect the GND's it stops!!! the pic keeps running and rs232 keeps working but hyper terminal just sits there! untill i remove ground but when i remove ground the data is rubish! any ideas?

 

[Ian Rogers]

I had this once.... Are you using a max232....If so I needed to ground out all other inputs / outputs... On a scope ( with the ground on ) it was bouncing all over the place... afterwards it was as stable as a rock.

 

[large_ghostman]

yes its a max232! i couldnt work out why one min it was ok then it wasnt!! but yes ive seen some horiable traces off it now and then! so i take it then every input or out put needs to be grounded? do i need a resistor? and do i do it on both the pic side of the max and the pc side?

 

[Ian Rogers]

Yes sorry!! the TTL side.... I can't remember how I did the HV side.... I hope someone else here can help. This is from wiki...

Because both ends of the RS-232 circuit depend on the ground pin being zero volts, problems will occur when connecting machinery and computers where the voltage between the ground pin on one end, and the ground pin on the other is not zero. This may also cause a hazardous ground loop. Use of a common ground limits RS-232 to applications with relatively short cables. If the two devices are far enough apart or on separate power systems, the local ground connections at either end of the cable will have differing voltages; this difference will reduce the noise margin of the signals. Balanced, differential, serial connections such as USB, RS-422 and RS-485 can tolerate larger ground voltage differences because of the differential signaling.[6]
Unused interface signals terminated to ground will have an undefined logic state. Where it is necessary to permanently set a control signal to a defined state, it must be connected to a voltage source that asserts the logic 1 or logic 0 level. Some devices provide test voltages on their interface connectors for this purpose.

 

[Wilksey]

I have never had an issue with a MAX232, I now tend to use Sipex 232 (pin compatible) as they are about £2 cheaper!
Good to know that the 2nd driver has to be grounded though.

I had a very annoying issue once, I had a dial up modem connected to a PIC and it wouldn't work correctly, it had 2 issues, first one was the 7805 Reg was regulating at 5.04V rather than the usual 4.98, so I put a diode in line to drop by 0.7V, that got the 232 working, then the modem wouldn't connect, just dropped the signal, that was a PSU issue, had a neutral return and caused a load on the ground reference, took a while to figure that one out!

 

[ghostman11]

i think its a more serious issue now. when it is plugged into my pc it all works so i think he has done something to the serial port on the laptop especialy as he had it working fine on there to start with! what i think he done was short some the pins out or worse still tried to solder the back on the serial plug that went to the cable and the soldering iron tends to have a voltage at the tip its not a ESD one.
maybe i should get him one like mine that is esd

 

[Ian Rogers]

i think its a more serious issue now. when it is plugged into my pc it all works so i think he has done something to the serial port on the laptop especialy as he had it working fine on there to start with! what i think he done was short some the pins out or worse still tried to solder the back on the serial plug that went to the cable and the soldering iron tends to have a voltage at the tip its not a ESD one.
maybe i should get him one like mine that is esd

Serial ports on PC's / laptop's are unforgiving..... Sometimes a reboot is necessary, once its screwed up.... Is this a FT232 serial port... ie USB to RS232?

 

[ghostman11]

no this is a real port

 

[Wilksey]

It is a bit of a bugger when that happens, I can't say i've managed to blow an RS232 port up as yet (there is still chance!!), but I would recommend getting a USB->232 converter or two, they are quite cheap, I have experienced issues with Prolific drivers in the past, not sure what they are like nowadays, the FTDI chips are quite good. Failing that get some free samples of 18F(2)(4)550 and go direct to USB, I expect other chips now have USB on them.

 

[kyru27]

I had some problems in getting strange data as I hadn't enoug voltage on pin 8 of the max 232, maybe it has something to do with this.

 

[ghostman11]

I had some problems in getting strange data as I hadn't enoug voltage on pin 8 of the max 232, maybe it has something to do with this.

except it was working fine and works good on another pc
as for getting usb to serial adapter i might get a couple but LG needs a serial port he connect to easilly with C#. i tried pic straight to usb and couldnt get it working

 

[Wilksey]

You can connect a USB-> serial device to C#, use CDC for the direct PIC->USB, Use the Microchip example, worked fine for me, just opens a virtual serial port which you can then use in C#.

Regards

Wilksey

 

[ghostman11]

cheers wilksey thats just the info i need, i didnt use CDC i used the HID one and it didnt work

 

[Wilksey]

Ahhh, no worries, HID is used for things like keyboard and mouse etc, if you look at the HID demo in Proteus (if you have Proteus that is) it moves your mouse around!

You can also use HID for security dongles, to protect your software, i'm going to give that a go one day, there are videos on how to remove a pen drive memory chip and replace with a USB PIC, very cool!

 

[ghostman11]

now that sounds like fun! i have ordered some 20 meg crystals as it looks like mine have gone missing! untill they come is there anyway to use internal clock with usb?

 

[Wilksey]

Which PIC are you using?

 

[ghostman11]

erm 18f4550 i think i also have 18f2550, i was wondering about running it at 32 meg by using the PLL

 

[Wilksey]

Just looked at the datasheet, section 2.3 is what you want to read, from that table, it seems you can't use the internal clock to drive the USB. USB Clock needs to be 6mhz for low rate, and 48mhz for full speed.

 

[ghostman11]

how come most the tutorials use a 20 mhz crystal? i dont understand how they get the right speed

 

[ghostman11]

i think i get it there is a divider, the datasheet is a bit confuseing on the usb section regarding the oscillator