Serial Port Device Control

[JonB]

I have a computer that controls a cash drawer as part of a Point of Sale system. The software vendor Tech Support has been pretty worthless, so lets see if anyone here knows much about serial port control.

The problem is that the old PC, a 286 based monster, finally died and I replaced it with a newer box. When you complete a transaction for a customer, it prints a receipt, then sends a signal to the cash drawer via the serial port. A big solenoid actuates and the drawer is SUPPOSED to open.

The problem is, the solenoid won't trigger reliably anymore. It used to with the old board. I have tried changing the baud rates, the parity, stop bits, etc.., but can't make it trigger more than 60% of first tries. Sometimes, it takes eight attempts before I finally hear the solenoid actuate.

What output lines do you think it uses? Does it sound like low voltage or possibly too short of a pulse to trigger the solenoid?

If so, how could I increase the voltage or lengthen the pulse? Is the new and significantly faster computer just too fast for its own good?

 

[blahblah99]

What gets connected to the serial port on the solenoid end? I'm sure there are more circuitry (perhaps a microcontroller, or some rectifying circuit and a couple transistors) to power that solenoid. A serial port is a serial port. It has a defined protocol and will interface the same whether the signal is coming out of an 286 PC or an Athlon PC.


1. If it uses a microcontroller, then you will need an oscope to check out the signals going into the micro-controller and the signals coming out to the solenoid. Check the clock to the microcontroller.


2. If it uses discrete components, get a multimeter and probe the input to the transistor (most likely at the base) and at the solenoid. There should be some sort of voltage pulse at the base of the transistor to activate the solenoid.

If the design uses #1 approach which I don't think, you might have a harder time troubleshooting. Now if it uses approach #2 which is a cheaper approach, I am guessing the transistor is sensing one of the data lines on the serial port. I forgot what's it called, but there is a line that goes from -15V to +15V indicating that the computer is ready to send. That line gets fed into the transistor which in turns powers the solenoid. Of course, I can be completely wrong on the design of the solenoid section.

OR your solenoid can just be bad.