Nicad analyser circuit problem

[Tim Duke]

Hi all,

Am having a little problem with a circuit that I have built and wonder if
someone here can help me. I recently purchased a programmed PIC to build a
nicad analyser. It connects to the PC and with the software provided, plots
a discharge curve enabling you to see the health and capacity of the pack.

Here is a link to the schematic:

http://www.tdmodels.co.uk/temporary/1.jpg

and here is a link to the circuit description:

http://www.tdmodels.co.uk/temporary/nicada.pdf


The problem I am having, is I believe the software or the interface to the
PIC. Voltage is supplied by the serial port to power the PIC and i have
tried powering externally just in case the PC didn't supply enough juice.
The software does instruct the PIC to discharge at the correct rate, but the
circuit does not reply properly.

Q1. How does this circuit get away without using a MAX232 chip to convert
the TTL signals to RS232 level ?

Q2. Anyone here built one of these and has managed to get it working (or
not) ?

Needless to say that I have contacted the guy who designs and sells the unit
but am not getting anything helpful back from him.

Here's hoping someone can help. Thanks in advance,


Tim

 

[hamilton]

Tim Duke wrote:

Hi all,

Am having a little problem with a circuit that I have built and wonder if
someone here can help me. I recently purchased a programmed PIC to build a
nicad analyser. It connects to the PC and with the software provided, plots
a discharge curve enabling you to see the health and capacity of the pack.

Here is a link to the schematic:

http://www.tdmodels.co.uk/temporary/1.jpg

and here is a link to the circuit description:

http://www.tdmodels.co.uk/temporary/nicada.pdf


The problem I am having, is I believe the software or the interface to the
PIC. Voltage is supplied by the serial port to power the PIC and i have
tried powering externally just in case the PC didn't supply enough juice.
The software does instruct the PIC to discharge at the correct rate, but the
circuit does not reply properly.

Q1. How does this circuit get away without using a MAX232 chip to convert
the TTL signals to RS232 level ?

Looking at the data sheet, (page 4) the 16c71 does NOT have a UART
built-in. So the code in the chip would have to bit bang the asnyc
serial protocol. The RS232 chips do two things, 1) level translation
(+-12V to 5V/Gnd) and 2) invert the signal. Current limiting with a
series resistor is not the best way to limit the current but it does
work. The inversion is done in the bit bang software.

Looking over the schematic this should work fine, but there is not much
room for error. If this device was on the end of a long cable, higher
voltage spikes could develop along the cable and blow the input of the
micro.

Are you running the circuit on a long cable ??
Is that why you are using an external supply ?

The response from the PIC maybe a problem over a long cable.
Limiting 12v to a 5V input can work, but 5V into a 12V input will be
hard to do. Be as close as possible to the PC and it should work OK.

Q2. Anyone here built one of these and has managed to get it working (or
not) ?

Needless to say that I have contacted the guy who designs and sells the unit
but am not getting anything helpful back from him.

Here's hoping someone can help. Thanks in advance,


Tim

 

[Tim Duke]

Looking at the data sheet, (page 4) the 16c71 does NOT have a UART
built-in. So the code in the chip would have to bit bang the asnyc
serial protocol. The RS232 chips do two things, 1) level translation
(+-12V to 5V/Gnd) and 2) invert the signal. Current limiting with a
series resistor is not the best way to limit the current but it does
work. The inversion is done in the bit bang software.

Looking over the schematic this should work fine, but there is not much
room for error. If this device was on the end of a long cable, higher
voltage spikes could develop along the cable and blow the input of the
micro.

Are you running the circuit on a long cable ??
Is that why you are using an external supply ?

The response from the PIC maybe a problem over a long cable.
Limiting 12v to a 5V input can work, but 5V into a 12V input will be
hard to do. Be as close as possible to the PC and it should work OK.

Thanks for the response Hamilton......

From the PCB to the D9 connector that connects to the serial port is no
longer than 2 inches ! And I only tried using an external supply as I was
running out of ideas. I will scope the Txd line from the circuit and post
the results.

All the best,

Tim