Any help or suggestions would be greatly appreciated - and if I have posted this in the wrong forum let me know.

I have a basic setup of a laptop and a receipt printer powered by a 24v lead acid battery (2 x 12v cells in series) - crude sketch attached. The printer is 24Vdc and so is powered directly from the battery, the laptop is 19v and so is powered from the battery via a dc-dc voltage converter - in this case it is a model from Maplin. The laptop is then connected to the printer using a USB-Parallel cable/adapter such as this one.

The problem that I am experiencing is whenever the dc converter is connected/disconnected from the battery it seems to blows the chip in the USB-Parallel cable, destroying it. Having gone through quite a few cables testing out various setups I have found that the data lead wont blow if:

1. Only connected to the laptop and not printer (open end).
2. Connected to the printer and laptop but the printer is powered from a separate battery.
3. The dc-dc converter and printer are connected to the same battery but the dc converter is powering a different laptop to the one that the data lead is plugged into.

Strangely the lead will blow if the printer and laptop are connected to the same battery, but the printer is switched off. So, lead will only seem to blow when the dc-dc converter is connected/disconnected if - the printer and computer are powered from the same battery and the data lead is connected to both devices.

To clarify, there is no loud bang, or smoke or melting of cables, it just seems that the pcb/chip in the USB-Parallel cable is damaged/fried so that it is no longer recognized as a USB device under Windows.

As a layman I would guess that connecting/disconnecting the dc-converter is creating some sort of surge or spike that is damaging the USB devices, but if anyone has any suggestions on what is causing this USB devices to blow and what the solution might be I would be very grateful.

 

Welcome to AAC!
I don't know whether the cable IC draws its power from the USB port or from the parallel port, but I'm wondering if, in the configuration which fries the IC, it loses its power supply but is still receiving data signals? Many ICs don't like that.

 

The dc converter you linked to indicates 12V input, not 24. Are you using a different model with 24V input, or are you running the converter from just one of the two batteries in order to provide the required 12V input?

 

Alec,

Thanks for the reply. If you plug just the data cable into a laptop, not connected to the printer, the IEEE-1284 Controller appears in the 'Devices and Printers' menu in Windows. I would assume therefore that the IC is powered by the USB bus.

As I mentioned in my original post, if the data lead is plugged into the laptop only (not connected to the printer), then connecting/disconnecting the dc-converter doesn't damage the lead. I don't know whether that fits with your thinking on if the IC is receiving data signals. I should have said that whilst testing what will or wont cause a data cable to fail, no print jobs were being sent to the printer.

I don't know if it is relevant, but your post re if the IC is powered from the parallel port got me wondering if the cable would fail if it was plugged into the parallel port only, and not the USB. I have just given it a quick test, both printer and laptop powered from the same battery, and I can't get it to kill the IC. It def seems something about completing the 'loop' of battery - dc converter - laptop - data cable - printer - battery that results in a blown parallel data cable if the dc-converter is connected/disconnected.

 

ebeowulf,

Thanks for the reply. The Maplin site only mentions 12v - I'm guessing because these are mostly used with car electrics, but the packaging for that model of dc-converter states that it will accept an input voltage from 10-30v, making it suitable for truck electrics too I guess. My apologies, I should have noticed that the webpage I linked to didn't state the input voltage range. I have attached a picture of the back of the device which shows this.

To confirm, since I think that it's OK for 24vdc, the converter is connected across both batteries.