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Please note : this is NOT a detailed 'how-to', just an idea for experimentation. Please don't ask me for detailed instructions. This modification should only be attempted by people who are comfortable with working on fine-pitch surface mount assemblies, or don't mind breaking stuff.

I needed a serial port for my nice new Sony laptop, which doesn't have one built in. Although USB to serial adapters are available, the chances of one working with my old dirty hardware-bashing DOS serial applications seemed pretty slim. PCMCIA serial port cards are available, but are hard to find and quite expensive due to the limited market.

I figured that old PCMCIA modems might use standard serial modem chips, and therefore have accessable serial data somewhere inside. I was right!
I pulled apart my old 14,4K IBM modem card, and plugged it into my laptop up using a PCMCIA card extender. Using some terminal software, I sent continuous characters to it, and probed around the chips with a 'scope until I saw the async. serial data on two pins of the chip I suspected was the UART. This was the data I was sending, plus the data echoed by the modem. I then hacked up some Turbo Pascal to twiddle the DTR and RTS lines, and quickly found these on the same chip. The input lines would be easier to find if the modem wasn't driving them.

Time to get ruthless .... I stripped off the 3 Rockwell modem chips and checked that the card was still recognised, and the serial data was still there.  I then tweaked my Pascal test code to display the state of the CTS/DSR/DCD control lines, and ran a lead from a 1Hz signal generator around the pins of the UART chip until I saw a line switching in time with the signal. (I didn't bother trying to find the RI line as I didn't need it).  All that was then necessary was to fit a RS-232 driver chip (I used a MAX241 because that's the first one I found in my heap of junk PCBs).

Not pretty, but it works!

Things to look for in a potential candidate for modification...

Old & slow (14,400 baud or less). Older units are more likely to have accessable serial lines internally, and are available very cheaply (<US$20) at computer fairs & radio rallies (hamfests) etc.
External DAA (Data Access arrangement or line interface), usually a 'lump' in the phone line cable. This implies lots of chips inside (i.e. seperate modem and uart), and usually means there are enough pins on the connector for all the serial lines you need.
Openable and re-closeable case - some are ultrasonically welded and impossible to open and subsequently re-close.
Cheap enough that you won't be too bothered if it turns out to not be modifiable, or you break it!


Make sure you only remove the modem chips (e.g. the ones that have 'Rockwell' or another modem chip maker on them). In particular don't remove any eprom or flash memory chips, as these may contain the PCMCIA data structure which is required for the PC to recognise the card - TSSOP chips are tricky to resolder! Take care when removing chips from a board with components on both sides, as it's easy to accidentally unsolder pins on chips on the underside.

Sorry, I didn't think to note the serial line pin numbers on the Rockwell chips before I removed them. The top-right one was either C5900-12 or C2900-12. I don't know the orientation, but  from the bottom-right corner, counting leftwards, the 6th, 7th and 8th pins are DCD, CTS and DSR respectively. Counting upwards, the 5th 7th,14th and 18th pins are DTR, RTS, RXD, TXD.

You may need to put some insulating tape on the inside cover to prevent shorting if headroom is tight.

Have fun!


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