If you ever have a problem or getting trouble how to set the the Samsung Galaxy S and other Galaxy S I900 or Galaxy S II I9100 and other series into 'Download Mode' if pressing the three button combination (Volume down+Power+OK) failed or not working for some reasons while flashing the phone using ODIN. This solution may help solved your problem.
All you need is a good soldering skills, a little bit effort and creativity and some components and tools below.
* micro USB cable
* a 300K and 1K = 301K 1/4 watts Resistors (A 300K will work also)
* Soldering iron at least 20Watts and a soldering lead
* a cutter or knife
Here's the block diagram to make a homemade Download Mode Jig dongle.
All you need is to solder the pin 4 (ID) and pin 5 (Ground) pins of the micro USB plug. Just cut out the sleeves of the USB plug, then attached and solder the 300K resistor across to pin 4 and 5. Just as like this finished product below:
How to use on Samsung Galaxy S while using ODIN.
1. Pull out the battery for about 2-5 minutes, then inplace it back again. Do not turn it ON yet.
2. Connect the jig you just made. Avoid touching the resistor and pins on the back or just put a a non inductive cover around it, an electrical tape works. The device will just turn into download mode in just a couple of seconds without pressing any buttons.
3. Once the Samsung Galaxy S displays the Downloading, disconnect the homemade jig USB plug and connect a regular USB cable from your PC to the phone, then you can flash the device using Odin.
Or to avoid any hassles or without any soldering skills ask Google where to find it.
It's very cheap to have it, but this USB Jig dongle will help you access to download mode even if your phone is 3-button combo disabled. If u plan on getting root access or adding ROM'S, you should have a dongle jig on hand just in case things do not go as planned. Download Mode Micro USB Jig will allow you to access the download mode on your Samsung Galaxy S Phone again.
This JIG also work with other phones below:
Samsung i896 Galaxy S Captivate Samsung T959 Vibrant Galaxy S Samsung Galaxy S Epic Samsung Omnia 7 Samsung i9000 Galaxy S Samsung Galaxy S Fascinate Samsung I897 Galaxy S Captivate. Others not tried yet but it might also.
Here's a video from NXjeff in his version of Homemade Samsung Galaxy S Download Mode Jig: