I assume that you already read the basic information about ISO8583. You can go to wikipedia for reference if you confused while reading my post here.
Here is ISO8583 sample message:
0200723A4001084180021666666612345678905610000000010010000327110332000981180332032703286011040014000980 000000000012123036123456 000012345678900000100000028000288
what is the content actually?
Here is the step:
- 0200 >>This is MTI. It means the message use ISO8583 with 1987 version and the process is Acquirer Financial Request.
- 723A400108418002 >> This is Bitmap. It will indicates which other data elements or data element subfields may be present elsewhere in a message. The step are:
- Take first Bitmap number. Then make it into hexa. In this example 7 will become 0111. The 0 of that hexa means that there is no bit65 present in this message. So we only need to take 16 part of the message. (16*4=64bit)
- Change all that 16 character to hexa. The 1 means the bit message are present. On this sample the message that present are on bit 2,3,4,7,11,12,13,15,18,32,37,42,48,49,63.
- The rest of message are data element which is contain the transaction information. For example, look at bit 63. The character can be alpha numeric or symbol have 999 characters (ans…999). So the message looks like 00214. This means that on bit 63 only have 2 characters and the message are 14.
Bits =  Bits =  Bits =  Bits =  Bits =  Bits =  Bits =  Bits =  Bits =  Bits =  Bits = [000980 ] Bits =  Bits = [123456 0000123456789000001000000] Bits =  Bits = 
I hope this article can help you 🙂