Tuesday, December 4, 2012

Caller ID project - part one

The first task in my Caller ID project is to research the USB Modem output to see what I have to work with.

I'm using a USR 5637 56 Kbps USB modem for my project because I had one on hand that hasn't seen use in a few years.  They're about $45 new, $25 on eBay.  I checked the specs and this one supports Caller ID as does my local phone company.  

I plugged the modem into the Mac, fired up ZTerm and connected to usbmodem0000001.

I typed ATH (the code for hang up) just to see whether ZTerm was connected to the Mac - I got the expected 'OK' response.

Then I typed AT#CID=2 - this sets the caller ID output to unformatted.  I figured this might be a little easier to parse. Apparently this setting is not "sticky" and needs to be set each time the USB modem powers on.

I plugged in a phone cable, called my number and saw this (this string has extra spaces that I put in for clarity):

MESG=80 27 01 08 31 32 30 33 31 39 32 34 02 0A 34 31 39 35 35 35 31 32 31 32 07 0F 43 65 6C 6C 20 50 68 6F 6E 65 20 20 20 4F 48 FF<CR>

The message is a string of Hexadecimal numbers followed by a Carriage Return.  The format of this message is:

message type
length of message
field type 1
field length 2
field 1
field type 2
field length 2
field 2

field type N
field length N
field N

The message type is '80' - MESG=80 will be a good indicator of the start of the message.

Field types:

  • 1 - date & time
  • 2 - phone number
  • 4 - number not present
  • 7 - name
  • 8 - name not present
If the calling party's number is not available or hidden, the name field contains an "O" or "P" respectively. Type 4 and type 8 are apparently always length 1 followed by an "O" or a "P".

So manually parsing the message above, I get:

MESG=80 - start of message 
39 character long message 
Type 1 - date & Time 
8 characters for the date and time  
12031924 (call was received on 12/03 at 19:24) 
Type 2 - phone number 
10 characters for the caller's number  
Type 7 - name 
15 characters for the name 
Cell Phone   OH (there are 3 spaces between "Phone" and "OH" 
Checksum: 255 

The USB Modem, sans case: 

I'm planning to encase it with the rest of the project components.

Next step - write a program to parse this.  Hmmm: C or Python?  I think C...

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