Jon,

first, it is great, please continue, just please don't overshoot it.

A couple of remarks, mostly formal:

• you mention DCE (and DTE). One of the most confusing thing for newbies is the usage of such abbreviations (and after "decoding" to "Data Communication Equipment" it does not get clearer IMHO). I would call it "modem-like", "slave" or anything else, with the "DCE" following in brackets for the advanced reader.
  
• formatting - I would recommend using block alignment of lines in paragraphs
  
• I assume you used Word and exported via Adobe PDF distiller. Word has a stupid feature switched on by default - it automatically capitalizes after a fullstop. E.g. on page 21: "It's recognized as a comm. Port,...".
  
• the tables on page 20 - I assume you have taken them over from some source, are you able to edit it? If yes, I would recommend to
  • rename TD and RD into the more common TxD and RxD
    
  • what they call there UART and should be DTE, call "PC(DTE)"; the "modem" is already there (maybe call it "modem(DCE)")
    
  • indicate the direction of signals in the table (PC->modem, PC<-modem)
    
  • in Functions table, group signals according to their function (and in order of importance - mentioning perhaps that the "less important" signals don't need to be connected in less demanding applications):
    
    
    • ground... well, ground is ground :-)
      
    • RxD+TxD - data (both directions)
      
    • RTS+CTS - hardware handshake ("throttling" data) (both directions)
      
    • DTR+DSR - indication of presence (both directions)
      
    • DCD+RI - in direction from modem to PC, used in real modems only
      
    
    
  • in the Functions table, the descriptions of signals are notoriously misleading. I would try to stress that the "names" of signals are from the viewpoint of PC and further use something like:
    
    
    
    • RxD - data flowing from modem to PC
      
    • TxD - data flowing from PC to modem
      
    • RTS - PC indicates to modem that it is ready or not to receive data
      
    • CTS - modem indicates to PC that it is ready or not to receive data
      
    • DTR - PC indicates to modem that it is present, switched on (if connected, sometimes used to clear a jam by simulating "non-presence" for a short time)
      
    • DSR - modem indicates to PC that it is present, switched on
      
    • RI - modem indicates to PC that it detects ringing on the connected phone line (somebody attempts to call the modem)
      
    • DCD - modem indicates to PC that a connection to an another modem has been established
      
  
  
• also it is worth playing with the figures' format outside Word, inserting ready-made pictures into it (don't even resize them within Word). Also for this style of pictures the jpg format is not too well suitable. Try gif, or better png. Yields less blurry pictures and smaller files. For simple pictures manipulation I use IrfanView.
  

I hope this was not too much.

Jan Waclawek