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Serial Cables

There are two different types of serial equipement, DCE and DTE:

DCE Data Communications Equipment, the RS-232 concept for equipment (like modems) that forwards data but doesn't generate data of its own.
DTE Data Terminal Equipment, the RS-232 concept for any equipment that generates and/or reads data and does something with it, which means pretty nearly any type of equipment except a modem.

This is the main reason we need so many different types of cables for serial. The original idea behind DCE and DTE was that there were two types of equipment — “terminal” type of equipment that generates and/or receives data of its own, and “communication” type equipment that only relays data generated by someone else. Computers, printers, and teletypes and glass TTYs were all of the first type (DTE) and modems were of the other type (DCE). Then for various reasons that are now fairly insignificant they decided that the 25-pin and 9-pin RS-232 connectors on these two types of equipment actually needed to be wired differently, with the result being that you then needed two different types of cables: one for connecting a DTE to a DCE and another for connecting two DTE's directly to each other.


IBM PC-AT serial ports - male

DB-9 male connector on PC's (originally the IBM PC-AT) serial ports. When looking at the back of your computer, the pin numbers are:

   1 2 3 4 5 
    6 7 8 9 

and the signal assignments are:

pinsignalsignal direction from computer's point of view
1 DCD - Data Carrier Detect in
2 RxD - Recieve Data in
3 TxD - Transmit Data out
4 DTR - Data Terminal Ready out
5 GND - Ground -
6 DSR - Data Set Ready in
7 RTS - Request to Send out
8 CTS - Clear to Send in
9 RI in


sex Male Female Female Male
1 No Connect DCD/DSR/DTR DCD/DSR/DTR No Connect
2 BF533 Transmit BF533 Transmit BF537 Transmit BF561 Transmit
3 BF533 Receive BF533 Receive BF537 Recieve BF561 Receive
4 No Connect DCD/DSR/DTR DCD/DSR/DTR No Connect
5 Ground Ground Ground Ground
6 No Connect DCD/DSR/DTR DCD/DSR/DTR No Connect
9 No Connect No Connect No Connect No Connect


straight through

To connect a PC to a STAMP Board, or EZKIT, all that is necessary is a straight through cable. Either a male ↔ Female cable for the BF537-STAMP and BF533-STAMP, or a Female ↔ Female cable for the BF533-EZKIT and the BF561-EZKIT.

Many Female ↔ Female cables are null modem cables, and will not work when connecting a PC to a Blackfin target


Null-modem cables are required when you need to swap pins 2 and 3 on either ends of the cable. This can be used to connect STAMP board to a STAMP board. There are three common types of null-modem cables. The simplest is the three-wire cable, used when neither side expects hardware handshaking:

  one end         other end 
   (DTE)            (DTE) 
    TxD ------------ RxD 
    RxD ------------ TxD  
    GND ------------ GND 

The second type is used when one end expects hardware handshaking but the other end cannot provide it. It “fakes” the hardware handshaking signals, allowing communication to happen but losing the benefit of hardware handshaking:

  one end         other end 
   (DTE)            (DTE) 
    TxD ------------ RxD 
    RxD ------------ TxD 
    RTS -.        .- CTS 
         |        | 
    CTS -'        `- RTS 
    DCD -.        .- DTR 
         |        | 
    DTR -'        `- DCD 
    GND ------------ GND 

The third type is a full handshake cable, and is used when both ends are doing hardware handshaking:

  one end         other end 
   (DTE)            (DTE) 
    TxD ------------ RxD 
    RxD ------------ TxD 
    RTS ------------ CTS 
    CTS ------------ RTS 
    DCD -+---------- DTR 
    DSR -' 
                  ,- DSR 
    DTR ----------+- DCD 
    GND ------------ GND 


If you think that you are having issues with the serial port, from your terminal:

  • check the buad rate
  • check parity, and number of stop bits
  • check the cable (short pins two and three - this should provide loopback - anything you type on the keyboard, should be echoed back)
  • for systems that support UART Boot - connect with your favourite terminal, reset the board, and hit the “space” bar. you should get 2 garbage char back. If you don't - it means your cable or terminal application is not functioning properly.

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