Bus Pirate

Dmesg

[25067.543315] ftdi_sio 2-1.2:1.0: FTDI USB Serial Device converter detected
[25067.543514] usb 2-1.2: Detected FT232RL
[25067.543521] usb 2-1.2: Number of endpoints 2
[25067.543526] usb 2-1.2: Endpoint 1 MaxPacketSize 64
[25067.543532] usb 2-1.2: Endpoint 2 MaxPacketSize 64
[25067.543537] usb 2-1.2: Setting MaxPacketSize 64
[25067.543927] usb 2-1.2: FTDI USB Serial Device converter now attached to ttyUSB0

Lsusb

Bus 002 Device 016: ID 0403:6001 Future Technology Devices International, Ltd FT232 USB-Serial (UART) IC

Use the bus pirate as a simple USB-Serial adaptor

Since the bus pirate is basically an FTDI chip + PIC, it is possible to set the software that runs on the PIC to behave as a simple TTL usb serial adaptor.

With screen, do the following:

HiZ>?
General                                 Protocol interaction
---------------------------------------------------------------------------
?       This help                       (0)     List current macros
=X/|X   Converts X/reverse X            (x)     Macro x
~       Selftest                        [       Start
#       Reset                           ]       Stop
$       Jump to bootloader              {       Start with read
&/%     Delay 1 us/ms                   }       Stop
a/A/@   AUXPIN (low/HI/READ)            "abc"   Send string
b       Set baudrate                    123
c/C     AUX assignment (aux/CS)         0x123
d/D     Measure ADC (once/CONT.)        0b110   Send value
f       Measure frequency               r       Read
g/S     Generate PWM/Servo              /       CLK hi
h       Commandhistory                  \       CLK lo
i       Versioninfo/statusinfo          ^       CLK tick
l/L     Bitorder (msb/LSB)              -       DAT hi
m       Change mode                     _       DAT lo
o       Set output type                 .       DAT read
p/P     Pullup resistors (off/ON)       !       Bit read
s       Script engine                   :       Repeat e.g. r:10
v       Show volts/states               .       Bits to read/write e.g. 0x55.2
w/W     PSU (off/ON)            <x>/<x= >/<0>   Usermacro x/assign x/list all
HiZ>m
1. HiZ
2. 1-WIRE
3. UART
4. I2C
5. SPI
6. 2WIRE
7. 3WIRE
8. LCD
9. DIO
x. exit(without change)

(1)>3
Set serial port speed: (bps)
 1. 300
 2. 1200
 3. 2400
 4. 4800
 5. 9600
 6. 19200
 7. 38400
 8. 57600
 9. 115200
10. BRG raw value

(1)>9
Data bits and parity:
 1. 8, NONE *default 
 2. 8, EVEN 
 3. 8, ODD 
 4. 9, NONE
(1)>1
Stop bits:
 1. 1 *default
 2. 2
(1)>1
Receive polarity:
 1. Idle 1 *default
 2. Idle 0
(1)>1
Select output type:
 1. Open drain (H=Hi-Z, L=GND)
 2. Normal (H=3.3V, L=GND)

(1)>2
Ready
UART>
UART>

Once you have the UART> prompt, just type "(1)", and it will transform itself into a usb-serial bridge.