Python Class for Fast USB-based Digital I/O
Using an FTDI USB-to-Parallel I/O Device
(Windows 64-bit)

Introducing the FTDI UM245R USB-to-Parallel Interface
UM245R Data Sheet      FT245R Data Sheet

A simple, reliable, fast and inexpensive approach to reading and writing to an 8-bit digital I/O interface can be achieved using the FTDI FT234R USB-to-Parallel interface module (cost: approximately $20).  A photograph of the UM245R development module (which contains a FT245R device with easily accessible pin outs) is shown in Figure 1. This device allows the developer to quickly read and/or update the states of 8 digital I/O lines over a USB interface.  The FTDI drivers and programming API are free of charge and easily downloadable from the FTDI website.  The 8 I/O lines are depicted as pins DB0-DB7 in Figure 2.  The directionality (i.e., input vs. output functionality) of these 8 lines is completely programmable (i.e., any combination of input and output lines can be configured by the user).  Detailed setup instructions and python code for accessing this device are presented below.

The following Python code snippet demonstrates how to use the um245() class:

#load the code extension module
from UM245R import um245 
# create a um245 interface object using standard default parameters
#     devno = 0 --> Uses first FTDI device enumerated during search of the computer
#     iomode = 0xf0 --> bits 4-7 = outputs, bits 0-3 = inputs
device = um245(); 
# verify successful connection to FTDI device (i.e., status = True)
status = device.isDeviceOpen()
print('Device ready = {}'.format(status))
# let's try reading an 8-bit value from the UM245R device
data = device.input()
print('Current state of I/O lines = %s' % format(data,'#08b'))
#
#  now let's activate the most significant bit on the data port (bit-7)
data = data | 128          #use bitwise OR operator to set most significant bit
device.output(data );   #update state of I/O lines
print('Updated state of I/O lines = %s' % format(data,'#08b'))
 

Parsing Individual Bits within an I/O Byte

When one reads an I/O port one is usually interested in the status of a single bit among the 8-bits returned by a call to <um245>.input(). Python provides a number of operators to deal with data on a 'bitwise' basis.  Some examples follow:

Activate Output Line DB7:

from UM245R import um245      #load software interface to FTDI UM245R device (i.e., um245() class)
#create interface object specifying that I/O port bits(0-3) are inputs and bits(4-7) are outputs
usb = um245(iomode = 0b11110000)
#read current state of I/O port lines
ioport = usb.input()
#use bitwise OR operator to set bit-7 (corresponding to DB7 output line)
ioport = ioport | 0b10000000
#update I/O port thus activating DB7 output line (but leaving all other bits unaffected)
usb.output(ioport)

Deactivate Output Line DB7:

from UM245R import um245      #load software interface to FTDI UM245R device (i.e., um245() class)
#create interface object specifying that I/O port bits(0-3) are inputs and bits(4-7) are outputs
usb = um245(iomode = 0b11110000)
#read current state of I/O port lines
ioport = usb.input()
#use bitwise AND operator to reset bit-7 (corresponding to DB7 output line)
ioport = ioport & 0b01111111
#update I/O port thus de-activating the DB7 output line (but leaving all other bits unchanged)
usb.output(ioport)

 Test the State of Digital Input Line DB0:

 #read current state of the 8 I/O port lines
ioport = usb.input()
#use a mask with a bitwise AND operator to isolate I/O port input DB0
state = ioport & 0b00000001
#check to see if DB0 input line is active (i.e., ON = logic '0' = input line 'grounded')
if state == 0:
    DB0_active = True   #DB0 is active because it is at logic level-0
else:
    DB0_active = False  #DB0 is inactive because it is at logic level-1 (internal pull-up resistor)

Search Computer for Installed FTDI USB Devices:   find_ftdi.py

Fig. 5: Sample Setup for Testing the UM245R Hardware [Sample circuit: 12v solenoid driver]

Python Code for Exercising the Test Circuit

from UM245R import um245
import time
#create interface to UM245R with DB0-3 = inputs and DB4-7 = outputs
usb = um245(iomode = 0b11110000)
#turn-on LED by making DB7 output level go 'high'
usb.output(128)
#wait for 3 seconds
time.sleep(3.0)
#turn-off LED by making DB7 output level go 'low'
usb.output(0)
#
#after reading the state of the I/O port determine the state
#of the Test Switch connected to the DB0 input line
ioport = usb.input()
if (ioport & 0b00000001) == 0:
    print('The Test Switch is CLOSED (i.e., DB0 is grounded)')
else:
    print('The Test Switch is OPEN')

Simple UM245R Timing Benchmark
HP ZBook G2 Laptop; i7-4810MQ CPU 2.8 GHz; Windows 10 Pro (Build 19045.2251)

Code snippet:

import time
from UM245R import um245
usb = um245()   #create interface to UM245R device
times = []
for i in range(500):  #500 iterations of input timing measurement
    startTime = time.perf_counter()   #read system clock (secs)
    data = usb.input()   #fetch input from UM245R device
    endTime = time.perf_counter()    #read system clock (secs)
    latency = (endTime-startTime) * 1000  #compute input read latency (millisec)
    times[i] = latency   #save result

Benchmark Results:

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Last revised: 10 December 2022

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