TTL TO SERIAL
MADE
EASY
INTRODUCTION
This page has been written to clarify the serial interface between the
FPGA
and the RS232 serial port.
BACKGROUND
To provide a serial interface from the FPGA to the RS232 serial port of a
computer the voltage levels must first be converted from TTL/CMOS logic
to
the voltage levels of the RS232 port (Table 1). Table 2 lists the
components required for the interface.
Buffering is done by using a MAX232A IC. The capacitor values used for
the MAX232A IC are listed in
Figure 2.
Table 1:Voltage Levels
|
Level
|
Transmitter
capable (V)
|
Receiver
capable (V)
|
Logical
space state
|
+5
... +15
|
+3
... +25
|
|
Logical
1
mark state
|
-5
... –15
|
-3
... -25
|
|
Undefined
|
-
|
-3
... +3
|
Table 2 : List of Components
|
Manufacturer |
Part Number |
| 9Pin
Female D-Sub Connector |
AMP |
A23292-ND |
| TTL/CMOS
to RS232 Logic |
Maxim |
MAX232A |
| 1
uF Electrolytic Capacitors |
N/A |
N/A |
| 1N4005 Diode |
N/A |
N/A |
In order to supply the Max232A chip with power, we could either use an
external power supply (5V) or "steal" the power from the serial port
of the computer. To do this we need to use a 7805 voltage regulator
(as configured in Figure 1) to
covert the 12 volt from the PC to 5V which the MAX232A needs.
Figure 1 : 7805 Voltage Regulator Pin Out
Figure 2 - Schematic of TTL to RS232 Interface (Click to
enlarge)
The Specification Sheet recommends a 0.1uF Electrolytic Capacitor, but
since only 1uF Electrolytic Capacitors are available, we used them
instead. The diode in the schematic is a just a general purpose
diode.
For more information please contact Timmy Li.
Group Members:
Darren Gonek
Guillermo Barreiro
Andrew Ling
Shyam Chadha
Timmy Li
Reid Orsten
Group AppNotes
TTL
to Serial
DC
Motor Control
CodeWarrior
Tutorial
Surviving VI
RS232 Protocol
RS232
Software Support
References:
Maxim Semiconductor (Free samples of
MAX232A IC)
RS232
Serial Port
MAX232A Spec Sheet
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