(Info extracted from here: ftp://ftp.demon.co.uk/pub/amiga/docs/cd32-pinouts.txt)
!!!! The People at CD32 Source refuse to take responsability
for ANY damage this might cause to your hardware !!!!!
level: RS232
-12V..+12V
|\ | Amiga CD32
TxD o-------| >|---.
Pin2 |/ | | level: /TTL
D1 | 0V..+5V
1N4001 .-.
| |
| | IC1 7400
R1 | | ........
(*) `-' 1: _ :
| ___| \ :3
*--*___| O-------->o /RxD Pin6
| : |_/ :
.-. 2: :
| | : :
| | : :
R2 | | : :
(*) `-' : :
| : :
| : :
| : :
=== : :
GND /// : _ :4
6: / |____
RxD o<-------------------O |____*----o /TxD Pin2
Pin3 : \_|
: :5
:.......
GND o---------------*-----------------o GND Pin3
Pin7 |
| ........
| 7: :IC1
*--:GND :
| 9: :
*--: \ :
|10: |not :
*--: |used:
|12: | :
*--: | :
|13: | :14
`--: / +5V:-.
:......: |
|
|
+5V o---------------------------'
Pin4
shield o=======================! [n.c.] o shield
Pin1
The RS232-TxD-signal carries +12V or -12V level. As the first step
in conversion the diode D1 blocks if the signal drops to -12V.
In this case the voltage divider (R1,R2) is only connected with
GND potential. So the gate input carries GND level, too.
When the RS232-TxD-signal changes to +12V the diode will pass it
through and the voltage divider now provides a +5V-level signal to
the gate input.
The backward conversion consists of simple negation of the cd32's
/TxD signal. In fact there is no level conversion.
While the conversion for the received signal does
work proper with any RS232 opponent, the sended
signal furthermore carries TTL-level.
The opponent hardware tries to regain the TTL-level signal from
its 'RS232' input. The conversion unit handles the provided
pseudo RS232-type signals correct (as we want ist to be:-).
(*) voltage divider R1-R2:
U U = 12V U = 5V
R1+R2 R1+R2 R1+R2 R2
------ = -----
U R2
R2
U
R1+R2
I = 5 mA ==> R1+R2 = ------ = 2400 Ohm
R1+R2 I
R1+R2
(R1+R2) * U
R2
U = 5V ==> R2 = ------------- = 1000 Ohm; R1= 1400 Ohm
R2 U
R1+R2
R1=1500 Ohm
R2=1000 Ohm ==> I=4.8 mA ==> U(R2)=4.8 V .. will be OK
IC1 = 74LS00 (4 NAND gates)
D1 = e.g. 1N4001