Date: Fri, 13 Jan 1995 18:31:00
Subject: isolated converter
From: Colin Crichton <colinc@hotlogical.win-uk.net>

To:  Everybody who requested the current loop ckt.

Please note that the circuit was drawn after it was
breadboarded so the drawing has never actually been used to
build a circuit from.  The design works well using a circuit
emulation package however, so i'm reasonably confident there
are no mistakes.

A power supply will be needed at the PC end if your PC does
not have a 12V o/p available.  The decoder should have a 5V Vcc
line capable of providing 200mA, but this may vary from model
to model, if you are in doubt, use a 5V PSU at that end as
well.


Some useful info from the original season 7.

Pin outs for 9 or 25 way d-types are as follows:


               Sub-D 25-pin    Sub-D 9-pin
  ---------------------------------------------------------
    TxD             2               3       transmit data
    RxD             3               2       receive data
    CTS             5               8       clear to send
    DSR             6               6       data set ready
    GND             7               5       ground
    DCD             8               1       carrier detect (reset)
    DTR             20              4       data terminal ready
 

Connections to the decoder smart card are as follows:

           ______________________________________
         /                                        \
        |                                          |
        |                                          |
        |    C1   C5                               |
        |    C2   C6                               |
        |    C3   C7                               |
        |    C4   C8                               |
        |                                          |
        |                                          |
        |                                          |
         \________________________________________/

         
The current loop transmission system is a much under-rated and
under-used format which gives extremely long transmission
distances and is resilient against interference in almost
every form (cable self capacitance is normally what kills it
after >600m). I have had to use dual transistor drivers on each
of the loops in order to combat the built in data inversion in
the MAX233.  Obviously, a MAX232+4xTantalums will do if a 233
is hard to get. The standard current loop drivers in the text
books normally recommend diodes connecting the current loop
low line to GND and Vcc but I have never seen any improvement
in signal quality from this. However, fit them if you like. Oh
and the 74LS07 should really be decoupled with a 0.1 uF fitted
close to pin 14. 

The circuit has not been tested at long distance. I would be
interested to hear of any attempts and what distances were
achieved.

Colin 

