[E3-hacking] Universal E2/E3 serial interface design

[Ian Stirling]

Andy wrote:
> Hiya,
> 
> As per my earlier posting I've now designed and built a universal E2/E3 
> interface board that should convert to/from legal RS232 levels. A jumper 
> is used to select E2 or E3 mode. I've also produced a circuit diagram 
> from my prototype and put this here:
> 
> http://www.cambridgewargames.org.uk/members/andy/emailer/
> 
> I would welcome any feedback on the design and would also appreciate 
> people sanity check I've not put any stupid mistakes in the circuit 
> diagram when copying from my stipboard prototype.
> 

The 1K resistors are not quite adequate protection on the input of IC1A.
A static spark could easily generate 2Kv, or a 2A pulse.
This will probably not kill the 4070 the first time.

Four diodes, one each between the tip/ring connections, and VCC/ground 
would be a good idea.
1Nn4148 or similar would be fine.