[GreenKeys] RS-232 Interface

[Bob McConnell]

lobasa wrote:
>   Could anyone explain more about the RS-232 Interface? I have
> seen the circuit that has circulated here on Greenkeys.  The 
> Teletypewriters are standart 5 bit Baudot.  The RS-232 is 8
> bit isn't it.  So is there an UART between the 8 Bit and 5 Bit
> Baudot?  I have been looking for a UART circuit to use.
>    Any information would be appreciated.
>                             James Tanton
> P.S.   I found a stack of 4n35 opto-insolators.  Couldn't they be
> used  as a substitute in the interface?

RS-232 only defines the electrical connection and some pin assignments 
for the interface. It defines a bipolar signal where anything in the 
3-25 volt range is a valid signal, anything between +3 and -3 V is 
undefined and should be ignored. That provided 6 volts of hysteresis 
back in the hollow state days when it was useful.

Most computers of the IBM PC persuasion use a set of RS-232 
driver/receiver chips between the UART and the external connectors. A 
few, like HP, try to get away with a 5 V neutral signal instead, 
originally to force you to buy their peripherals to use on their PC.

The UART behind that interface is usually a conventional device, 
descended from the 8250, that can be programmed for most combinations of 
five to eight bits with or without parity. A few can also handle eight 
data bits plus parity, which was someone's standard for an addressable 
bus a long time ago.

The machine I am typing this on has a pair of 16550's. Unfortunately, 
most of those are UART's, with the 'A' being Asynchronous. The original 
IBM interface could use an 8251 USART, which would also do synchronous 
connections, almost as well as the 8530 from the Z80 chip set.

HTH,

Bob McConnell
N2SPP