Date   : Tue, 28 May 1996 12:10:02 +0100 (BST)
From   : Philip Blundell <pjb27@...>
Subject: Re: Unidentified subject!

On Tue, 28 May 1996, Iain Hardcastle wrote:

> However, the RS232 of a PC is more complex. Its definition suggests (but 
> not necessarily prescribes) that the DTR pin has a function similar to 
> the Beeb's RTS pin and the DSR pin a function similar to the Beeb's CTS 
> pin. Most protocols implemented on the PC (and all I tried) adhere to 
> this. This is why you have to connect the Beeb's RTS to the PC's DSR and 
> the Beeb's CTS to the PC's DSR. 
>      
> But things are more complex. The RS232 also has a DCD (data carrier 
> detect) pin, which can be used to detect if there is a device connected 
> to it. Most (but not all) protocols implemented on a PC use this. The 
> easiest way to "circumvent" this is to connect this pin to the PC's DSR 
> pin, so that every time it receives a DSR-signal, the PC automatically 
> knows there is also a device connected. 
>      
> To make things even more complex, the RS232 also has an RTS and a CTS 
> pin, albeit with different functions than on a Beeb. Some protocols 
> implemented on the PC use these for a second handshake after the 
> handshake with DTR/DSR/DCD. (Don't ask me what it is good for.) The 
> easiest way to "circumvent" this, is to connect the PC's RTS pin simply 
> to its own CTS pin.

Just to clarify, yes, the PC does indeed (usually) have all of RTS, CTS, 
DSR, DTR and DCD. 

The usual way to handle this situation is to loop back DTR to DSR and DCD 
locally, and cross-wire RTS and CTS in the same way as RD and TD.  Your 
solution is probably fine as well, albeit a bit unconventional.  You can 
substitute CTS and DSR, as well, which is useful if you might be using a 
broken machine like an Archimedes which has trouble with CTS.

> - Find out which PC's RS232 port is COM1 and which one is COM2. I thought
>   COM1 is always the 9-pin socket and COM2 always the 25-pin socket, but 
>   on the PC I used it was exactly the other way round! (Btw. Does anyone 
>   know of an easy way to find this out?)

There is no easy way.  In general, you just have to try both and see 
which works.  If you have a trustworthy cable on hand, this shouldn't be 
a problem.

P.