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Perdition · Guest

I heard from a technician that the following is incorrect:

DTR is always on at the DTE. When dsr arrives from the DCE to the DTE,
the rts isn't sent in response.

He believes that in order to get an rts you must not only provide a DSR
but also transmit from TX. I believe that dsr is enough to get an rts.
What is the correct procedure?

Thanks alot guys :)

Perdition · Guest

oh and the interface in question is rs232c, thanks again ^_^

Aaron Leonard · Guest

Do you want to know what is or is not "correct", or what some actual device
in fact DOES?

In any case, it would be good to get clear on whether you are interested
in a half duplex or full duplex device. RTS/CTS handshaking behaves very
differently in the two cases.

As far as DTR and DSR - my view is that they should simply denote that
the respective devices (DTE and DCE) are powered on and operational.
They may or may not be required for data flow depending on whether
the device of interest is paying attention.

DSR indicates to the DTE that connectivity to its local DCE is present,
as distinguished from DCD which indicates that connectectivity to the
REMOTE DCE (i.e. carrier) is present. (Having said that, I will note
however that the standard for Cisco async DTEs is for DSR as such
to be unused, and for DCD to be called "DSR" internally.)

Cheers,

Aaron

Perdition · Guest

well i'm refering to a connection between an ibm computer through a
parallel port to a modem using an rs232c cable. i've learnt that the
DTR pin (pin 20) has voltage coming from it as of the time the computer
is operating. Once the dtr pin is asserted and the signal arrives at
the DCE, dsr should be sent in response. then the computer will assert
rts either constantly for constant carrier or only when it needs to
transmit for switched carrier, regardless CTS would be the reply from
the DCE to allow transmission. At any point is pin 2 necessary in this
local procedure to get CTS? And how is it DCD is asserted from the
remote station sending information if this entire procedure doesn't
involve pin 2?

Aaron Leonard · Guest

On 30 Nov 2005 11:43:12 -0800, "Perdition" <nhnmp@walla.co.il> wrote:

~
~ Aaron Leonard wrote:
~ > ~ I heard from a technician that the following is incorrect:
~ > ~
~ > ~ DTR is always on at the DTE. When dsr arrives from the DCE to the DTE,
~ > ~ the rts isn't sent in response.
~ > ~
~ > ~ He believes that in order to get an rts you must not only provide a DSR
~ > ~ but also transmit from TX. I believe that dsr is enough to get an rts.
~ > ~ What is the correct procedure?
~ > ~
~ > ~ Thanks alot guys :)
~ >
~ > Do you want to know what is or is not "correct", or what some actual device
~ > in fact DOES?
~ >
~ > In any case, it would be good to get clear on whether you are interested
~ > in a half duplex or full duplex device. RTS/CTS handshaking behaves very
~ > differently in the two cases.
~ >
~ > As far as DTR and DSR - my view is that they should simply denote that
~ > the respective devices (DTE and DCE) are powered on and operational.
~ > They may or may not be required for data flow depending on whether
~ > the device of interest is paying attention.
~ >
~ > DSR indicates to the DTE that connectivity to its local DCE is present,
~ > as distinguished from DCD which indicates that connectectivity to the
~ > REMOTE DCE (i.e. carrier) is present. (Having said that, I will note
~ > however that the standard for Cisco async DTEs is for DSR as such
~ > to be unused, and for DCD to be called "DSR" internally.)
~ >
~ > Cheers,
~ >
~ > Aaron
~
~ well i'm refering to a connection between an ibm computer through a
~ parallel port

"serial port", I assume ...

~ to a modem using an rs232c cable. i've learnt that the
~ DTR pin (pin 20) has voltage coming from it as of the time the computer
~ is operating. Once the dtr pin is asserted and the signal arrives at
~ the DCE, dsr should be sent in response.

The DCE sends DSR in response to it seeing DTR? OK.

~ then the computer will assert
~ rts either constantly for constant carrier

What you are calling "constant carrier" here I would call full duplex ...

~ or only when it needs to
~ transmit for switched carrier,

I would not call this "switched carrier" but half duplex.

(A Google search for "switched carrier" turned up a number of devices
that do indeed appear to use this as a synonym for half duplex, also
a Cisco page that uses this to denote transport over a switched circuit
as opposed to a leased circuit.)

~ regardless CTS would be the reply from
~ the DCE to allow transmission. At any point is pin 2 necessary in this
~ local procedure to get CTS?

By "pin 2" I assume you mean TX, i.e. data transmitted by the DTE.
If you are using half duplex CTS/RTS, then the DCE asserting CTS
does not depend on any signal on pin 2, but the other way around:
the DTE can't signal on pin 2 (i.e. transmit data) till it sees CTS
from the DCE.

~ And how is it DCD is asserted from the
~ remote station sending information if this entire procedure doesn't
~ involve pin 2?

Didn't follow that. DCD is not exactly "asserted from the remote station",
rather it is a signal asserted by a DCE to its local DTE signifying that
carrier is present, i.e. presumably that a communications channel is open
to the remote DCE/DTE.

Aaron

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