Hi Art,
RS485 is generally not directly supported by the uart. The UART can be set to
send and receive with our without flow control, but the HW flow control logic
is typically used in RS485 circuits. Do you know that the RS232 circuit is
supposed to have a level converter/driver? That driver is connected onto the
input/output of the UART between the DB9 and the UART. For RS485, one of the
UART's output signals is connected to an enable line of the driver (this enable
line is not needed for RS232). Uarts that have hardware flow control will
assert the RTS signal when there is a character to send and wait for the
receiver to assert their CTS signal (which is connected to your RTS) before
sending. When the recieve buffer gets filled up, the receiver will not assert
CTS even if the RTS is asserted. This is a purely hardware function, so you
will not have any control over this besides just setting it up.
For RS485, the RTS line from your UARTs trasmit logic is often just wired to
the line driver enable. This means that the RS485 driver is on as long as your
UART wants to send stuff. If your system works like this and you disable HW
flow control, then the RTS line will either be always on or always off. Either
nothing gets out or nothing gets in.
There is a slight chance that a different control line runs the driver, but
this would require software support in the serial driver, so you would probably
know about that from the card manufacturer already.
I think you want the HW flow control on.
Mike
-----Original Message-----
>From: Art <>
>Sent: Aug 23, 2006 5:02 PM
>To:
>Subject: [ts-7000] Re: rs232 vs rs485
>
>Ok,
>
>That means that the Com2 may be locked into a transmit mode until It
>gets put into receive?. This makes sence but I do not set the
>transmition state and I thought it would default to the RS232
>functions.
>
>--- In Michael L Howard <> wrote:
>>
>> Hi Art,
>>
>> RS422 and RS485 are multi-drop serial buses. This means that
>multiple senders could conceivably be transmitting characters at the
>same time. In practice, the devices take turns transmitting, but
>this requires that each device must be able to shut off its' driver
>circuitry when idle. I believe that it is typical to use the RTS
>output of the handshake circuit to turn the transmit driver off and
>on automatically.
>>
>> The RS485 line will have some way to control the enable/disable of
>the driver circuit.
>>
>> Mike
>>
>>
>> -----Original Message-----
>> >From: Art <>
>> >Sent: Aug 23, 2006 12:47 PM
>> >To:
>> >Subject: [ts-7000] Re: rs232 vs rs485
>> >
>> >Yan
>> >
>> >Thanks for the quick responce. The question I was asking was
>intent
>> >on getting some information on a com port that has the ability to
>run
>> >rs485 vs a com port that can just run rs232. I have a TS-7200
>with
>> >rs485 capable port. I have it setup to run rs232. I am running
>> >rs232 on the other end. I have hardware and software flow control
>> >disabled on both ends. Yet it seems to stop sending/receving.
>But
>> >what I was asking is for the diffrences in hardware on the ts-7200
>> >for just rs232 vs (rs232 and rs485) on a com port. I get the same
>> >software to run fine on COM1 vs COM2(get's stuck).
>> >
>> >--- In "Yan Seiner" <yan@> wrote:
>> >>
>> >> --- In "Art" <kingartjr@> wrote:
>> >> >
>> >> > Is there any diffrences in a rs485 configured port that would
>> >prevent
>> >> > it from functioning as a standard rs232 port? I then ask what
>> >are the
>> >> > diffrences? it seems event when I turn the flow control off on
>> >this
>> >> > port it still functions as if it were in place. Getty doesn't
>> >seem to
>> >> > work well on com2.
>> >> >
>> >> rs232c and rs485 specify different electrical standards. I am
>not
>> >> sure what you are asking.
>> >>
>> >> Typically rs485 is run on 2 or 4 wires, with no flow control
>lines.
>> >> I'm not that familiar with the 4 wire setup; I've only used
>> >> half-duplex (2 wire). Each station turns its transmitter on
>when it
>> >> wants to say something; the other stations are supposed to
>respect
>> >> that and not transmit.
>> >>
>> >> RS232c is point-to-point, and provides for all sorts of
>redundancy
>> >> (the full spec has secondary RX and TX lines, even...) There are
>> >> liens that each station is supposed to turn on and off when it is
>> >> ready. There are also signal lines for special events like
>carrier
>> >> detect.
>> >>
>> >> Basically, RS232c was designed to talk short distances to a
>modem,
>> >an
>> >> RS485 was designed to talk long distances to multiple peers.
>Also,
>> >> an RS485 bus is (I believe) a voltage differential setup, whereas
>> >> RS232 is referenced to 0, but I could be getting confused with
>> >another
>> >> bus I am working with...
>> >>
>> >> So when you switch a port from rs232 to rs485, you lose all
>hardware
>> >> flow control, since there is none.
>> >>
>> >> --Yan
>> >>
>> >
>> >
>> >
>> >
>> >
>>
>
>
>
>
>
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