I only have one usb port on the device I am designing and I really need
RS232 to come out on it along with USB. I'm trying to figure out a way
to
allow this either by muxing or selectively enabling/disabling the MAX232
chip. The data is going out over a USB cable. Anyone have experience with
this?
This is the closest mux I could find, but it doesn't have have a low
enough
negative voltage throughput to allow the RS232 signal:
http://www.intersil.com/data/fn/FN6515.pdf
Is it possible run USB lines through a FET (note: I only need low-speed USB)
USB and RS232 line sharing with a mux
Started by ●June 19, 2008
Reply by ●June 19, 20082008-06-19
If I read this right, you want to use one cable for USB sometimes,
other times RS232?
The signals are so different that I would reach for a relay to do this.
Rgds,
Martin
other times RS232?
The signals are so different that I would reach for a relay to do this.
Rgds,
Martin
Reply by ●June 19, 20082008-06-19
Russ Prentice wrote:
> I only have one usb port on the device I am designing and I really need
> RS232 to come out on it along with USB. I'm trying to figure out a way to
> allow this either by muxing or selectively enabling/disabling the MAX232
> chip. The data is going out over a USB cable. Anyone have experience with
> this?
I haven't used this ... but here is a UART+USB mux from Maxim:
MAX3349EA USB 2.0 Full-Speed Transceiver with UART Multiplexing Mode
http://datasheets.maxim-ic.com/en/ds/MAX3349EA.pdf
http://www.maxim-ic.com/quick_view2.cfm/qv_pk/5354
I'm pretty sure it'll only deal with UART logic levels, not
RS-232 levels. So you'd want to use an external level
translator when working in UART mode (if you really want
RS-232 levels).
Cheers,
Dave
> I only have one usb port on the device I am designing and I really need
> RS232 to come out on it along with USB. I'm trying to figure out a way to
> allow this either by muxing or selectively enabling/disabling the MAX232
> chip. The data is going out over a USB cable. Anyone have experience with
> this?
I haven't used this ... but here is a UART+USB mux from Maxim:
MAX3349EA USB 2.0 Full-Speed Transceiver with UART Multiplexing Mode
http://datasheets.maxim-ic.com/en/ds/MAX3349EA.pdf
http://www.maxim-ic.com/quick_view2.cfm/qv_pk/5354
I'm pretty sure it'll only deal with UART logic levels, not
RS-232 levels. So you'd want to use an external level
translator when working in UART mode (if you really want
RS-232 levels).
Cheers,
Dave
Reply by ●June 19, 20082008-06-19
Yeah, I found that UART/USB mux too but, like you mentioned, it wont output the
RS232 logic levels which is what I need.
If I can just turn off the usb lines to protect them from the RS232 levels that would be ideal. Could I use a couple of FETs to do this, similar to a solid-state relay?
If I can just turn off the usb lines to protect them from the RS232 levels that would be ideal. Could I use a couple of FETs to do this, similar to a solid-state relay?
Reply by ●June 19, 20082008-06-19
Hi Russ,
> Yeah, I found that UART/USB mux too but, like you mentioned,
> it wont output the RS232 logic levels which is what I need.
Ok.
> If I can just turn off the usb lines to protect them from the
> RS232 levels that would be ideal. Could I use a couple of FETs
> to do this, similar to a solid-state relay?
Yes, I am sure you could do something like that.
However, I would be concerned that you would not be
able to detect when RS232 voltages exist on the
USB connector versus USB voltages. So when would
you enable/disable the FETs? You'd want that decision
logic to be implemented in hardware, eg. a comparator.
Starts getting complicated ...
Is this device so small that you cannot use a USB connector
for USB only, and say a micro-DB9 connector for serial?
Eg. the Model micro-D connectors look like small DB9s
Look at the right of this photo, and you'll see a micro-D
connector next to an RJ45 gigabit ethernet connector.
http://www.ovro.caltech.edu/~dwh/carma_board/carma_board.jpg
Cheers,
Dave
> Yeah, I found that UART/USB mux too but, like you mentioned,
> it wont output the RS232 logic levels which is what I need.
Ok.
> If I can just turn off the usb lines to protect them from the
> RS232 levels that would be ideal. Could I use a couple of FETs
> to do this, similar to a solid-state relay?
Yes, I am sure you could do something like that.
However, I would be concerned that you would not be
able to detect when RS232 voltages exist on the
USB connector versus USB voltages. So when would
you enable/disable the FETs? You'd want that decision
logic to be implemented in hardware, eg. a comparator.
Starts getting complicated ...
Is this device so small that you cannot use a USB connector
for USB only, and say a micro-DB9 connector for serial?
Eg. the Model micro-D connectors look like small DB9s
Look at the right of this photo, and you'll see a micro-D
connector next to an RJ45 gigabit ethernet connector.
http://www.ovro.caltech.edu/~dwh/carma_board/carma_board.jpg
Cheers,
Dave
Reply by ●June 19, 20082008-06-19
> > If I can just turn off the usb lines to protect
them from the
> > RS232 levels that would be ideal. Could I use a couple of FETs
> > to do this, similar to a solid-state relay?
>
> Yes, I am sure you could do something like that.
>
> However, I would be concerned that you would not be
> able to detect when RS232 voltages exist on the
> USB connector versus USB voltages. So when would
> you enable/disable the FETs? You'd want that decision
> logic to be implemented in hardware, eg. a comparator.
>
> Starts getting complicated ...
>
> Is this device so small that you cannot use a USB connector
> for USB only, and say a micro-DB9 connector for serial?
>
> Eg. the Model micro-D connectors look like small DB9s
> Look at the right of this photo, and you'll see a micro-D
> connector next to an RJ45 gigabit ethernet connector.
>
> http://www.ovro.caltech.edu/~dwh/carma_board/carma_board.jpg
>
> Cheers,
> Dave
>
I am not too worried about detecting the RS232 signal levels, that
will be microprocessor controlled based on looking at one of the pins
on the USB connector (ID pin will be pulled low when the RS232 is
plugged in).
Is there a specific requirement I should look for in the FETs other
than a short propogation delay (drain-source) because USB is so fast?
> > RS232 levels that would be ideal. Could I use a couple of FETs
> > to do this, similar to a solid-state relay?
>
> Yes, I am sure you could do something like that.
>
> However, I would be concerned that you would not be
> able to detect when RS232 voltages exist on the
> USB connector versus USB voltages. So when would
> you enable/disable the FETs? You'd want that decision
> logic to be implemented in hardware, eg. a comparator.
>
> Starts getting complicated ...
>
> Is this device so small that you cannot use a USB connector
> for USB only, and say a micro-DB9 connector for serial?
>
> Eg. the Model micro-D connectors look like small DB9s
> Look at the right of this photo, and you'll see a micro-D
> connector next to an RJ45 gigabit ethernet connector.
>
> http://www.ovro.caltech.edu/~dwh/carma_board/carma_board.jpg
>
> Cheers,
> Dave
>
I am not too worried about detecting the RS232 signal levels, that
will be microprocessor controlled based on looking at one of the pins
on the USB connector (ID pin will be pulled low when the RS232 is
plugged in).
Is there a specific requirement I should look for in the FETs other
than a short propogation delay (drain-source) because USB is so fast?
Reply by ●June 19, 20082008-06-19
Hi,
I am not sure what you really need but i think it may be useful for you
to have a look at the ISP1110 datasheet from NXP. It is a Universal
Receiver Transmitter. It automatically switches usb or uart mode
depending on the signal to its specific pins. I recommend you have a
look at that data sheet. I hope this helps.
Best regards,
Firat Kocak
David Hawkins wrote:
>
> Hi Russ,
>
> > Yeah, I found that UART/USB mux too but, like you mentioned,
> > it wont output the RS232 logic levels which is what I need.
>
> Ok.
>
> > If I can just turn off the usb lines to protect them from the
> > RS232 levels that would be ideal. Could I use a couple of FETs
> > to do this, similar to a solid-state relay?
>
> Yes, I am sure you could do something like that.
>
> However, I would be concerned that you would not be
> able to detect when RS232 voltages exist on the
> USB connector versus USB voltages. So when would
> you enable/disable the FETs? You'd want that decision
> logic to be implemented in hardware, eg. a comparator.
>
> Starts getting complicated ...
>
> Is this device so small that you cannot use a USB connector
> for USB only, and say a micro-DB9 connector for serial?
>
> Eg. the Model micro-D connectors look like small DB9s
> Look at the right of this photo, and you'll see a micro-D
> connector next to an RJ45 gigabit ethernet connector.
>
> http://www.ovro.caltech.edu/~dwh/carma_board/carma_board.jpg
> Cheers,
> Dave
>
>
I am not sure what you really need but i think it may be useful for you
to have a look at the ISP1110 datasheet from NXP. It is a Universal
Receiver Transmitter. It automatically switches usb or uart mode
depending on the signal to its specific pins. I recommend you have a
look at that data sheet. I hope this helps.
Best regards,
Firat Kocak
David Hawkins wrote:
>
> Hi Russ,
>
> > Yeah, I found that UART/USB mux too but, like you mentioned,
> > it wont output the RS232 logic levels which is what I need.
>
> Ok.
>
> > If I can just turn off the usb lines to protect them from the
> > RS232 levels that would be ideal. Could I use a couple of FETs
> > to do this, similar to a solid-state relay?
>
> Yes, I am sure you could do something like that.
>
> However, I would be concerned that you would not be
> able to detect when RS232 voltages exist on the
> USB connector versus USB voltages. So when would
> you enable/disable the FETs? You'd want that decision
> logic to be implemented in hardware, eg. a comparator.
>
> Starts getting complicated ...
>
> Is this device so small that you cannot use a USB connector
> for USB only, and say a micro-DB9 connector for serial?
>
> Eg. the Model micro-D connectors look like small DB9s
> Look at the right of this photo, and you'll see a micro-D
> connector next to an RJ45 gigabit ethernet connector.
>
> http://www.ovro.caltech.edu/~dwh/carma_board/carma_board.jpg
> Cheers,
> Dave
>
>
Reply by ●June 19, 20082008-06-19
> Is there a specific requirement I should look for in
the FETs other
> than a short propogation delay (drain-source) because USB is so fast?
I can't help with a discrete implementation. I'd probably
start by playing with circuits in PSPICE, or Linear
Technology's free SwitcherCAD and figure it out that way.
If you have +/-12V supplies, or the RS232 level translator's
supplies can be used, you might want to consider using
an analog multiplexer. Just make sure to use a dual-rail
one, and it'll happily pass the +/-12V from the RS-232,
or the 5V USB, and its bidirectional.
A quick search on Analog devices shows up this guy:
http://www.analog.com/en/prod/0,,768_836_ADG1209%2C00.html
something along that line might work for you.
Cheers,
Dave
> than a short propogation delay (drain-source) because USB is so fast?
I can't help with a discrete implementation. I'd probably
start by playing with circuits in PSPICE, or Linear
Technology's free SwitcherCAD and figure it out that way.
If you have +/-12V supplies, or the RS232 level translator's
supplies can be used, you might want to consider using
an analog multiplexer. Just make sure to use a dual-rail
one, and it'll happily pass the +/-12V from the RS-232,
or the 5V USB, and its bidirectional.
A quick search on Analog devices shows up this guy:
http://www.analog.com/en/prod/0,,768_836_ADG1209%2C00.html
something along that line might work for you.
Cheers,
Dave
Reply by ●June 20, 20082008-06-20
Do you want to connect the board with a DB9 connector?
If NO, I think you can implement a USB CDC device. Once it is connected to PC, the device will act as a RS232 device.
BR
Rick
----- Original Message ----
From: Russ Prentice
To: l...
Sent: Friday, June 20, 2008 12:55:18 AM
Subject: [lpc2000] USB and RS232 line sharing with a mux
I only have one usb port on the device I am designing and I really need
RS232 to come out on it along with USB. I'm trying to figure out a way to
allow this either by muxing or selectively enabling/disabling the MAX232
chip. The data is going out over a USB cable. Anyone have experience with
this?
This is the closest mux I could find, but it doesn't have have a low enough
negative voltage throughput to allow the RS232 signal:
http://www.intersil .com/data/ fn/FN6515. pdf
Is it possible run USB lines through a FET (note: I only need low-speed USB)
If NO, I think you can implement a USB CDC device. Once it is connected to PC, the device will act as a RS232 device.
BR
Rick
----- Original Message ----
From: Russ Prentice
To: l...
Sent: Friday, June 20, 2008 12:55:18 AM
Subject: [lpc2000] USB and RS232 line sharing with a mux
I only have one usb port on the device I am designing and I really need
RS232 to come out on it along with USB. I'm trying to figure out a way to
allow this either by muxing or selectively enabling/disabling the MAX232
chip. The data is going out over a USB cable. Anyone have experience with
this?
This is the closest mux I could find, but it doesn't have have a low enough
negative voltage throughput to allow the RS232 signal:
http://www.intersil .com/data/ fn/FN6515. pdf
Is it possible run USB lines through a FET (note: I only need low-speed USB)
