Just looking quickly at the datasheet.
This is a bidirectional device. Is it possible the IOCOM_RXD signal is
forcing the input (TXD)  to ground?
What happens if the OE pin is pulled low - does the TXD signal at the
input pin now appear correctly on the scope?

RP

2009/6/4 Philip Pemberton <piclist@philpem.me.uk>:
> Hi guys,
> =A0 Has anyone here been using the the Texas Instruments TXB0104 (and oth=
er
> members of that product line, e.g. TXB0108)?
>
> =A0 I've designed one into a GPS controller board, and I'm seeing some re=
ally
> odd behaviour. The GPS chipset feeds the TXB like this:
>
> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 _________
> TXD o----[220R]------| TXB0104 |-------> IOCOM_RXD
> RXD o----[220R]------| =A0 =A0 =A0 =A0 |-------< IOCOM_TXD
> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0|_________|
>
> In this diagram, "Side A" is on the left, and powered by a National
> Semiconductor LP5951MF-1.8V (1.8V LDO). "Side B" is on the right, powered=
 by a
> USB port. Both Vcc's are bypassed with 100nF ceramics within millimetres =
of
> the chip, grounding straight to the topside ground plane. The 5V input
> additionally has a 4u7 multilayer ceramic near the input to the 3.3V volt=
age
> regulator for the GPS chipset.
>
> IOCOM_RXD and TXD go straight to an FTDI FT232R USB-to-RS232 controller
> (actually an FTDI TTL-232R cable). Drive strength on those is about 20mA,=
 and
> drive strength on the GPS side is similar.
>
> As far as I can tell, both sides are operating within spec per Vcca/Vccb,=
 but
> the TXB isn't translating properly. If I put a scope probe on TXD (before=
 the
> resistor), I can see the NMEA data blitting across the serial link. If I =
put
> it after the resistor (on the "A" side of the TXB), I see a constant low.=
 If I
> scope the output of the TXB0104 (B-side), I see what appears to be 0V/5V =
noise.
>
> AIUI, my GPS chip is driving with 20mA at 1.8V. Ohm's Law says the series
> resistor will bring that down to (worst case) 8mA. Even so, the TXB0104
> datasheet states that it should work correctly as long as the driving dev=
ice
> has a drive capacity of at least 2mA each way.... Hmm...
>
> I'd really appreciate a sanity-check here...
>
> Thanks,
> --
> Phil.
> piclist@philpem.me.uk
> http://www.philpem.me.uk/
> --
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