Q10028 Mounting termination resistors on boards
My signals require a termination network. I originally planned to place the termination resistors at the connector, but with the high-density D-connectors on the ends of my termination boards, this is not a practical option. Is there a better way?
If you are using a MSXB086 4-20 mA receiver board, you do not need to worry about termination resistors. A high-precision loading and termination network is already present on the board.
If you are using MSTB009, MSTB010, or MSXB037 boards, these boards have a termination breadboarding area specifically intended for your purpose. There are through-hole pads adjacent to the connector terminals where you can mount the termination components.
The image on the left shows an MSTB009 board, illuminated to
make the traces as visible as possible, though this distorts the
apparent colors. You can see part of the labels for the signals,
in the sequence G0, S0, G1, S1, G2, S2,
etc.,
matching up with the slots in the Wago quick-connect strips. The
other board models have similar pads and traces.
The traces are easiest to see near the center, at roughly the
G2 - S2
position. For each pair of quick connect terminals, there are
three traces, each with multiple connection pads. In the center, a
trace carrying the signal from the end connector extends out to
make a T
shape, with three connected through-hole pads
below each side of the T
. Upward and left from this, a
trace extends from the G
terminal of the quick connector
to four through-hole pads. Upward and right, a trace extends up from
the S
terminal of the quick connector to another four
through-hole pads.
It is easy to see on your termination board, but hard to see in
this photo, that there is an additional trace that connects the upper
right corner of the T
to the connector pad just above it.
It is obscured in the photo because of a printed X symbol
directly on top of it. This trace completes the normal connection
between the end connector signal pin and the quick-connect terminal
for the signal. To put a component in series with the signal path, you
will need to cut away this trace at the board surface — cutting
any trace voids the warranty, but there is little risk, because the
termination boards are tough and have few sensitive electronic
components. If you later regret making this modification, it
is a simple matter to attach a small jumper wire across the pads to
restore the original connection.
Example: You have a single-ended input for a 4-20 mA current
loop. To convert this signal into a voltage range within the -5V to +5V range
limits, you need the current drive signal from the connector to pass through
a load resistor to ground. You can mount this resistor in the solder
pads on the left side. The link connecting to the input trace remains in
place, routing the signal to the S
terminal. You must have
a return path for your current loop, so you can connect this return wire
to one of the other ground-side pads on the left, and run this to the
current loop supply return point.
Example: You have a differential voltage input and you need
to give it a balanced 100K resistive loading to ground on each side as
a balanced discharge path to ground. This involves a pair of inputs. Suppose these
are signal pins marked as S2
and S3
on the
termination board. These are the same signals that will be recognized as
the differential pair D1
when you adjust your input sampling
configuration for differential operation. You mount one 100K resistor in
pads between the central signal trace and the ground trace on the left
side of S2
. Moving over one slot, you mount one 100K resistor
in pads between the central signal trace and the ground trace on the left
side of S3
.
L23955
See the MSXB037 hardware manual for more information about the layout and usage of its termination pads, with application examples.