diff --git a/applications/test/parallel-nbx2/Make/files b/applications/test/parallel-nbx2/Make/files
new file mode 100644
index 0000000000..262936c21f
--- /dev/null
+++ b/applications/test/parallel-nbx2/Make/files
@@ -0,0 +1,3 @@
+Test-parallel-nbx2.C
+
+EXE = $(FOAM_USER_APPBIN)/Test-parallel-nbx2
diff --git a/applications/test/parallel-nbx2/Make/options b/applications/test/parallel-nbx2/Make/options
new file mode 100644
index 0000000000..18e6fe47af
--- /dev/null
+++ b/applications/test/parallel-nbx2/Make/options
@@ -0,0 +1,2 @@
+/* EXE_INC = */
+/* EXE_LIBS = */
diff --git a/applications/test/parallel-nbx2/Test-parallel-nbx2.C b/applications/test/parallel-nbx2/Test-parallel-nbx2.C
new file mode 100644
index 0000000000..27bb661e0d
--- /dev/null
+++ b/applications/test/parallel-nbx2/Test-parallel-nbx2.C
@@ -0,0 +1,227 @@
+/*---------------------------------------------------------------------------*\
+  =========                 |
+  \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
+   \\    /   O peration     |
+    \\  /    A nd           | www.openfoam.com
+     \\/     M anipulation  |
+-------------------------------------------------------------------------------
+    Copyright (C) 2023 OpenCFD Ltd.
+-------------------------------------------------------------------------------
+License
+    This file is part of OpenFOAM.
+
+    OpenFOAM is free software: you can redistribute it and/or modify it
+    under the terms of the GNU General Public License as published by
+    the Free Software Foundation, either version 3 of the License, or
+    (at your option) any later version.
+
+    OpenFOAM is distributed in the hope that it will be useful, but WITHOUT
+    ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+    FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+    for more details.
+
+    You should have received a copy of the GNU General Public License
+    along with OpenFOAM.  If not, see <http://www.gnu.org/licenses/>.
+
+Application
+    Test-parallel-nbx2
+
+Description
+    Test for send/receive data
+
+\*---------------------------------------------------------------------------*/
+
+#include "List.H"
+#include "argList.H"
+#include "Time.H"
+#include "IPstream.H"
+#include "OPstream.H"
+#include "IOstreams.H"
+#include "Random.H"
+
+using namespace Foam;
+
+// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
+
+int main(int argc, char *argv[])
+{
+    argList::noCheckProcessorDirectories();
+    argList::addBoolOption("non-blocking", "Test with non-blocking receives");
+
+    #include "setRootCase.H"
+
+    const bool optNonBlocking = args.found("non-blocking");
+
+    if (!Pstream::parRun())
+    {
+        Info<< "\nWarning: not parallel - skipping further tests\n" << endl;
+        return 0;
+    }
+
+    Info<< "\nTesting with non-blocking receives: " << optNonBlocking << nl;
+
+
+    const int tag = (UPstream::msgType() + 314159);
+    const label comm = UPstream::worldComm;
+
+    Random rnd(20*UPstream::myProcNo());
+
+    // Looks a bit like a DIY PstreamBuffers...
+    Map<DynamicList<char>> sendBufs;
+    Map<DynamicList<char>> recvBufs;
+
+    DynamicList<UPstream::Request> sendRequests(10);
+    DynamicList<UPstream::Request> recvRequests(10);
+
+    if (!Pstream::master())
+    {
+        // Send some random length to master
+
+        const int toProci = UPstream::masterNo();
+
+        label len = rnd.position<label>(10, 20);
+        if (UPstream::myProcNo() && (UPstream::myProcNo() % 3) == 0) len = 0;
+
+        scalarField fld(len, scalar(UPstream::myProcNo()));
+
+        // Format for sending
+        if (!fld.empty())
+        {
+            auto& buf = sendBufs(toProci);
+            UOPstream os(buf);
+            os << fld;
+        }
+
+        // Start nonblocking synchronous send to process dest
+
+        if (sendBufs.found(toProci) && !sendBufs[toProci].empty())
+        {
+            Pout<< "send: [" << sendBufs[toProci].size() << " bytes] "
+                << flatOutput(fld) << endl;
+
+            // Has data to send
+            UOPstream::write
+            (
+                sendRequests.emplace_back(),
+                UPstream::masterNo(),
+                sendBufs[toProci],
+                tag,
+                comm,
+                UPstream::sendModes::sync
+            );
+        }
+    }
+
+
+    // Probe and receive
+
+    UPstream::Request barrierReq;
+
+    for (bool barrier_active = false, done = false; !done; /*nil*/)
+    {
+        std::pair<int, int> probed =
+            UPstream::probeMessage
+            (
+                UPstream::commsTypes::nonBlocking,
+                -1,  // ANY_SOURCE
+                tag,
+                comm
+            );
+
+        if (probed.second > 0)
+        {
+            // Message found and had size: receive it
+
+            const label proci = probed.first;
+            const label count = probed.second;
+
+            if (optNonBlocking)
+            {
+                recvBufs(proci).resize_nocopy(count);
+
+                // Non-blocking read
+                UIPstream::read
+                (
+                    recvRequests.emplace_back(),
+                    proci,
+                    recvBufs[proci],
+                    tag,
+                    comm
+                );
+                // Pout<< "Done: "
+                //     << UPstream::finishedRequests(recvRequests) << endl;
+            }
+            else
+            {
+                IPstream is
+                (
+                    UPstream::commsTypes::scheduled,
+                    probed.first,
+                    probed.second,
+                    tag,
+                    comm
+                );
+
+                scalarField fld(is);
+
+                Info<< "from [" << probed.first
+                    << "] : " << flatOutput(fld) << endl;
+            }
+        }
+
+        if (barrier_active)
+        {
+            // Test barrier for completion
+            if (UPstream::finishedRequest(barrierReq))
+            {
+                done = true;
+            }
+        }
+        else
+        {
+            // Check if all sends have arrived
+            if (UPstream::finishedRequests(sendRequests))
+            {
+                UPstream::barrier(comm, &barrierReq);
+                barrier_active = true;
+            }
+        }
+    }
+
+    Pout<< "pending receives: " << recvRequests.size() << endl;
+
+    // Wait for receives to complete
+    UPstream::waitRequests(recvRequests);
+
+    // It could be we need this type of synchronization point
+    // if the receives are non-blocking
+    if (optNonBlocking)
+    {
+        UPstream::barrier(comm);
+    }
+
+    if (!recvBufs.empty())
+    {
+        Pout<< "Receives from: " << flatOutput(recvBufs.sortedToc()) << endl;
+
+        forAllConstIters(recvBufs, iter)
+        {
+            Pout<< "proc:" << iter.key() << " len:" << iter.val().size() << nl;
+
+            if (!iter.val().empty())
+            {
+                UIPstream is(iter.val());
+                scalarField fld(is);
+
+                Pout<< "recv:" << iter.key()
+                    << " : " << flatOutput(fld) << nl;
+            }
+        }
+    }
+
+    Info<< "\nEnd\n" << endl;
+    return 0;
+}
+
+
+// ************************************************************************* //
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/IPBstreams.C b/src/OpenFOAM/db/IOstreams/Pstreams/IPBstreams.C
index 5540777f09..a43f73116e 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/IPBstreams.C
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/IPBstreams.C
@@ -5,7 +5,7 @@
     \\  /    A nd           | www.openfoam.com
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
-    Copyright (C) 2022 OpenCFD Ltd.
+    Copyright (C) 2022-2023 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -75,13 +75,12 @@ Foam::IPBstream::IPBstream
         commsType,
         fromProcNo,
         Pstream::transferBuf_,
-        transferBufPosition_,
+        UIPstreamBase::storedRecvBufPos_,   // Internal only
         tag,
         comm,
         false,  // Do not clear Pstream::transferBuf_ if at end
         fmt
-    ),
-    transferBufPosition_(0)
+    )
 {}
 
 
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/IPstream.H b/src/OpenFOAM/db/IOstreams/Pstreams/IPstream.H
index 43152570b1..5dda9b6e6e 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/IPstream.H
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/IPstream.H
@@ -56,11 +56,6 @@ class IPstream
     public Pstream,
     public UIPstream
 {
-    // Private Data
-
-        //- Receive index into Pstream::transferBuf_
-        label transferBufPosition_;
-
 public:
 
     // Constructors
@@ -90,11 +85,6 @@ class IPBstream
     public Pstream,
     public UIPBstream
 {
-    // Private Data
-
-        //- Receive index into Pstream::transferBuf_
-        label transferBufPosition_;
-
 public:
 
     // Constructors
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/IPstreams.C b/src/OpenFOAM/db/IOstreams/Pstreams/IPstreams.C
index caabf931d7..d82a270238 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/IPstreams.C
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/IPstreams.C
@@ -5,7 +5,7 @@
     \\  /    A nd           | www.openfoam.com
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
-    Copyright (C) 2022 OpenCFD Ltd.
+    Copyright (C) 2022-2023 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -91,6 +91,16 @@ Foam::UIPstream::UIPstream(const int fromProcNo, PstreamBuffers& buffers)
 }
 
 
+Foam::UIPstream::UIPstream
+(
+    const DynamicList<char>& recvBuf,
+    IOstreamOption::streamFormat fmt
+)
+:
+    UIPstreamBase(recvBuf, fmt)
+{}
+
+
 Foam::IPstream::IPstream
 (
     const UPstream::commsTypes commsType,
@@ -107,13 +117,12 @@ Foam::IPstream::IPstream
         commsType,
         fromProcNo,
         Pstream::transferBuf_,
-        transferBufPosition_,
+        UIPstreamBase::storedRecvBufPos_,   // Internal only
         tag,
         comm,
         false,  // Do not clear Pstream::transferBuf_ if at end
         fmt
-    ),
-    transferBufPosition_(0)
+    )
 {}
 
 
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/OPstreams.C b/src/OpenFOAM/db/IOstreams/Pstreams/OPstreams.C
index 9f1d70f42f..fa913a868c 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/OPstreams.C
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/OPstreams.C
@@ -52,6 +52,16 @@ Foam::UOPstream::UOPstream(const int toProcNo, PstreamBuffers& buffers)
 {}
 
 
+Foam::UOPstream::UOPstream
+(
+    DynamicList<char>& sendBuf,
+    IOstreamOption::streamFormat fmt
+)
+:
+    UOPstreamBase(sendBuf, fmt)
+{}
+
+
 Foam::OPstream::OPstream
 (
     const UPstream::commsTypes commsType,
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/UIPstream.H b/src/OpenFOAM/db/IOstreams/Pstreams/UIPstream.H
index bcb989cf3a..68e3f26bdc 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/UIPstream.H
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/UIPstream.H
@@ -85,19 +85,30 @@ protected:
 
     // Protected Data
 
-        int fromProcNo_;
-
-        DynamicList<char>& recvBuf_;
-
-        label& recvBufPos_;
+        //- Source rank for the data
+        const int fromProcNo_;
 
+        //- Message tag for communication
         const int tag_;
 
-        const label comm_;
+        //- The communicator index
+        const int comm_;
 
+        //- The message size, read on bufferIPCrecv or set directly
+        int messageSize_;
+
+        //- Receive position in buffer data, if ony
+        //- If there is no external location for recvBufPos_
+        label storedRecvBufPos_;
+
+        //- Clear the receive buffer on termination (in the destructor)
         const bool clearAtEnd_;
 
-        int messageSize_;
+        //- Reference to the receive buffer data
+        DynamicList<char>& recvBuf_;
+
+        //- Reference to the receive position in buffer data
+        label& recvBufPos_;
 
 
     // Protected Constructors
@@ -120,10 +131,17 @@ protected:
         //- Construct given buffers
         UIPstreamBase(const int fromProcNo, PstreamBuffers& buffers);
 
+        //- Construct for an externally obtained buffer.
+        //  The parameter is allowed to be const (since reading will not
+        //  affect it), but must reference a concrete variable.
+        UIPstreamBase
+        (
+            const DynamicList<char>& receiveBuf,
+            IOstreamOption::streamFormat fmt
+        );
 
 public:
 
-
     //- Destructor. Optionally clears external receive buffer.
     virtual ~UIPstreamBase();
 
@@ -238,6 +256,16 @@ public:
         //- Construct given buffers
         UIPstream(const int fromProcNo, PstreamBuffers& buffers);
 
+        //- Construct for reading from a standalone buffer that has
+        //- been obtained externally by the caller.
+        //  The parameter is allowed to be const (since reading will not
+        //  affect it), but must reference a concrete variable.
+        explicit UIPstream
+        (
+            const DynamicList<char>& recvBuf,
+            IOstreamOption::streamFormat fmt = IOstreamOption::BINARY
+        );
+
 
     //- Destructor
     virtual ~UIPstream() = default;
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/UIPstreamBase.C b/src/OpenFOAM/db/IOstreams/Pstreams/UIPstreamBase.C
index 228332528f..97685b3022 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/UIPstreamBase.C
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/UIPstreamBase.C
@@ -159,12 +159,13 @@ Foam::UIPstreamBase::UIPstreamBase
     UPstream(commsType),
     Istream(fmt),
     fromProcNo_(fromProcNo),
-    recvBuf_(receiveBuf),
-    recvBufPos_(receiveBufPosition),
     tag_(tag),
     comm_(comm),
+    messageSize_(0),
+    storedRecvBufPos_(0),
     clearAtEnd_(clearAtEnd),
-    messageSize_(0)
+    recvBuf_(receiveBuf),
+    recvBufPos_(receiveBufPosition)
 {
     setOpened();
     setGood();
@@ -180,12 +181,13 @@ Foam::UIPstreamBase::UIPstreamBase
     UPstream(buffers.commsType()),
     Istream(buffers.format()),
     fromProcNo_(fromProcNo),
-    recvBuf_(buffers.accessRecvBuffer(fromProcNo)),
-    recvBufPos_(buffers.accessRecvPosition(fromProcNo)),
     tag_(buffers.tag()),
     comm_(buffers.comm()),
+    messageSize_(0),
+    storedRecvBufPos_(0),
     clearAtEnd_(buffers.allowClearRecv()),
-    messageSize_(0)
+    recvBuf_(buffers.accessRecvBuffer(fromProcNo)),
+    recvBufPos_(buffers.accessRecvPosition(fromProcNo))
 {
     if
     (
@@ -205,6 +207,32 @@ Foam::UIPstreamBase::UIPstreamBase
 }
 
 
+Foam::UIPstreamBase::UIPstreamBase
+(
+    const DynamicList<char>& receiveBuf,
+    IOstreamOption::streamFormat fmt
+)
+:
+    UPstream(UPstream::commsTypes::nonBlocking), // placeholder
+    Istream(fmt),
+    fromProcNo_(UPstream::masterNo()),      // placeholder
+    tag_(UPstream::msgType()),              // placeholder
+    comm_(UPstream::selfComm),              // placeholder
+    messageSize_(receiveBuf.size()),        // Message == buffer
+    storedRecvBufPos_(0),
+    clearAtEnd_(false),   // Do not clear recvBuf if at end!!
+    recvBuf_
+    (
+        // The receive buffer is never modified with this code path
+        const_cast<DynamicList<char>&>(receiveBuf)
+    ),
+    recvBufPos_(storedRecvBufPos_)          // Internal reference
+{
+    setOpened();
+    setGood();
+}
+
+
 // * * * * * * * * * * * * * * * * Destructor  * * * * * * * * * * * * * * * //
 
 Foam::UIPstreamBase::~UIPstreamBase()
@@ -517,8 +545,7 @@ void Foam::UIPstreamBase::print(Ostream& os) const
 {
     os  << "Reading from processor " << fromProcNo_
         << " using communicator " << comm_
-        <<  " and tag " << tag_
-        << Foam::endl;
+        <<  " and tag " << tag_ << Foam::endl;
 }
 
 
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/UOPstream.H b/src/OpenFOAM/db/IOstreams/Pstreams/UOPstream.H
index a3f86d3281..02f6f85037 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/UOPstream.H
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/UOPstream.H
@@ -92,16 +92,21 @@ protected:
 
     // Protected Data
 
-        int toProcNo_;
-
-        DynamicList<char>& sendBuf_;
+        //- Destination rank for the data
+        const int toProcNo_;
 
+        //- Message tag for communication
         const int tag_;
 
-        const label comm_;
+        //- The communicator index
+        const int comm_;
 
+        //- Call bufferIPCsend on termination (in the destructor)
         const bool sendAtDestruct_;
 
+        //- Reference to the send buffer data
+        DynamicList<char>& sendBuf_;
+
 
     // Protected Constructors
 
@@ -122,6 +127,12 @@ protected:
         //- Construct given buffers
         UOPstreamBase(const int toProcNo, PstreamBuffers& buffers);
 
+        //- Construct for externally obtained buffers
+        UOPstreamBase
+        (
+            DynamicList<char>& sendBuf,
+            IOstreamOption::streamFormat fmt
+        );
 
 public:
 
@@ -310,6 +321,14 @@ public:
         //- Construct given buffers
         UOPstream(const int toProcNo, PstreamBuffers& buffers);
 
+        //- Construct for writing into a standalone buffer.
+        //- Data transfer is handled externally by the caller.
+        explicit UOPstream
+        (
+            DynamicList<char>& sendBuf,
+            IOstreamOption::streamFormat fmt = IOstreamOption::BINARY
+        );
+
 
     //- Destructor, usually sends buffer on destruct.
     virtual ~UOPstream();
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/UOPstreamBase.C b/src/OpenFOAM/db/IOstreams/Pstreams/UOPstreamBase.C
index 73d4c83939..b865858bad 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/UOPstreamBase.C
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/UOPstreamBase.C
@@ -145,10 +145,10 @@ Foam::UOPstreamBase::UOPstreamBase
     UPstream(commsType),
     Ostream(fmt),
     toProcNo_(toProcNo),
-    sendBuf_(sendBuf),
     tag_(tag),
     comm_(comm),
-    sendAtDestruct_(sendAtDestruct)
+    sendAtDestruct_(sendAtDestruct),
+    sendBuf_(sendBuf)
 {
     setOpened();
     setGood();
@@ -160,16 +160,36 @@ Foam::UOPstreamBase::UOPstreamBase(const int toProcNo, PstreamBuffers& buffers)
     UPstream(buffers.commsType()),
     Ostream(buffers.format()),
     toProcNo_(toProcNo),
-    sendBuf_(buffers.accessSendBuffer(toProcNo)),
     tag_(buffers.tag()),
     comm_(buffers.comm()),
-    sendAtDestruct_(buffers.commsType() != UPstream::commsTypes::nonBlocking)
+    sendAtDestruct_(buffers.commsType() != UPstream::commsTypes::nonBlocking),
+    sendBuf_(buffers.accessSendBuffer(toProcNo))
 {
     setOpened();
     setGood();
 }
 
 
+Foam::UOPstreamBase::UOPstreamBase
+(
+    DynamicList<char>& sendBuf,
+    IOstreamOption::streamFormat fmt
+)
+:
+    UPstream(UPstream::commsTypes::nonBlocking), // placeholder
+    Ostream(fmt),
+    toProcNo_(UPstream::masterNo()),        // placeholder
+    tag_(UPstream::msgType()),              // placeholder
+    comm_(UPstream::selfComm),              // placeholder
+    sendAtDestruct_(false),   // Never sendAtDestruct!!
+    sendBuf_(sendBuf)
+{
+    sendBuf_.clear();  // Overwrite into buffer
+    setOpened();
+    setGood();
+}
+
+
 // * * * * * * * * * * * * * * * * Destructor  * * * * * * * * * * * * * * * //
 
 Foam::UOPstreamBase::~UOPstreamBase()
@@ -394,8 +414,8 @@ void Foam::UOPstreamBase::rewind()
 
 void Foam::UOPstreamBase::print(Ostream& os) const
 {
-    os  << "Writing from processor " << toProcNo_
-        << " to processor " << myProcNo() << " in communicator " << comm_
+    os  << "Writing to processor " << toProcNo_
+        << " from processor " << myProcNo() << " in communicator " << comm_
         << " and tag " << tag_ << Foam::endl;
 }