diff --git a/dev-requirements.txt b/dev-requirements.txt
index a5adf1678..58edc2bb1 100644
--- a/dev-requirements.txt
+++ b/dev-requirements.txt
@@ -3,3 +3,4 @@ pytest==6.1.1
 pytest-mock
 hypothesis==4.28.2
 pylint==2.4.4
+pyvisa-sim
diff --git a/instruments/abstract_instruments/comm/visa_communicator.py b/instruments/abstract_instruments/comm/visa_communicator.py
index d32cbb5b8..154fde4a4 100644
--- a/instruments/abstract_instruments/comm/visa_communicator.py
+++ b/instruments/abstract_instruments/comm/visa_communicator.py
@@ -29,9 +29,6 @@ class VisaCommunicator(io.IOBase, AbstractCommunicator):
     def __init__(self, conn):
         super(VisaCommunicator, self).__init__(self)
 
-        if pyvisa is None:
-            raise ImportError("PyVISA required for accessing VISA instruments.")
-
         version = int(pyvisa.__version__.replace(".", "").ljust(3, "0"))
         # pylint: disable=no-member
         if (version < 160 and isinstance(conn, pyvisa.Instrument)) or \
@@ -118,7 +115,7 @@ def read_raw(self, size=-1):
 
         elif size == -1:
             # Read the whole contents, appending the buffer we've already read.
-            msg = self._buf + self._conn.read()
+            msg = self._buf + self._conn.read_raw()
             # Reset the contents of the buffer.
             self._buf = bytearray()
         else:
@@ -136,10 +133,10 @@ def write_raw(self, msg):
         self._conn.write_raw(msg)
 
     def seek(self, offset):  # pylint: disable=unused-argument,no-self-use
-        return NotImplemented
+        raise NotImplementedError
 
     def tell(self):  # pylint: disable=no-self-use
-        return NotImplemented
+        raise NotImplementedError
 
     def flush_input(self):
         """
diff --git a/instruments/tests/test_comm/test_visa_communicator.py b/instruments/tests/test_comm/test_visa_communicator.py
new file mode 100644
index 000000000..2bccd7f44
--- /dev/null
+++ b/instruments/tests/test_comm/test_visa_communicator.py
@@ -0,0 +1,179 @@
+#!/usr/bin/env python
+# -*- coding: utf-8 -*-
+"""
+Unit tests for the VISA communication layer
+"""
+
+# IMPORTS ####################################################################
+
+
+import pytest
+import pyvisa
+
+from instruments.units import ureg as u
+from instruments.abstract_instruments.comm import VisaCommunicator
+
+
+# TEST CASES #################################################################
+
+# pylint: disable=protected-access,redefined-outer-name
+
+# create a visa instrument
+@pytest.fixture()
+def visa_inst():
+    """Create a default visa-sim instrument and return it."""
+    inst = pyvisa.ResourceManager("@sim").open_resource("ASRL1::INSTR")
+    return inst
+
+
+def test_visacomm_init(visa_inst):
+    """Initialize visa communicator."""
+    comm = VisaCommunicator(visa_inst)
+    assert comm._conn == visa_inst
+    assert comm._terminator == "\n"
+    assert comm._buf == bytearray()
+
+
+def test_visacomm_init_wrong_type():
+    """Raise TypeError if not a VISA instrument."""
+    with pytest.raises(TypeError) as err:
+        VisaCommunicator(42)
+    err_msg = err.value.args[0]
+    assert err_msg == "VisaCommunicator must wrap a VISA Instrument."
+
+
+def test_visacomm_address(visa_inst):
+    """Get / Set instrument address."""
+    comm = VisaCommunicator(visa_inst)
+    assert comm.address == visa_inst.resource_name
+    with pytest.raises(NotImplementedError) as err:
+        comm.address = "new address"
+    err_msg = err.value.args[0]
+    assert err_msg == (
+        "Changing addresses of a VISA Instrument is not supported."
+    )
+
+
+def test_visacomm_terminator(visa_inst):
+    """Get / Set terminator."""
+    comm = VisaCommunicator(visa_inst)
+    comm.terminator = ("\r")
+    assert comm.terminator == "\r"
+
+
+def test_visacomm_terminator_not_string(visa_inst):
+    """Raise TypeError if terminator is set with non-string character."""
+    comm = VisaCommunicator(visa_inst)
+    with pytest.raises(TypeError) as err:
+        comm.terminator = 42
+    err_msg = err.value.args[0]
+    assert err_msg == (
+        "Terminator for VisaCommunicator must be specified as a single "
+        "character string."
+    )
+
+
+def test_visacomm_terminator_not_single_char(visa_inst):
+    """Raise ValueError if terminator longer than one character."""
+    comm = VisaCommunicator(visa_inst)
+    with pytest.raises(ValueError) as err:
+        comm.terminator = "\r\n"
+    err_msg = err.value.args[0]
+    assert err_msg == (
+        "Terminator for VisaCommunicator must only be 1 character long."
+    )
+
+
+def test_visacomm_timeout(visa_inst):
+    """Set / Get timeout of VISA communicator."""
+    comm = VisaCommunicator(visa_inst)
+    comm.timeout = 3
+    assert comm.timeout == u.Quantity(3, u.s)
+    comm.timeout = u.Quantity(40000, u.ms)
+    assert comm.timeout == u.Quantity(40, u.s)
+
+
+def test_visacomm_close(visa_inst, mocker):
+    """Raise an IOError if comms cannot be closed."""
+    io_error_mock = mocker.Mock()
+    io_error_mock.side_effect = IOError
+    mock_close = mocker.patch.object(visa_inst, 'close', io_error_mock)
+    comm = VisaCommunicator(visa_inst)
+    comm.close()
+    mock_close.assert_called()  # but error will just pass!
+
+
+def test_visacomm_read_raw(visa_inst, mocker):
+    """Read raw data from instrument without size specification."""
+    comm = VisaCommunicator(visa_inst)
+    mock_read_raw = mocker.patch.object(
+        visa_inst, 'read_raw', return_value=b'asdf'
+    )
+    comm.read_raw()
+    mock_read_raw.assert_called()
+    assert comm._buf == bytearray()
+
+
+def test_visacomm_read_raw_size(visa_inst, mocker):
+    """Read raw data from instrument with size specification."""
+    comm = VisaCommunicator(visa_inst)
+    size = 3
+    mock_read_bytes = mocker.patch.object(
+        visa_inst, 'read_bytes', return_value=b'123'
+    )
+    ret_val = comm.read_raw(size=size)
+    assert ret_val == b'123'
+    mock_read_bytes.assert_called()
+    assert comm._buf == bytearray()
+
+
+def test_visacomm_read_raw_wrong_size(visa_inst):
+    """Raise ValueError if size is invalid."""
+    comm = VisaCommunicator(visa_inst)
+    with pytest.raises(ValueError) as err:
+        comm.read_raw(size=-3)
+    err_msg = err.value.args[0]
+    assert err_msg == (
+        "Must read a positive value of characters, or -1 for all characters."
+    )
+
+
+def test_visacomm_write_raw(visa_inst, mocker):
+    """Write raw message to instrument."""
+    mock_write = mocker.patch.object(visa_inst, 'write_raw')
+    comm = VisaCommunicator(visa_inst)
+    msg = b'12345'
+    comm.write_raw(msg)
+    mock_write.assert_called_with(msg)
+
+
+def test_visacomm_seek_not_implemented(visa_inst):
+    """Raise NotImplementedError when calling seek."""
+    comm = VisaCommunicator(visa_inst)
+    with pytest.raises(NotImplementedError):
+        comm.seek(42)
+
+
+def test_visacomm_tell_not_implemented(visa_inst):
+    """Raise NotImplementedError when calling tell."""
+    comm = VisaCommunicator(visa_inst)
+    with pytest.raises(NotImplementedError):
+        comm.tell()
+
+
+def test_visacomm_sendcmd(visa_inst, mocker):
+    """Write to device."""
+    mock_write = mocker.patch.object(VisaCommunicator, 'write')
+    comm = VisaCommunicator(visa_inst)
+    msg = 'asdf'
+    comm._sendcmd(msg)
+    mock_write.assert_called_with(msg + comm.terminator)
+
+
+def test_visacomm_query(visa_inst, mocker):
+    """Query device."""
+    mock_query = mocker.patch.object(visa_inst, 'query')
+    comm = VisaCommunicator(visa_inst)
+    msg = 'asdf'
+    comm._query(msg)
+    mock_query.assert_called_with(msg + comm.terminator)