Field of the Invention
[0001] The invention relates to data processing, in particular to computer monitors for
use with personal computers. More specifically, the invention relates to an implementation
of an interface protocol for use with existing monitors which do not themselves support
that protocol.
Background of the Invention
[0002] Computer monitors in current use are connected to personal computers using an interface
cable, one end of which is usually attached to the monitor internally. The interface
cable has a connector at the other end, which is plugged into a mating connector,
usually located on the rear of the personal computer, or on an adapter card, which
is plugged into the personal computer.
[0003] Since the introduction of IBM and IBM-compatible personal computers, various different
connectors have been used to connect the interface cable to the personal computer.
In general, as each type of monitor requiring different interface signals has become
available, a corresponding new adapter card was available to work together with the
monitor as a combination. Examples of these combinations include:
The IBM Monochrome Display Adapter (MDA) and the IBM Monochrome Display, which
provided 80 columns and 25 rows of alphanumeric, monochrome text;
The IBM Colour Graphics Adapter (CGA) and the Colour Graphics Display, which also
provided graphics having a resolution of 640 dots by 200 dots; and
The IBM Enhanced Graphics Adapter (EGA) and Enhanced Colour Display, which provided
graphics having a resolution of 640 dots by 350 dots, as well as making 64 colours
available.
[0004] The EGA card could also have the Monochrome Display or Colour Graphics Display connected
to it, but relied on small switches mounted on the card itself, inside the personal
computer itself, to configure it for the different displays. A user of the personal
computer changing the type of monitor plugged into the card had to remove the cover
of the computer and change these switches.
[0005] When the IBM Video Graphics Array (VGA) and the 8514/A adapter card were introduced,
a range of monitors was offered, both monochrome and colour. One of the monitors supported
an additional mode, available only from the 8514/A adapter card. The other monitors
did not support this mode. The mechanical connections of the monitors were common,
and it was intended that any monitor should work with any adapter, without the user
needing to change any switches, or answer any questions asked by an installation program.
In addition, applications did not have to know whether a monochrome or colour monitor
was attached, the signals provided by the VGA being adapted by the personal computer
to provide readable displays on either monochrome or colour displays without user
intervention.
[0006] This was achieved by the use of ID bits in the interface connector. Inside the connector
attaching the monitor to the personal computer, the three ID pins were either open
circuit, or connected to a ground signal. A sensing circuit in the personal computer
could then determine whether a monitor was plugged in and if so, whether the monitor
was monochrome or colour, and whether it supported the higher resolution capabilities
of the 8514/A adapter card.
[0007] For example, ID bit 0 was grounded for a monochrome monitor, otherwise open circuit.
ID bit 1 was grounded for a colour monitor, otherwise open circuit. ID bit 2 was grounded
if the monitor supported the higher resolution capabilities of the 8514/A adapter
card.
[0008] Later, when the number of monitor types proliferated, a fourth ID pin was added,
and in addition to the pins being connected to a ground signal or open circuit, some
connectors had the pins internally connected to either the vertical or horizontal
synchronising signal.
[0009] With the advent of multiple mode monitors, capable of supporting more than one of
these combinations, switches on the monitor, or software to manually override the
auto-detected monitor type were required. Either of these solutions to the problem
of identification of the monitor type connected involved the manual setting of the
monitor type.
[0010] With a view to overcoming this problem, the Video Electronic Standards Association
(VESA) have defined what it calls the Data Display Channel (DDC) which has as its
purpose communicating, between the monitor and the personal computer or adapter card
to which it is attached, what modes each can support and then deciding on the optimum
mode or modes which will be used. A similar identification scheme is described in
US Patent 5,276,458.
[0011] Many users have monitors which are perfectly functional and have adequate performance
for the user. However they are unlikely to have the function required to support DDC.
So it would be advantageous to provide an addition to those monitors to allow them
to support DDC.
[0012] DDC re-assigns the pins which were previously used for monitor ID bits to provide
a serial communications link having a clock and a data signal. ID bit 3 becomes a
Clock signal, ID bit 1 becomes a Data signal. Power is provided from the personal
computer on what was previously the Keying pin. As mentioned before, existing personal
computers and adapter conforming the VGA specification use this pin as a polarising
or keying pin which is blanked off, so there is no pin to be used to provide power.
[0013] If an existing monitor is plugged into a personal computer or adapter which conforms
to the DDC specification, various signals will be short circuited.
[0014] The VESA scheme does not remove existing mechanical compatibility with the interface
cable plug and therefore it is to be expected that the above monitors will be attached
to new DDC systems. It is also reasonable to expect that when this is done, due to
the short circuits imposed, damage will result either to the monitor or the personal
computer/adapter card, or both. It would therefore be advantageous to have an interface
circuit that allowed a monitor to be plugged into a personal computer or adapter card
which would not damage that personal computer or adapter card if the adapter card
had DDC, but would nonetheless display the correct ID bits if the personal computer
or adapter card only supported ID bits.
[0015] There are a number of problems with providing this function. The interface circuit
for DDC needs to work regardless of whether the monitor is powered on. The solution
needs to be low cost, have simple logistics and minimal changes to reduce agency approval
times and costs.
[0016] Power for the interface circuit may be taken from the pin previously used for keying,
but existing systems have a polarising pin in this position which is blanked off.
[0017] Power may be obtained from the monitor, but the monitor may not be powered on when
the personal computer is powered on. The personal computer will not then receive any
response to its requests for data from the monitor via the DDC. The personal computer
will interpret this as meaning that a monitor which does not support DDC is attached.
A partial solution to this problem is to provide standby power from monitor, even
when the monitor is powered off.
[0018] A further disadvantage with this method is that if the personal computer is powered
off and on, DDC sees a 1 to 0 transition on ID bit 3, which may cause the monitor
to wait for data from the personal computer. This causes the system to hang up as
monitor awaits instructions. This situation also occurs if the personal computer turns
off power to the video circuits during power management. In summary, this solution
only works when the monitor is powered on first, the personal computer second, the
monitor is off whenever the personal computer is off, and it does not cope with personal
computers having power management functions.
Disclosure of the Invention
[0019] In accordance with the present invention, there is now provided apparatus for adding
a display data channel to a display device, the apparatus comprising: a memory for
storing display identification data; an I/O connector releasably connectable to a
display output port of a personal computer for receiving display drive signals from
the display output port; first communication means for transferring the display drive
signals to the display device; second communication means for communicating the display
identification data stored in the memory to the personal computer via the I/O connector;
and power receiving means for receiving electrical power to power the second communication
means independently of the display device.
[0020] The present invention solves the aforementioned problems associated with the prior
art by providing apparatus for introducing DDC capability to a display device. Because
electrical power is provided to the second communication means independently of the
display device, data from the memory can be read by a host personal computer via DDC
even when the display device is turned off.
[0021] Preferably, the power supply means is adapted to receive electrical power from the
personal computer via the I/O connector. It will however be appreciated that the power
supply means may alternatively comprise a battery or similar power source.
[0022] The apparatus preferably comprises a housing containing the memory. In one preferred
embodiment of the present invention, the I/O connector is remote from the housing
and connected to the memory via an intermediate cable. In another preferred embodiment
of present invention, the I/O connector is integral to the housing. It will be appreciated
that the apparatus may be in the form of a "dongle". The dongle of the present invention
advantageously enables an existing display device to be upgraded to DDC compatible
display device, simply by plugging the display device into one end of the dongle and
the personal computer into the other end.
[0023] In a particularly preferred embodiment of the present invention, the apparatus comprises
selection means for accessing different portions of the display identification data
for communication to the personal computer in dependence on one or more identification
bits (ID Bit 0-3) supplied by the display device.
[0024] The first communication means may comprise an output connector releasably connectable
to an interface cable of the display device. It will be appreciated that, in other
embodiments of the present invention, the housing may be integral to the interface
cable. This permits existing display devices to be upgraded to DDC compatible display
devices simply by exchanging the existing interface cable for a cable containing apparatus
of the present invention. The apparatus of the present invention thus has minimal
impact to modification of existing display devices. Furthermore, the apparatus of
the present invention can be made compatible with existing ID schemes and cabling.
[0025] It will be appreciated that the present invention extends to a display device comprising:
a display screen; means for generating a picture on the display screen in response
to display drive signals from a personal computer; and apparatus as described above
for connecting the display device to the personal computer.
Brief Description of the Drawings
[0026] Embodiments of the invention will now be described, by way of example only, with
reference to the accompanying drawings, in which:
Figure 1 shows a computer monitor with which the present invention may be used;
Figure 2 shows a first embodiment of the invention, the module being an extension
to a connector located at an end of an interface cable;
Figure 3 shows a second embodiment of the invention, the module being a box located
and fixed to the interface cable;
Figure 4 shows a third embodiment of the invention, the module being a separate box
interposed between the interface cable and the personal computer/adapter card;
Figure 5 is a schematic diagram of a first functional embodiment of the interface
circuit contained in the modules of either figure 1 or figure 2; and
Figure 6 is a schematic diagram of a second functional embodiment of the interface
circuit contained in the modules of either figure 1 or figure 2.
Detailed Description of the Invention
[0027] A commonly used concept when applied to serial communications links etc. is to provide
what is called a "dongle", or an interface module that is placed between two ends
of a communications link. Such a dongle may carry out the function of a translation
of dat, change in interface characteristics such as voltage levels, or may be more
intelligent and contain a small amount of processing power. In the present invention,
the dongle is used to "fool" the personal computer/adapter card into believing that
a monitor fully supporting DDC is connected to it. The monitor is "fooled" into thinking
that it is connected to an existing type of personal computer/adapter card. In this
way damage to either piece of equipment is avoided and compatibility is also achieved.
[0028] Two mechanical embodiments of the interface module will now be described. The electrical
characteristics of the circuitry contained within the interface module will be described
later with reference to figure 3.
[0029] Figure 1 shows a computer monitor 100 with which the present invention may be used.
The monitor includes an interface cable assembly 102 having at the end furthest from
the monitor a connector 104 for connection to a personal computer or adapter card.
[0030] Figure 2 shows a first embodiment of the interface module, in which the connector
104 is enlarged to form a connector assembly 202 to take a circuit card, or a flexible
circuit substrate containing the interface circuit (described later). This embodiment
results in a very compact implementation of the interface module and allows use of
existing types of bulk interface cable.
[0031] Figure 3 shows a second embodiment of the interface circuit. An interface cable assembly
300 comprises a cable 102 which is connected to the monitor at a first end. A small
box 302 containing the interface circuitry forms the module, and is mounted on the
interface cable 102, such that the interface cable 102 from the monitor enters the
box 302 at a first end 304 and exits at a second end 306. Connections are made from
the interface cable 102 to the interface circuitry contained within the box 302. This
embodiment takes up a minimum of space at the rear of the personal computer/adapter
card and allows a standard style of connector to be used to attach to the personal
computer/adapter card.
[0032] Figure 4 shows a third embodiment of the interface circuit. A separate module 400
is used and the existing display interface cable connector 104 plugs into a first
end of the module. The second end of the module plugs into the personal computer/adapter
card. The interface circuit is contained within the module. This embodiment may be
used as an upgrade to existing standard monitors without any internal modification.
[0033] Figure 5 shows a schematic diagram of an interface circuit, suitable for use between
a monitor and a personal computer/adapter card to solve this problem. The function
of two functional embodiments of the interface circuit will be described.
[0034] Video signals for the Red, Green and Blue channels 501, 502, 503, and Horizontal
synchronising signal 504 pass through the interface circuit unchanged and are not
used in any way by the interface circuit. Vertical synchronising signal 505 and the
ground signal 512 pass through the interface circuit unchanged, but are used by the
interface circuit. The test line 506, which is an input to the monitor, is grounded,
so that the monitor believes that it is connected to a personal computer/adapter and
does not operate in a test mode. The key pin 507 has no connection.
[0035] In a first embodiment of the functional aspects of the circuit, ID bits 0 through
3, (508, 509, 510, 511) from the monitor are not connected, the identification data
for the particular monitor being predetermined within the Read Only Memory (ROM) contained
in the interface circuit. Different part numbers of interface circuit are used for
monitors having different identification information.
[0036] A second functional embodiment of the functional aspects of the circuit is shown
in figure 6. ID bits 0 through 3,
from the monitor (508, 509, 510, 511) are connected to the circuit, the identification data for the
particular monitor being determined by the 4 bit ID obtained from the ID bits and
then translated into the relevant data to be sent to the personal computer/adapter
card. The 4 bit ID may be used as an offset into the data contained in the ROM, thus
determining which of the multiple sets of data stored in the ROM is communicated to
the personal computer/adapter card. The special cases of ID bits 2 and 3 having a
synchronisation signal present can be handled by a suitable monostable circuit detecting
the transitions in the sync signal and providing an additional one or more address
bits.
[0037] The Extended Display Identification Data (EDID) circuit 520 contains a ROM, which
has the information necessary to describe the monitor to the personal computer/adapter
card. The EDID requires the following signals:
o Ground
o +5v power (this is from either or both of ID bits 0 and 2, and optionally from the
monitor via the interface cable).
o ID Bit 3 from the personal computer/adapter card (used as a clock line)
o ID Bit 1 from the personal commuter/adapter card (used as a data line)
o Vertical Sync
The ground connection is used by the EDID circuit as a power and a signal ground.
Power is preferably drawn from the personal computer/adapter card via the ID bit 0
and/or the ID bit 2 connections. In the personal computer/adapter card, these are
connected through pull-up resistors, typically of value 1kOhm to a +5v supply. Diodes
530 and 532 are used to power the circuit from either or both of these ID bits. Because
of the low power consumption of the interface circuit, the voltage drop across the
1kOhm resistors does not affect operation of the circuit. Optionally, an additional,
or a replacement, power connection may be made to a supply from the monitor. Use of
ID bits 0 and 2 to supply power has the advantage that the monitor timing data will
be available even if the monitor has no power applied (or indeed not even connected!).
This is actually an advantage over a "proper" DDC implementation with the ROM and
interface circuit contained within the monitor. As mentioned above, power may optionally
be supplied from the monitor via the interface cable. Power does not need to be supplied
from the adaptor card using the means mentioned above.
[0038] In a first mode of operation, called DDC1 by VESA, the circuit continually sends
128 bytes of information, containing details about the capability of the monitor,
to the personal computer/adapter card using ID bit 1 as a data line, and the Vertical
Sync line as a clock line.
[0039] In a second mode of operation, called DDC2B by VESA, the clock and data signals are
used by circuits in the personal computer/adapter card to retrieve information from
the ROM concerning the description of modes supported by the monitor. Examples of
the mode description information can be found in table 1 below. Note that DPMS stands
for Display Power Management Signalling.
[0040] In an optional third mode of operation, called DDC2AB by VESA, there is bi-directional
communication between the personal computer/adapter card and the monitor. This allows,
for example, the personal computer keyboard to be plugged into the monitor and keystroke
data transferred from the monitor via the interface circuit. It also allows for access
by the personal computer/adapter card to controls inside the monitor for the adjustment
of, for example, image size and shape.
[0041] Either of the embodiments of the functional aspects of the circuit may be extended
to include other information from the monitor, such as its serial number, date of
manufacture etc.
1. Apparatus for adding a display data channel to a display device (100), the apparatus
comprising: a memory (520) for storing display identification data; an I/O connector
(104) releasably connectable to a display output port of a personal computer for receiving
display drive signals from the display output port; first communication means (501-505)
for transferring the display drive signals to the display device; second communication
means (Clock, Data) for communicating the display identification data stored in the
memory to the personal computer via the I/O connector; and power receiving means (+5V)
for receiving electrical power to power the second communication means independently
of the display device.
2. Apparatus as claimed in claim 1, wherein the power supply means is adapted to receive
electrical power from the personal computer via the I/O connector.
3. Apparatus as claimed in claim 1 or claim 2, comprising a housing (202;302;400) containing
the memory.
4. Apparatus as claimed in claim 3, wherein the I/O connector is remote from the housing
(302) and connected to the memory via an intermediate cable.
5. Apparatus as claimed in claim 3, wherein the I/O connector is integral to the housing
(202;400).
6. Apparatus as claimed in any preceding claim, comprising selection means (508-511)
for accessing different portions of the display identification data for communication
to the personal computer in dependence on one or more identification bits (ID Bit
0-3) supplied by the display device.
7. Apparatus as claimed in any preceding claim, wherein the first communication means
comprises an output connector releasably connectable to an interface cable (102) of
the display device.
8. Apparatus as claimed in claim 7, in the form of a dongle.
9. Apparatus substantially as hereinbefore described with reference to Figures 2, 3,
4, 5, or 6 of the accompanying drawings.
10. A display device comprising: a display screen; means for generating a picture on the
display screen in response to display drive signals from a personal computer; and
apparatus as claimed in any preceding claim for connecting the display device to the
personal computer.