Circuit and method for automatic input-output configuration through local area network detection

Abstract: The invented circuit includes an I/O driver (14) capable of supplying current to an I/O connector (13), a current detection circuit that may include a current limit resistor (16) and a differential amplifier (20), a comparator (22) to compare the level of current to a predetermined level and to produce an output signal based thereon, and a translator (25) to convert the output signal from the comparator (22) into a transistor-transistor logic signal. The device that includes the invented circuit then interprets the logic signal as identifying either a serial I/O system or a LAN I/O system.

Description: Technical Field This invention relates to a device that may operate in either a serial input-output system or a local area network input-output system. More particularly, this invention relates to a device that automatically detects whether it is part of a serial input-output system or a local area network input-output system. Background Art A device such as a computer printer is connected to a host computer by a cable. The cable is attached at one end to a port on the printer and at the other end to a port on the computer. Both the printer and the host computer have input-output ("I/O") drivers to receive and transmit signals through the cable. When a single printer serves a single computer in a stand-alone system, the signals received and transmitted between them are often asynchronous, transmitted serially in time and transmitted only on one channel. That type of I/O is commonly known as serial I/O. Alternatively, a single printer may be part of a local area network ("LAN") and service several computers. A LAN is simply a group of interconnected devices or nodes. Devices operating in a LAN typically transmit and receive data between themselves by using LAN I/Os. LAN I/Os operate with synchronous communication and data transmission that is different and much faster than the transmission used by serial I/O systems. To be able to use a device such as a printer on both a serial I/O system and a LAN I/O system, the device must be able to distinguish between the systems because of the different data transmission characteristics. This has been accomplished in previously existing devices by using a mode switch to toggle between serial and LAN modes. Thus, to use a printer on a LAN, the user would have to configure the system by manually setting a switch. This usually involves remembering that the system needs to be configured, referring to a manual, finding the switch on the device, determining whether it needs to be changed and if so, changing it. Often the product has to be dismantled to some extent to access the switch. If the user changes back to a serial I/O system, the switch would need to be changed back. US-A-4 647 912 discloses a coupling discriminator for Ethernet standard local area network interface equipment monitors the electrical characteristics of an incoming differential signal pair, discriminates between a signal pair that is direct coupled and a pair that is transfer coupled, and configures the interface equipment for operation with the type of coupling detected. Features of the invention are defined by Claims 1 and 8 respectively. The invention can be embodied so that it automatically recognizes whether a device is connected to a serial I/O system, or to a LAN I/O system and configures the device accordingly. The user does not have to set any switches or remember to configure the system. Additionally, the cost of the components in the invention is less than the cost of a suitable switch to perform the same function. In one embodiment of the invention, there is provided an I/O driver capable of supplying current to an I/O connector, a current detection circuit that may include a current limit resistor and a differential amplifier, a comparator to compare the level of current to a predetermined level and to produce an output signal based thereon, and a translator to convert the output signal from the comparator into a transistor-transistor logic signal whereby the device interprets the logic signal to identify either a serial I/O system or a LAN I/O system. Brief Description of the Drawings Figure 1 is a schematic diagram of a preferred embodiment of the invention. Figure 2 is a simple block diagram showing the steps of the method of the preferred embodiment. Detailed Description and Best Mode for Carrying Out the Invention Figure 1 shows generally at 11 a circuit constructed according to the invention. Circuit 11 may be employed in a printer, modem or any device that may operate in either a serial I/O system or a LAN I/O system. Circuit 11 includes an I/O connector 13 to connect with other devices. Connector 13 may simply be the port on a printer that receives a cable connected to a computer, and may be referred to as I/O connector means. Circuit 11 also includes a driver means, including an I/O driver 14 that supplies the signals to connector 13 that are to be transmitted to connected devices. In Figure 1, driver 14 is a 26LS30 integrated circuit. The standard symbols explaining the driver are shown in the drawing to facilitate understanding but need not be discussed to understand the invention. Between connector 13 and driver 14 is a current detection circuit including a first current limit resistor 16 and a second resistor 18. These resistors create a voltage drop between driver 14 and connector 13. In the preferred embodiment, resistors 16 and 18 each have a resistance of 24-ohms, as shown in the schematic. When driver 14 is enabled, it may send an output signal or current to connector 13 across resistor 16. The current through resistor 16 is measured by differential amplifier 20. The output of amplifier 20 is compared with a reference signal by a comparator means including a comparator 22 and then translated into a transistor-transistor logic ("TTL") signal 26 by a translator means including a translator 25. The translator means may also be referred to as TTL circuitry. In the preferred embodiment, translator 24 is an LM339 integrated circuit. The TTL signal can then be fed into a logic input and interpreted to identify either a serial I/O system or a LAN I/O system. Figure 1 depicts a preferred embodiment of the invented circuit and shows the values of the circuit's components. The specific components are familiar to a person skilled in the art of electric current detection and therefore are not discussed in detail. Other values may also be used for the components shown in Figure 1. In fact, numerous other current detection circuits may be used to determine the current from the driver to the connector. Such current detection circuits may also be referred to as detection means. The invented circuit works because a serial I/O connection to connector 13 has different properties than a LAN I/O connection. A direct serial I/O connection has a high impedance and therefore results in a low current across resistor 16 when driver 14 is enabled. In the preferred embodiment, the current across resistor 16 generally does not exceed 3.25-mA when connector 13 is connected to a serial I/O system. A LAN I/O connection has a low impedance because it includes a transformer and therefore, a high current and large voltage drop results across resistor 16 when driver 14 is enabled. In the preferred embodiment, the current across resistor 16 is generally 30-mA to 40-mA when connector 13 is connected to a LAN I/O system. As explained, the current across resistor 16 changes depending on whether connector 13 is connected to a serial I/O or a LAN I/O. Amplifier 20 detects the current and outputs a signal to comparator 22, which then compares it with a predetermined reference level. If the reference level is met, translator 25 outputs a TTL signal that identifies the type of I/O system. For example, if connector 13 is connected to a serial I/O, then the current across resistor 16 is low and signal 26 is low or zero. If connector 13 is connected to a LAN I/O, then the current across resistor 16 is high and signal 26 is high or one. Operation The steps that are taken by circuit 11 are illustrated in Figure 2 by a block diagram shown generally at 40. Initially, driver 14 is enabled at step 42, usually by simply turning on the device that includes circuit 11 and supplying power to the driver. The invented method waits at step 44 before detecting current across resistor 16 in order to let the circuit settle. Typically, the method waits 50-µs to 100-µs. As represented by step 46, the method then measures the current, determines whether it is high or low across resistor 16 and generates the appropriate TTL signal, which may also be referred to as the LAN bit. If the current is low across resistor 16, then the method does not generate a LAN bit and the device identifies a serial I/O system and runs in serial mode, as shown by step 48. If the current is high across resistor 16, then the method does generate a LAN bit and the device identifies a LAN I/O system and runs in LAN mode, as shown by step 51. Industrial Applicability The invented circuit and method are applicable to any device that may operate in either a serial I/O system or a LAN I/O system. Specifically, the invented circuit and method are applicable to printers, modems, computers, and other similar devices. While the best mode and preferred embodiment of the invention have been disclosed, changes may be made without departing from the scope of the invention, as defined by the appended claims.

Claims: Apparatus for allowing a device to detect whether it is connected to a serial input-output system, or to a local area network input-output system, the apparatus comprising: an input-output connector (13) adapted for connection to such a system; driver means (14) operatively associated with said connector for supplying a signal to the same under circumstances where the connector (13) is connected to such a system; and detection means (16, 18, 20, 22, 25) operatively associated with said connector (13) for measuring the level of said signal supplied from said driver means and for comparing the level of the measured signal with a predetermined level in order to interpret whether the device is connected to said serial input-output system or to said local area network. The apparatus of Claim 1, wherein said detection means includes translator means for identifying said serial input-output system when the level of the measured signal does not meet the predetermined level, and for identifying said local area network system when the level of the measured signal meets the predetermined level. The apparatus of Claim 1, wherein the signal is electric current, and said detection means includes a current limit resistor (16) and a differential amplifier (20) to measure the current through said resistor (16). The apparatus of Claim 3, wherein said differential amplifier (20) produces an output signal, and said detection means further includes a comparator (22) that compares the output signal of the differential amplifier (20) with a predetermined level to produce a comparator voltage. The apparatus of Claim 4, wherein said detection means includes a translator (25) that converts the comparator voltage into transistor-transistor logic levels. The apparatus of Claim 5, wherein said translator (25) identifies a serial input-output system when the logic level is low, and identifies a local area network input-output system when the logic level is high. Apparatus according to any preceding claim when used in a printer connectable to either a serial input-output system or to a local area network input-output system. A method of allowing a device to detect whether it is connected to a serial input-output system, or to a local area network input-output system, the method comprising: supplying a signal from driver means (14) to an input-output connector (13) connected to said system; measuring the level of said signal supplied from said driver means (13) and comparing the level of the measured signal with a predetermined level in order to interpret whether the device is connected to said serial input-output system or to said local area network. A method according to Claim 8, wherein the signal is an electric current. A method according to Claim 8 or 9, wherein a comparator voltage is converted into transistor-transistor logic levels and a serial input-output system is identified when the logic level is low, and a local area network input-output system is identified when the logic level is high. A method according to any of Claims 8-10 when used with a printer connectable to either a serial input-output system or to a local area network input-output system.

Inventors: PATTON DAVID B, PATTON, DAVID B.
Applicants: HEWLETT PACKARD CO, HEWLETT-PACKARD COMPANY
Application year: 1990
Main classification: H04L12
IPC codes: A61H39, G06F3, A47G27, H04L12, H01R13, A61N5