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ML4644 Scheda tecnica(PDF) 9 Page - Micro Linear Corporation |
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ML4644 Scheda tecnica(HTML) 9 Page - Micro Linear Corporation |
9 / 11 page 9 ML4644 ML4644 ML4644 ML4642 Figure 2. Block Diagram of an Eight port Multiplexer. FUNCTIONAL DESCRIPTION Figure 1 is a block diagram of a Four Port Multiplexer using the ML4644. All AUI interfaces are transformer coupled as required in an AUI connection. RECEPTION The receive function consists of detecting data on receive differential data input pair of the MAU port (RxIN) and transmitting this data out of the RxOUT ports of all the DTE port. This data will only be passed onto the DTE port if it meets the unsquelch criteria of the AUI receiver circuit. This provision prevents any noise on the AUI cable from being misinterpreted as data and transmitted to the RxOUT pins. The receiver squelch circuit rejects signal typically with pulse width less than 20ns or a voltage level more positive than –250mV. Once the receiver is unsquelched, it remains so until reception of the idle signal which is more positive than –170mV for longer than 180ns. TRANSMISSION The transmit function consists of detecting data on any of the four differential data input pairs (TxIN1, TxIN2, TxIN3, and TxIN4) and transmitting this data out of the TxOUT pair of the MAU port. Only data that meets the unsquelch criteria of the AUI receiver circuit will be passed onto the TxOUT port. LOOPBACK If the ML4644 is connected to a MAU, or connected to other 4642/44s that are connected to a MAU, the MAU should be used to provide loopback. The loopback function of the MAU will loop data back to the RxIN port which will pass is to all FxOUTs and the REC pin will go low. In some configurations, there will be no MAU, and a ML4644 will be the last chip in the Tx path. In this case, the RxIN port will never be active, since it is not connected, and the REC pin has no function. The REC pin may be grounded and this will put the ML4644 in loopback mode, and data on the TxIN pair will be looped back to all RxOUT ports. COLLISION There are two possible collision scenarios. 1. Collision from the network connected to the MAU. 2. Collision between two or more DTEs attached to the multiplexer. In the case of a network collision, the MAU will send a collision presence signal to the multiplexer. The ML4644 will propagate this signal to the CDOUT pins of each of the DTE ports. The collision signal is a 10MHz ±0.01% signal. When a collision event occurs between two or more DTE ports, the ML4644 will send the collision presence signal to each of the DTEs via the collision ports. At the same time a 5MHz JAM signal is sent to the network. SQE TEST FUNCTION The Signal Quality Error (SQE) test function allows the DTE to determine whether or not the collision port is functional. After each transmission, during the interpacket gap time, the collision signal will be activated on the CDOUT port of the same DTE port data has been transmitted, for typically 1µs. The SQE function is not activated at the DTE ports that are in jabber state. The SQE function of each port can be disabled by tying the SQEN pin high. JABBER FUNCTION The jabber function prevents a malfunction transmitter from continuous transmission and thus loaded down the network. Within the ML4644, there is a jabber timer. The timer starts at the beginning of a received packet and resets at the end of each packet. If the packet lasts longer than 7 to 20ms the jabber circuit will disable the offending TxIN receiver (the transmission of excessively long packet is thus terminated) and generates a collision signal to the collision port of the offending DTE port. The DTE port will exit the jabber state when the transmission goes idle. LED DRIVERS The ML4644 has ten LED drivers. Each DTE port has a transmit LED and a jabber LED and the MAU port has a receive LED. Additionally there is a collision LED which indicates the presence of a collision condition. All LED drivers are active low open collector driver. All LED drivers except the jabber have pulse 1ms pulse stretchers. The pulse stretchers provide adequate on time for the LED to be visible. CASCADING THE ML4644 FOR EIGHT PORT MULTIPLEXER APPLICATION An 8 port multiplexer can be realized by using two ML4644s and one ML4642. In this configuration (see fig. 2), the SQE function of the ML4642 should be disabled to prevent false collision signalling. The SQE function in this configuration is performed by each DTE port of the ML4644 independently. Only one crystal is required, since X3 of the ML4644 with the crystal can drive X1 of the other chip. |
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