Method and apparatus for automatically sensing and configuring a termination in a bus-based network

A method and an apparatus for automatically sensing and configuring a termination in a network bus configuration are provided by input logic signals indicating whether or not a device connected to the network bus is located at an end position. If the device is at an end position, the network is automatically terminated by a matched load impedance located in the device. Because the load resistance at the ends of the network match the impedance of the communication medium, data loss due to signal reflections is eliminated.

BACKGROUND OF THE INVENTION 
This invention relates to local area networks and particularly to 
connections between nodes in bus-type local area networks. When signals 
are transmitted in a local area network (LAN) certain problems arise due 
to imperfect electrical termination of the wire medium used for 
transmitting the signals. In particular, data errors may result from 
signal reflections caused by a mismatch between the characteristic 
impedance of the wire communications medium and the impedance at the ends 
of the network. In order to overcome this problem, the ends of the network 
must be terminated properly in an impedance which matches the 
characteristic impedance of the medium. Proper termination may be 
accomplished by inserting a resistive impedance load having the same 
impedance as the characteristic impedance of the transmission wire at each 
end of the network. When this resistive load is driven by a reference 
voltage, it matches the characteristic impedance of the transmission wire, 
thus effectively removing reflections which may cause data loss. For 
example, if a transmission cable is used which has an impedance of 100 
.OMEGA., then a 100 .OMEGA. termination resistor may be inserted at the 
ends of the cable. 
A problem arises where an end of a LAN is moved due to the connection of 
another device to the chain of devices which form the network. A common 
way that this problem has previously been overcome has been the manual 
placement of a terminator cap on the end of the chain following the last 
device. This adds extra costs and the possibility for damage or error due 
to the need to manually identify the end of the chain. Moreover, the 
terminator caps may be inadvertently removed or disconnected, threatening 
the performance of the entire LAN. 
What is needed is a mechanism for eliminating the source of potential 
failure in a LAN connected through a wire medium. 
SUMMARY OF THE INVENTION 
According to the invention, in a local area network (LAN) having a wire 
communications medium between a plurality of devices, the wire 
communications medium having a first end and a second end each requiring 
impedance-matched termination, an apparatus in each one of the devices 
provides automatic sensing of position and insertion of a matched load for 
selective impedance-matched termination. A specific embodiment employs a 
matched load termination for the wire communications medium, means coupled 
to the matched load termination for connecting the matched load 
termination to the wire communications medium, and means coupled to the 
connecting means for enabling the connecting means where the device is 
most closely adjacent one of the first end and the second end, and for 
disabling the connecting means where the device is not most closely 
adjacent one of the first end and the second end. 
Also disclosed is a method for providing selective impedance-matched 
termination at any one of the devices of the LAN comprising steps of 
sensing at each one of the devices whether the device is most closely 
adjacent one of the ends and thereupon connecting a matched load 
termination within the device to the wire communications medium; otherwise 
isolating the matched load termination in the device from the wire 
communications medium where the device is not most closely adjacent one of 
the first end and the second end. 
In a specific embodiment, the apparatus is daisy chained, and two 
terminations are provided in each device to be connected to the LAN.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 illustrates a local area network having a wire communications medium 
10. The wire communications medium 12 may comprise a backplane, a printed 
circuited board (PCB) or a cable. In the diagram, the communications 
medium 12 has a characteristic impedance Z equal to 100 .OMEGA.. In order 
to prevent reflections, the two ends of the network are terminated with 
resistive loads 14 and 16 (R.sub.1 and R.sub.2), which match the 
characteristic impedance of the communications medium 12. The terminating 
resistive loads 14 and 16 are connected to ground 18 and 20. 
FIG. 2 illustrates a LAN 10' comprised of four devices, 100, 200, 300 and 
400 (BOX-1 through BOX-4 respectively) which incorporate the present 
invention. Each box is interconnected with a next adjacent box to form a 
network 10'. Each box is interconnected to the next box via two inputs 
TCHAIN1 (102, 202, 302, and 402) and TCHAIN2 (104, 204, 304, 404) forming 
two daisy chained control paths. These inputs indicate the presence or 
absence of a preceding or subsequent device in the network. 
The forward daisy chain begins with control input 102 of device 100 
(TCHAIN1 of BOX-1) which is pulled up to a logic high signal by a pull up 
106 (PU) located in device 100 (BOX-1). The pull up may be any pull up 
circuit or equivalent known in the art. The forward daisy chain continues 
with connection between the ground output 108 of device 100 (BOX-1) and 
control input 202 of device 200 (TCHAIN1 of BOX-2). It further continues 
with the connection between the ground output 208 of device 200 (BOX-2) 
and the control input 302 of device 300 (TCHAIN1 of BOX-3). Thus, the 
first control inputs 202 and 302 to devices 200 and 300 are both held low. 
The forward daisy chain ends with control input 402 of device 400 (TCHAIN1 
of BOX-4) which is held low by connection to ground output 308 of device 
300. Thus, the only device in which TCHAIN1 remains high is device 100 
(BOX-1), which has no preceding device in the network. 
The backward daisy chain is connected via second control inputs 104, 204, 
304 and 404 (TCHAIN2 of BOX-1 through BOX-4). The input 404 to the last 
device in the chain 400 is internally pulled high by pull up 406 (PU). 
Because device 400 (BOX-4) is the last device in the chain, there is no 
connection to input 404 which would indicate a subsequent device. 
Therefore, control input 404 remains high. However, because devices 100, 
200, and 300 (BOX-1, BOX-2 and BOX-3) are followed in the chain by a 
subsequent device, the inputs to these devices, 104, 204, and 304, are 
held low by connection to the ground outputs 210, 310, and 410 of the 
respective subsequent devices. 
In the four device network illustrated, devices 100, 200, 300 and 400 can 
be used to form a repeater 10'. According to this embodiment, each of the 
four devices is a twelve port repeater. Linked together in a hub, the 
network forms a forty-eight port repeater in which each repeater 100, 200, 
300 and 400 can communicate among each other through the connecting medium 
50. However, the present invention is not limited to use in conjunction 
with any one type of device, nor any specific number of devices. 
FIG. 3 further illustrates an embodiment in which control signals 
selectively terminate the ends of the LAN with a resistive load impedance 
located within the devices 100, 200, 300 and 400, depending on the logic 
state of their respective control inputs TCHAIN1 and TCHAIN2. In this way, 
the signal being transmitted through the LAN (illustrated at points 120, 
122, 130, 132, 140, and 142) along medium 50 is properly terminated. 
In this embodiment, because input 102 of device 100 is high, three-state 
buffer 112 coupled between the reference voltage 114 and matched load 
resistor 118 is enabled. Switch 116 then closes a path to the signal at 
point 120, and reference voltage 114 drives resistor 118 which has an 
impedance of approximately the same value as the characteristic impedance 
of the wire communications medium 50. 
Continuing forward along the daisy chain, the control input 202 of device 
200 is connected to ground output 108 of preceding device 100, and 
therefore after the signal at input 202 is inverted, a high logic signal 
is input to three-state buffer 216 which is disabled. Switch 216 is thus 
open, and the reference voltage 214 can not drive matched impedance load 
resistor 218. Accordingly, the load resistor 218 is isolated from the 
signal at point 220. The control inputs 302 and 402 of both devices 300 
and 400 similarly detect a preceding device and therefore disable buffers 
316 and 416. This removes the matched load resistors 318 and 418 from the 
signal at the points 320 and 420. 
The backward daisy chain determines whether there is a subsequent device, 
and either inserts or removes a termination in the same manner. Because 
device 100 is connected to a subsequent device 200 by control input 104 
and ground output 210, the input 104 to the three state buffer 130 is low. 
Buffer 130 inverts this signal, thereby disabling three-state buffer 126, 
and isolating matched load resistor 124 from the signal 122. The same 
occurs in devices 200 and 300. However, because there is no device 
subsequent to device 400, buffer 426 is enabled and reference voltage 428 
drives matched load resistor 424, effectively terminating the other end of 
the network. 
In this manner, a signal being transmitted through the repeater is properly 
terminated at both ends of the LAN 10'. Additional signals transmitted in 
a bus network may likewise be terminated by using apparatuses equivalent 
to that shown in FIG. 3. 
The invention has now been described with reference to specific 
embodiments. Other embodiments will be apparent to those of ordinary skill 
in the art. It is therefore not intended that this invention be limited, 
except as indicated in the appended claims.