Low level serial transceiver

A transceiver has a transmitter portion using serially coupled FETs coupled o low level positive and negative bias sources to enable a low power drain conversion of TTL input signals to low level serial data output signals for a coaxial data transmission cable and vice versa The transceiver also provides a constant 50 ohm impedance for the coaxial or triaxial transmission cable during active transmission periods, stand-by periods and power-off periods to provide an inexpensive method to transmit and receive 10 Megabit coded data via a coaxial or triaxial cable.

BACKGROUND OF THE INVENTION 
Some transceiver circuits which are coupled to a coaxial cable have the 
requirement to provide a constant impedance for the cable at all time to 
maintain the integrity of the communication system. In addition many times 
a transceiver of this type is remotely deployed for data gathering and is 
unattended for long periods of time and, as a consequence, must have a low 
power consumption. It is not uncommon that such a transceiver is one of a 
number of like transceivers in a interrelated network which is designed to 
share the transceiver function with a host of other related electronic 
functions. A design obstacle that frequently arises is that the various 
electronic functions might be provided in compact integrated circuits or 
microminiaturized packages where the heat dissipation problems associated 
with undue, high power consumption can and do impose severe operational 
limitations. 
A typical, contemporary transceiver operating in the standby mode sinks 110 
milliamps from its positive power supply and about 88 milliamps from its 
negative power supply. Some applications require as 45 amps from their 
negative supplies for just the transceivers This increased power 
consumption levels are largely attributed to the design complexity of the 
contemporary transceivers which also is reflected in a considerably 
increased cost per unit. 
Thus, there is a continuing need in the state of the art for a transceiver 
improvement that has a low power consumption presenting substantially the 
same impedance to a coaxial cable during the active and standby modes of 
the transceiver. 
SUMMARY OF THE INVENTION 
The present invention is directed to providing an improved low power level 
serial transceiver that provides a substantially constant characteristic 
impedance at all time. A low level serial transmitter portion and a 
receiver portion are coupled to the cable by a transformer to provide DC 
isolation. A FET switch coupled to the transformer provides line impedance 
maintained at 50 ohms when the transceiver is powered off ( i.e. no power 
to the transceiver). Thus, reflection of input signals due to mismatch in 
impedance will be eliminated during power off periods. The transmitter 
portion receives two separate TTL data signals and has a pair of serially 
arranged FETs converting the two signals to tri-level signals during the 
23 active mode. The tri-level signals are amplified by a fast op-amp to 
drive a 50 ohm load of the cable. When the transmitter is active or 
transmitting data, the receiver portion of the transceiver is disabled. 
This is done by a TTL controlled signal that disables comparators of the 
receiver portion. When the transmitter is not in the active mode (not 
transmitting data), the transceiver is in the stand-by mode and the 
receiver can receive data or be awaiting data. During power off periods, 
the transceiver has the same impedance as during the active and stand-by 
modes. 
An object of the invention is to provide a more power efficient serial 
transceiver. 
Another object is to provide a power efficient serial transceiver that is 
more cost-effective than contemporary designs. 
Yet another object is to provide a serial transceiver providing a 
relatively constant line impedance at all time to an interconnected 
coaxial or triaxial cable. 
These and other objects of the invention will become more readily apparent 
from the ensuing specification and claims when taken in conjunction with 
the appended drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to FIG. 1 of the drawings, low level serial transceiver 10 
includes a transmitter portion 11 and a receiver portion 12 coupled via an 
isolator portion 13 (coupling transformer) to a triaxial or coaxial data 
transmission cable 14. The cable extends to remote portions of a 
communication system and may be interconnected to a number of low level 
serial transceivers fabricated in accordance with this inventive concept 
to provide an inexpensive method of utilizing, for example, coaxial or 
triaxial cable to transmit and receive 10 Megabit Manchester coded data. 
The cable joined to an interconnected system having, for example, a number 
of transceivers, requires a constant line impedance at each transceiver. 
Accordingly, it has been a main consideration to assure that a constant 
characteristic impedance is presented for cable 14 while each transceiver 
10 is operating in the active mode as it is transmitting standby mode as 
it is receiving or awaiting the next message or when the transceiver is 
powered off. An impedance of 50 ohms+or-2 ohms at the isolator portion is 
measured from 10 MHz to 80 MHz to assure that no more than 10% of a 10 MHz 
input signal is reflected, see FIGS. 2 and 3. 
When the active mode (transmitting mode) is invoked, TTL to LLS (low level 
serial) converter 15 is actuated and FET switch 17 also is actuated to 
turn off by the application of the -5.0 power to the gate of FET Q3. 
Simultaneously, while in the active or transmitting mode, Q1 and Q2 of 
converter 15 encode the two TTL inputs TXDATN and TXDAT to a three-level 
signal of +v, -v and ground. This three-level signal is amplified by fast 
operational amplifier U1 of coaxial cable driver 16 to drive a 50-ohm 
impedance load R.sub.L which is associated with the distal end of cable 
14. 
FET Q3 of switch 17 is uniquely fabricated to function as an impedance 
matcher between driver circuit 16 and load R.sub.L in a manner to maintain 
a unique feature of this transceiver, namely, that its impedance maintains 
a 50 ohm magnitude in either the active mode (while transmitter portion 15 
is transmitting), the stand-by mode (during receiving by receiver portion 
12 or awaiting data), or is in a power-off mode (no power to the 
transceiver). When transmitter portion 11 is power on (transceiver 10 is 
in the active mode), the negative 5 volt to the gate of FET Q3 will turn 
off FET Q3 and make switch 17 act like an open circuit. In the stand-by 
mode, resistors R8 and R20 form the impedance of the transceiver and will 
make the transceiver appear to be 50 ohm. When there is no power to the 
transceiver, the Q3 transistor will be turned on because the absence of 
the negative 5 volts to the gate of this FET Q3 transistor. When this 
transistor turns on, there is a resistance of 120 ohm across to ground. 
The high impedance of the driver (Ul) (when there is no power) will 
present an open circuit at that point to the interface so the resistance 
to the transistor and R20 will be a component that makes the transceiver 
appear to be 50 ohm. In this manner, low level transceiver 10 maintains 50 
ohms impedance looking into the transceiver from the interconnecting end 
of cable 14 while in the active mode (while transmitter portion 15 is 
transmitting), the stand-by mode (during receiving by receiver portion 12 
or awaiting data), or is in a power-off mode (no power to the 
transceiver). 
When transmitting or operating in the active mode (transmitter portion 11 
transmitting) data will not be echoed back into receiver portion 12 since 
an RXSTROB signal circuit 18 applies an RXSTROB signal to receiver portion 
12 during transmission. The RXSTROB signal acts as a control signal to 
disable a comparator U.sub.2 circuit in the receiver portion. In the 
receiving mode or the standby mode, the RXSTROB signal is absent so that 
low level serial (LLS) data signal coming in over cable 14 is fed to a 
dual comparator circuit U.sub.2 whose outputs are TTL signals RXDAT and 
RXDATN. 
During the active mode, that is when converter 15 is activated by applying 
the +5 volts and -5 volts across the serially connected FETs Q1 and Q2, an 
adequate and appropriate power low level power is provided for a suitable 
data transmission. During the stand-by mode of operation, that is when 
converter 15 is not transmitting, the transceiver sinks to about 80 
milliamps from the +5 volts supply and 24 milliamps from the -5 volts 
power supply. This represents a savings of about 70% power when compared 
to other transceivers with the resultant power saving and greatly reduced 
power dissipation which might compromise associated integrated circuits. 
This power saving is attributed to the simplicity of the design which 
makes this transceiver use much less power. The components for the design 
of this circuit were selected to assure the use of less power and for 
increased reliability. 
Obviously, many modifications and variations of the present invention are 
possible in the light of the above teachings. It is therefore to be 
understood that within the scope of the appended claims, the invention may 
be practiced otherwise than as specifically described.