What is missing from today's Communications Channel Encoders

I have to say that the simplicity and power of wireline communication encoders have not been replicated in wireless communications systems. When examining systems that use LSFR scramblers , RS encoding, Constitutional Encoding, Turbo Product Coders, LDPC encoders, nothing really matches the features and simplicity of your garden variety 8B10B encoder and decoder.

Sadly the one needed feature of channel encoders is the ability to have a data space and control space. K characters allow the link to separate the messages passed into unique spaces such as a data space and control space. This feature allows the in-band control to be sent co-mingled with the data payloads. The current systems force an upper level protocol to make a header that may not be unique then encapsulate control data. Byte or word synchronization is always an issue with the wireless channel encoders. On scrambled systems there always exists a time shifted version of the synchronizing sequence. In an effort to solve the problem longer sync sequences are selected. Possibly with CRCs to validate the sync header.

Using 8b10b carries a hefty penalty though, for 10 available bit times only 8 are transmitted unless you count the "isK" to be one of the bits received then 9 bits are received in the span of a 10 bit time. Therefore  "bits received" / "bit times available" = 8/10 or 80% of the bandwidth is utilized for actual data. 8B/10B codes are derived from 5B/6B and 3B/4B codes. The filing date for the 5B/6B codes is 1991. So nothing new for 23 years now.

64/66 bit codes is a sad attempt to do the same thing but suffers from DC bias wondering based data pattern as a result of the scrambling. DC bias is controlled on a word by word basis on a 8B/10B system by calculating the running disparity. Also 64/66 bit encoding is not generally used on wireless communications. One can argue that DC bias is not needed in wireless system since in today's modern communications systems are phase and amplitude modulated and many sub carriers are used (QAM, OFDM)

I'm still waiting for the perfect channel coders that deliver the features that we love about wireline encoding systems and also provide the error correction close in performance of the current lineup of LDPC FEC. 

Leonard Dieguez

J1772 - pilot signal

Your driving around in your new home grown electric vehicle  (EV) and then you notice that your charge gauge is running just a bit on the low side. This senario can strike fear into the the hearts of electric car owners. The senario is equivalent to being on a long stretch of road and knowing that you passed the last gas station 50 miles back and you spot a sign that says next service in 50 miles. Your first instinct is normally look at your gas gauge, your 2nd would be to pray.

But what if there was a service station just up the road a bit but you could not a fill your tank because your car did not have a standard filling inlet. Yours was too small or square or in the case of an electric vehicle you needed standard J1772 plug but you have a NEMA 5-15 or a NEMA 14-50 receptacle installed. At this point you kind of out of luck.  

So you are developing an electric plug-in vehicle and you want to be J1772 compliant?  Well this blog is to help you do just that. The standard level 1 and level 2 J1772 connectors have 5 signals 2 phases , a gound , a pilot signal and a proximity signal. The pilot signal and proximity signals are there for safety reasons.

Since this blog is about the control pilot signal that is what we will be focused on. The pilot signal has a wealth of functions that it is used for in the EV charging system. Listed they include:

-- lets the J1772 charging station know that the connector is plugged into a car.
-- lets the J1772 charging station know that the EV is ready to accept a charge.
-- lets the J1772 charging station comunicate to the EV how much current it can supply.
-- lets the EV know that it is plugged into a charging station.
-- lets the EV comunicate to the charging station that it is ready to accept a charge.

Let us examine the pilot signal. Using LTspice I simulated the basic pilot signal from the charging station through the connector and into the EV. The model is show below.

Control Pilot signal spice model

The source V1 on the left is a 1 KHz signal generator toggling from -12V to +12V. The resistor R1 is the source impedance of the pilot signal that the EV would see. R1 is also used by the charging station to detect a voltage drop thus indicating that the J1772 connector is plugged in to a EV. Vpilot _conn is the signal that we will look at and is the monitored signal by the charging station. The signal after the diode is the signal the EV sees and we are calling that signal Vpilot_diode. Switch S2 is simulating the connector being plugged into the receptacle. Switch S1 is controlled by the BMS or EV charger and is closed when the EV is ready to accept a charge and opened when the EV is fully charged. Below is a Spice simulation of the waveforms on these 2 signals. 

Pilot control signal at the connector and in the EV

From the above waveforms we see the pilot signal is initially toggling between +12V and -12V. When the connector is inserted into the EV (simulated by SW2 closing) the Pilot_conn then toggles from +9V  to -12V. In the EV the pilot_diode signal goes active and is one diode drop below +9V (~8.3V) and zero. Then when the EV is ready to accept the charge the EV charger controller will close SW1 thus shunting current through R3 a 1.3K ohm resistor. After that even the charging station sees the pilot signal toggle between  +6V to -12V. The EV sees the same signal but with a diode drop from 6V. 

The task of the EV designer is to turn the pilot signal that the EV sees into a usable control signal. This information coupled with the battery charge state and the readiness of the charger is used to control SW1. One of the easiest ways to do this is to turn the PWM toggling signal from the charge station into a steady state logic signal using a mono-stable multivibrator , or a one shot. In my next post I will discuss and simulate a few variations of  a mono-stable multivibrator. Leonard Dieguez