Help in transistor polarity

Thread Starter


Joined Apr 19, 2012
Hi everyone,
Sorry for my poor English.
I have a problem in understanding the polarity of transistors.Considering the circuit inside an RS485 driver and receiver
in the attachment file.
In this figure illustrate when DI is in high logic level (+5V).Because the driver in trasmitting mode therefore,
DE pins is in high logic level and RE pin is in low logic level => Q1, Q4 switch on and Q2, Q3 switch off.
The voltage on line A causes Q6 to switch on.Current flows into Q6 and returns to the driver via the ground wire.In a similar
way, the low voltage on line B cause Q7 to switch on and current flows from Q7 into Q4,
returning to the receiver via the ground wire. Line A is more
positive than line B, and the result is a logic high at the receiver’s TTL output
I read this in the book but I do not really understand it.
For me, I think that if the diode in this figure is silic then the voltage drops in it will be 0.7V and therefore
the voltage in A line is 5-2x0.7= 3.6V.And with Q5 is NPN, Q6 is PNP transistor I guess that Q6 is conduct and Q5 is not conduct
but I can not to explain and unserstand cleary why it is.
Can anyone help me explain in detail why each diode is conducted or not?
And help me expalin the role of each diode and resistor in this figure?
Thanks in advance.