Z80 INFO II

Eagle: Serial TTL/FTDI, RS232 Schematic

Serial Signals: RS232

RS232 Info:

We have attached an o`scope to the 2x8 header to examine the TTL SIN (RxD)signal from the Tera Term terminal emulation program running on a PC connected to the 16550; the MAX232A is not attached. (Schematic is left adjacent).

According to the left-adjacent diagram, Serial Signals, when the ASCII letter `U` is sent out as hex value 0x55, the binary value is 0101 0101.

The Least Significant Bit (LSB) is sent out first (after the Start Bit) and the Most Significant Bit (MSB) is sent out last, followed by the Stop Bit. This effort of putting the littlest bit of the byte onto the wire first is called Little Endian bit ordering. (Ethernet does it, too.)

As per the left-adjacent diagram, the idle state is a 5v signal. We can see that in the right-adjacent o`scope screenshot (click it to zoom).

The Start Bit is the first low-going bit on the left, followed by the LS bit of the byte. So byte 01010101 is transmitted as Start_10101010_Stop.

Get more info on RS232:

https://www.sparkfun.com/tutorials/215

Z80 Assembly Language Programming

Visit Z80 Info X to learn how the Assembly code works for this circuit.

 Letter `U`, ASCII 0x55, Binary 01010101