Documentation   :     F18 I/O    

ANALOG OUTPUT:

The digital to analog converter (DAC) is a programmable current source that can be used to generate a voltage across a resistance to ground, or to source an op-amp. By writing the xor of a desired value and hex 155 into the low nine bits of io that value controls the DAC output. A value of 1FF (written as 0AA) sets the DAC for maximum current; a value of 0 (written as 155, the reset state) sets minimum current, high impedance output. The typical DAC transfer functions, in mV versus DAC values, into 75,
50, 37.5 and 8 Ohms are shown at right; as the resistance decreases, the voltage decreases and the function becomes
more linear. Analog nodes in F18A based designs have both ADC and DAC connected with separate pins.

THE IO REGISTER: An 18-bit control and status register, named io and diagrammed below, is each F18A’s interface with its I/O circuitry (if any) and comm port handshake lines. In the “READ” line of the table below, the green background indicates signals that, if they exist, come from other nodes or the world outside

IO Register  :        9-bits   D/A value  XOR  x155     Bits 8 7 6 5 4 3 2 1 0

A/D MODE :      Bits 14  13

In ArrayForth :    Communication with GA144

host  load panel

with node 117  ( 117 Analog  117.Ao , pin 50)

talk 0 117 hook upd

analog output  :

[0..511]   {decimal}     155  {Hexa}    OR   io   r!   

     Example :       100   155   OR   io  r!  

100

115 (Hexadecimal)

Press  F1   (decimal  -->  Hexadecimal)

XOR

XOR  result :

io

r!

You can measure between  117.ao   (Pin 50) and  Vss    --->   V = 0.245 v      for 100 (decimal)

The schematic :

Several Measures :   Analog Output  117.ao   (Pin 50)

Decimal Value	Hexadecimal	Hexadecimal XOR $155	Measure (Volts)  value  R= 50 ohms
0	0	155	0
1	1	154	0.002
10	A	15F	0.026
50	32	167	0.125
100	64	131	0.244
200	C8	19D	0.47
300	12C	79	0.665
400	190	C5	0.837
500	1F4	A1	0.977
511	1FF	AA	0.99