ISA Bus Servo I/O Card Hardware Manual

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ISA Servo Hardware Manual5.5 Analog Output SectionThese write-only registers provide access to the digital-to-analog converters (DACs) as 16 bitwords. Accessing the registers as bytes is invalid. The registers are located at even offsets beginning atlocation 0x10 and ending at 0x1E. The digital-to-analog converters have a 13 bit resolution. The outputvoltage has a -10V to +10V range. Additionally, Opto-22 compatible digital outputs corresponding to thesign of the analog signal are automatically generated by the hardware. These outputs are located on theP2 connector. Upon a hardware reset, and at power on, the DAC output is set to 0V by the hardware.Below are some example DAC values:Value Written Output Voltage0x0000 -10V0x10000V0x1FFF +10VThese values are inverted, compared to the datasheet for the MAX547B because an invertingamplifier buffers the output of the MAX547B.Two optional potentiometers may be installed to adjust the reference voltage (zero offset) for theanalog input as well as the analog output circuits. Also, the board can be configured so that the DACs areall latched simultaneously by the hardware at the time of the periodic interrupt. Refer to Section 2.3 –Jumper J3, for more information.Copyright 1995. 1996 Servo To Go, Inc. 30
Servo To Go, Inc.5.6 Analog Input SectionThe analog-to-digital converters (ADCs) are controlled by a set of registers located at evenoffsets beginning at location 0x10 and ending at 0x1E in the high bank of I/O addresses. They must beaccessed as 16 bit words. To read an analog channel:1. Read from the ADC. This is a dummy read which only sets the input multiplexer to the properchannel. Ignore the data, just the act of reading sets the multiplexer. For example, to get thevoltage on channel 2, read 16 bits from 0x14 in the high bank of I/O addresses.2. Write to the STARTCONV & LATCHn register to start the conversion on a particular channel.For example, to get the voltage on channel 2, write 0x0000 to offset 0x14 in the high bank of theI/O addresses.3. The software must wait (see table below) for the conversion to finish. Other necessary operationscan be performed by the software during this time. Refer to the ADC12441 data sheet tocalculate the exact time (note: the clock is 1.7978 MHz). The End-Of-Conversion bit can beconfigured to cause an interrupt. Alternatively, the EOC bit always sets its interrupt request flag,even if the EOC interrupt is disabled, so the software may poll this bit to determine if theconversion has finished.4. Read 16 bits from the ADC location that you read from in step 1. The data read will be thenewly converted channel input.Status of the Auto-zero (AZ) Bit in Register INT.C Polling Delay Not Polling Delay0 = Auto-zero 19 µS 34 µS1 = Don’t Auto-zero 4 µS 19 µSNote: It will take the “polling delay” amount of time for the EOC bit to go to zero after the startconversion command is issued. When the bit transitions from a zero to a one, the conversion is finished.Polling without auto-zero is probably the preferred method, therefore, after waiting 4 µS, check EOC, if itis a one then the conversion is done.The value read from the ADC is a normal 2’s compliment value. Below are some exampleswhich assume the input range (set by jumper J2) is set at -10 to +10 Volts:Value Read Input Voltage0x0FFF 9.9975V0x0000 0.0000V0x1FFF -0.0024V0x1000 -10.0000VPlease see section 2.2 for information on how to set the input range.31 Copyright 1995. 1996 Servo To Go, Inc.
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