Real Time Devices AD1200 User Manual page 35

Data Transfer
The converted data can be tansfened through the PC data bus to PC memory in one of two ways: by using the
microprocessor or by using direct memory access (DMA). Data bus transfers take more processor time to e*ecute.
They use polling and intemtpts to determine when data has been acquired and is ready for transfer. DMA places
da0a directly into the PC's memory, one byte at a time, with minimal use of processor time. DMA transfers are
managed by the DMA controller as a background function of the PC, letting you operate at higher throughput rates.
The maximum throughput rate of the 1200 is 125 kHz.
D/A Converters (ADA1200 Only)
Two independent l2-bitanalog output channels are included on the ADAI200. The analog outputs are gener-
ated by two l2-bit D/A converters with independent jumper-selectable output ranges of +5, +10, 0 to +5, orb to +10
volts. The 110 volt range has a resolution of 4.88 millivolts, the +5 and 0 to +10 volt ranges have a resolution of
2.44 millivolts, and the 0 to +5 volt range has a resolution of 1.22 millivolts.
Timer/Counters
An8254 programmable interval timer provides three 16-bit, 8 MHz timer/counters to support a wide range of
timing and counting functions. Two of the timer/counters, TCO and TCl, are cascaded so that they can be used for
the pacer clock. The pacer clock is described in Chapter 4. You can use the remaining timer/counter, TC2, for
counting applications, or cascade it to TCO and TCI for timing applications. Figure 3-2 shows the timer/counter
circuiry.
Each timer/counter has two inputs, CLK in and GATE in, and one output, timer/counter OUT. They can be
programmed as binary or BCD down counten by writing the appropriate datato the command word, as described in
Chapter 4. The command word also lets you set up the mode of operation. The six programmable modes are:
Mode 0
Event Counter (Intemrpt on Terminal Count)
Mode I
Hardware-Retriggerable
One-Shot
Mode 2
Rate Generator
Mode 3
Square Wave Mode
!-t ru i - - - - - - l
1200
I/O CONNECTOR
? 2
I
I
I F-
8 MHz
EXT CLK
EXT GATE 1
TO A/D
T R I G G E R
XTAL
E C 1
prr{ gglrnrecen rN
L - - - - _ _ - _ _ . ;
T I T E R /
C O U I { T E R
C L K
0
GATE
OUT
rt* o11116 er1l .,
C L K 2
Fig. 3-2 - 8254 Timer/Gounter
Circuit Block Diagram