29-05-2022, 05:45 PM
When the DG680 & DG640 were designed, many TTL Inputs were left floating under the presumption that they'd float high.
This is actually poor design practice and it is recommended that the appropriate signal treatment is applied.
For example. IC33 and IC36 are 74LS257's and their A inputs (pins 2,5, 11 & 14) are disconnected and left to float when their corresponding POJ DIP Switch is in the open position.
A 10k pullup resistor on each of these inputs is a viable solution. An 8 x 10k SIP resistor network with a single common pin is a convenient package to use.
Other examples include:
IC3 74LS90 pin 5
IC8 74LS90 pin 3, pin 14, these can be tied to +5V directly or connected with pull up resistors.
(Remember that +5V is not on pin 14 of the 74LS90, it is pin 5.
This is actually poor design practice and it is recommended that the appropriate signal treatment is applied.
For example. IC33 and IC36 are 74LS257's and their A inputs (pins 2,5, 11 & 14) are disconnected and left to float when their corresponding POJ DIP Switch is in the open position.
A 10k pullup resistor on each of these inputs is a viable solution. An 8 x 10k SIP resistor network with a single common pin is a convenient package to use.
Other examples include:
IC3 74LS90 pin 5
IC8 74LS90 pin 3, pin 14, these can be tied to +5V directly or connected with pull up resistors.
(Remember that +5V is not on pin 14 of the 74LS90, it is pin 5.