Part 2

Plug-In Assembly Configuration 54 7Design Suggestions for the Digital SectionThe following paragraphs provide some suggestions and precautions for designingcustom circuits using the digital section. See Chapter 6 for commands to control thedigital section.+ 5V Digital Power SupplyThere is only one + 5V (& .25V) power supply provided for the digital section. It isreferenced to the outguard section of the 3497A mainframe and appears on thebackplane edge connector as follows: + 5V at pins BB and CC and GND at pins 16and T.Maximum power dissipation allowed by a card is 2 watts, including when bothanalog and digital section power supplies are used. Circuit traces bring the + 5V andGND close to their respective buses so that only short jumpers are needed to thebuses. Take care not to mount a component on a bus. IC power supply connectionsshould be as shown in Figure 157.It is recommended that bypass and filter capacitors always be used between + 5Vand ground. A good rule to follow is to use one 1pF tantalum filter capacitor andtwo .O1 pF ceramic bypass capacitors for every eight (or less) ICs.Buffering the Digital Reset LineA location is provided on the breadboard card to mount a SN74LS14 IC to bufferthe Digital Reset Line (pin U on the backplane connector). This IC provides a hightrue and low true version of the Digital Reset Line, as shown in Figure 158.Buffering the Data BusThe data bus drive capability is limited and may require buffering depending on yourapplication. If the data direction is always from the 3497A to the breadboard card(write only), data latches can be placed directly on the data bus as shown in Figure159 (a).Whenever the data direction is from the breadboard card to the 3497A (read), thedata bus must be buffered with a tri-state device(s1. The buffering device must beable to drive 20 LS loads (20 Low Power Schottky inputs) such as the SN74LS24XSERIES or 81LS97. A typical data bus configuration for read operation is shown inFigure 159 (b).Artisan Technology Group - Quality Instrumentation ... Guaranteed | (888) 88-SOURCE | www.artisantg.com