Part 2

634 Interface ConceptsAn asynchronous modem simply means that it operates at random speeds. That is,the character generation is random and the arrival of a character is unpredictable.This is illustrated in the Figure A-6.RANDOH TIMEBETWEEN CHARACTERSSTARTBITSTART/ BITSTOPBITSTOPBITFigure A-6. Asynchronous Bit FlowA full duplex modem is capable of transmitting and receiving data simultaneously.A half duplex modem can transmit and receive data, but it can't do both at thc, sametime. You may wonder why full duplex operation is required in a data acquisitionsystem since, typically, the 3497A will be instructed to perform some specified taskand then transmit the results back to the computer. However, two-way simultaneoustransmission is required when the 3497A sends a break. That is, when the 3497Asends a break, it may do so while the computer is still sending commands.EchoThe 3497A does not support echo, and any computer used in the system shouldhave this feature turned off. Where echo is supported, the receiving device transmitseach character it receives back to the transmitting device. The returned characteris usually then displayed on a CRT or printed on a printer, permitting a visual checkto be made of the character transmission. However, characters echoed back to the3497A will be interpreted as commands. Since they will not be understood, the3497A will set the "Message Not Executed" Bit in its status register and, if enabledto do so, send a break. Another likely occurrence will be an input buffer overflow.Artisan Technology Group - Quality Instrumentation ... Guaranteed | (888) 88-SOURCE | www.artisantg.com