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E3645A rs232 handshaking

Question asked by IAtech on Nov 20, 2017

     Short question:  Why does it not work to jumper the DTR and DSR pins on the serial port in order to bypass the Rs232 handshaking on an E3645A power supply?

Now the long explanation if anyone has the time:   

I am working on an application where I need to establish serial communications between a E3645A Power Supply and a PLC.  This PLC has a Rs232 comm port available that I can easily use to transmit and receive ascii strings but has no handshaking/flow control capability. Only Tx, Rx, and Gnd (pins 2,3,& 5).  I have verified with Windows hyperterminal that it is sending and receiving strings perfectly. 

     It transmits commands and communicates fine with the power supply unless I send any query command which requires a response.  I get no response and if I send another query command I get an error having to do with an interrupted response/uncleared buffer.  If I connect the power supply to a windows PC using hyperterminal and a cable which uses only pins 2, 3, 5 it does the same thing.  If I add pins 4 and 6 (DTR & DSR) to the cable (crossed-over) it responds properly to queries.  Pins 1,7,8, and 9 can remain disconnected and it will communicate back & forth fine with hyperterminal.

     After a quick google search I discovered the easy solution would be to jumper pins 4 and 6 to "fake out" the handshaking but this does not work.  Anyone know why?

Thanks in advance

Scott

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