< Wouter van Ooijen took issue with my contention regarding the IBM PC when I said> > Two examples of this behavior [requiring control lines] are the > Rockwell R6551AP UART chip and probably the original > IBM PC RS-232 adapter card as specified in the legendary > "Blue Book." I disagree, Wouter. And although regrettably I suffer your learned disapprobation, I enjoy the company of one of the more prolific authors about serial communications at the time. As though anticipating this thread, Joe Campbell in "C Programmer's Guide to Serial Communications", Howard Sams & Co., 1978, remarks as follows on page 144: "The IBM PC family represents the height of folly in this regard, where the system -- DOS and ROM BIOS -- does not contain simple unadorned functions to perform character-by-character serial I/O. For every byte the serial output system perfunctorily asserts the Request To Send (pin 4 [25 pin]) and Data Terminal Ready (pin 20 [25 pin]) outputs, then tests both the Clear To Send (pin 5 [25 pin]) and Data Set Ready (pin 6 [25 pin]) inputs are enabled before sending this byte. Likewise, the serial input function refuses to read data from the serial port unless the Data Set Ready is asserted. If these constraints are not bad enough, the system provides no mechanism to defeat them." For those of us who dropped down to port calls Joe Campbell's observation proved to be only the tip of the iceberg. The serial port adapter was simply not able to kick the habit of requiring handshaking. If I recall correctly (without checking) the problem was with how IBM chose to strap the NS1645 UART, thus making one of the best arguments to quit engineering and go to law school. Aza D. Oberman -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist