brian@sdcsvax.UUCP (05/04/86)
Recently, whilst installing our Ungerman-Bass NIUs on one of the VAXen here at UCSD, I discovered that certain models of comm line multiplexors can do hardware flow control. In particular, the Emulex CS-11 (which does a pretty good job of looking like a DH11) has a switch for each line located in the multiplexor panel that when turned on causes the UART transmitter to pay attention to Clear-To-Send. The manual for the CS-11 is very clear on this, but I'd never looked. (Stupid, hah!) This hardware flow control is completely invisible to the software. What it does is when CTS is not asserted, it makes the transmitter stop at the end of the current character - so you get essentially instantaneous flow control. The software (at least, most drivers I've poked at) don't look at the CTS bit in the status register, so they just seem to think that the current character is taking an extraordinarly long time to send. For most applications, this works extremely well. If you have had problems with getting output stopped to avoid overrunning buffers in some peripheral (like lan boxes, terminals, buffered modems, etc) because the VAXen sometimes will send as many as 64 characters AFTER you ship it an X-OFF, this may provide an answer. Moral: check your hardware manual. It might well be that the particular comm multiplexor you're using might do the right thing, even though the software doesn't. Brian Kantor UC San Diego decvax\ brian@sdcsvax.ucsd.edu ihnp4 >--- sdcsvax --- brian ucbvax/ Kantor@Nosc "There is more harmony in films than in life." - Francois Truffaut
nichols@uwmacc.UUCP (Dennis Nichols) (05/05/86)
> Recently, whilst installing our Ungerman-Bass NIUs on one of the VAXen > here at UCSD, I discovered that certain models of comm line multiplexors > can do hardware flow control. In particular, the Emulex CS-11 (which > does a pretty good job of looking like a DH11) has a switch for each > line located in the multiplexor panel that when turned on causes the > UART transmitter to pay attention to Clear-To-Send. ... > > ... For most applications, this works extremely well. > > Brian Kantor UC San Diego We purchased Emulex muxes with the hope of using this feature. VMS didn't like it because writes would timeout if Clear-To-Send was held off too long. Worse than that, the Emulex would sometimes lose characters from a write with the switch in the 'use CTS' position. Because of problem one we did not push too hard to get problem two fixed (seemed to be a firmware bug having to do with length of line, not with the CTS signal actual going low). Emulex may well have corrected the situation by now. Dennis Nichols University of Wisconsin-Madison Academic Computing Center
david@varian.UUCP (David Brown) (05/06/86)
> Recently, whilst installing our Ungerman-Bass NIUs on one of the VAXen > here at UCSD, I discovered that certain models of comm line multiplexors > can do hardware flow control. In particular, the Emulex CS-11 (which > does a pretty good job of looking like a DH11) has a switch for each > line located in the multiplexor panel that when turned on causes the > UART transmitter to pay attention to Clear-To-Send. The manual for the > CS-11 is very clear on this, but I'd never looked. (Stupid, hah!) > The Able DH/DM multiplexor also has some jumpers (located in the upper right hand corner) to enable hardware flow control; unfortunately, the manual is *not* very clear on this. The mux will stop sending when it senses pin 22 (RING) go low; we made up special cables at the computer end tying together pins 22 and 8 (we chose to flow control on CARRIER (crossed with pin 20 - DTR) on printer lines rather than CTS). -- David Brown (415) 945-2199 Varian Instruments 2700 Mitchell Dr. Walnut Creek, Ca. 94598 {zehntel,dual,amd,fortune,rtech,lll-crg,rtgvax,ptsfa,csi}!varian!david