david@varian.UUCP (10/12/83)
Speaking of the RING indicator, prowling through the manual for the Able DH/DM multiplexor, I found an obscurely-documented but very useful use of it: Section 7.4 (pg. 7-5): In applications where modem control is not required, the RING line can be used as a device handshake signal which stops transmission to that device in a manner that is completely transparent to the software ... A jumper placement connects the RING line to the clear-to-send input of the UART ... the UART clear-to-send input is removed when the RING signal is placed at the inactive level. The loss of CTS inhibits the UART from raising its transmitter-ready flag and thus the next character is not loaded into the transmit holding register until the CTS signal has been restored by reactivation of the RING signal. I was able to use this feature to connect a serial printer that uses DTR control to UNIX without any software modifications (it seems to me that mdmbuf in the 4.1BSD tty driver doesn't work; am I right?). I connected the jumper (I had to phone Able to find out where they are - if you look at the Figure 3-1 on pg 3-3, there are 16 pairs of circles marked "RESERVED FOR FUTURE USE"; each pair corresponds to one of the 16 lines; jumpering pins 28 and 29 together will affect line 0, ... 60 and 61 affect line 15). I then made a special cable for the computer end which connected RING (pin 22) to the DTR output of the printer (which was now CARRIER (pin 8), having gone through a null modem, as both the printer and the multiplexor are DTE equipment). I created a new version of the printer queuing software with all the names changed (lpr-->lp2, lpd-->lpd2), linked /dev/ttyxx to /dev/lp2, and that was it. If anyone has questions about what I did, feel free to call or send me mail. I don't know if all Able multiplexors have this feature; we received ours in February 82, and the manual is dated July 81. We also have an older Able DMAX/16, which is a 3 board set (the DH/DM is a single board), and I haven't found anything in the manual about such a feature. David Brown Varian Instruments 2700 Mitchell Dr. Walnut Creek, Ca. 94598 (415) 945-2199 ...!decvax!sytek!zehntel!varian!david ...!ucbvax!menlo70!sytek!zehntel!varian!david ...!ihnp4!zehntel!varian!david ...!tektronix!zehntel!varian!david ...!fortune!varian!david ...!amd70!varian!david
thomas@utah-gr.UUCP (Spencer W. Thomas) (10/13/83)
Regarding the use of RI on Able DH/DMs for flow control. We have been doing this for over a year now with our LocalNet units. This allows us to have totally out-of-band flow control, which is great if the unit on one end is talking 4800 baud to a modem, while the unit on the other end is talking 9600 baud to the computer (as mine is at this very moment). The only problems we encountered were with our DEC-20, which thinks it knows how long it should take for a string to be transmitted. If it is flow controlled for too long by this method (which is totally invisible to the host), it gets upset. =Spencer