CAMPBELL%UTOROCI.BITNET@cornellc.cit.cornell.edu (05/27/89)
Many weeks ago, I posted a request (to SunSpots, actually) for info about hooking up a Matrix Multicolor video camera (aka Dunn 638HSC) to our Sun 3/260, which also has a TAAC, and which also has longish (say 20 ft) video cables between the system and the monitor. We were seeing ghosting on the monitor when the TAAC was hooked into the video lines (whether or not it was active) and the Matrix was connected. The Matrix has a switch for either 75ohm or Hi-Z termination, but it seemed that it was doing something funny to the video even when told to be Hi-Z. Anyhow, we got several responses, all of which basically said, "Let us know what you find out." The best lead was a phone call from a chap at Transcept (as in the T in TAAC) - and I hadn't even put my phone number in the posting! A comparison of this to reports of U.S. Sun support will be left as an exercise to the reader. (Canadian Sun support is good, by the way). So this Transcept guy and our electronics shop conversed, and results forthcame in the form of two active boxes, one at the TAAC output and the other in-line near the monitor. The fact that we wanted a switch unit to select any of several source systems as input to the Matrix complicates matters a bit, but not much. What follows are the notes made by our electronics shop at my request for this posting. The schematic renderings are mine. . . . The Sun and TAAC appear to have the following output configurations: Sun: Taac: ------75ohm---o (to 75 ohm monitor) -----0 ohm---o (to 75ohm monitor) | | (2V) (V) | | --- --- - - In either case the cable is terminated with 75ohm at the monitor, after about 20 feet of cable. We think the Sun configuration is better, effectively terminating the cable with 75ohms at both ends. The TAAC output appears to be a bit non-standard. We built two devices, one mounted close to the TAAC output, the other near the monitors. The downline unit is actually a switchbox, switching the output of several devices (The Sun/TAAC, a MegaVison image processing unit, and an HP 9000 workstation) to a Matrix Multicolor video camera, without disturbing the feed to the monitors. The video buffers used a Comlinear CLC 400AJP operational amplifier. The first unit effectively standardized the TAAC output (s and c mark signal and common (shield) inputs from the TAAC). ------270ohm- | | |---270ohm---| - \ | --- | \ | - | >-----68ohm---o (output) | / --- (shield) s---------------| + / | | | x 75 ohm | | | c-|-------------------------------- | --- - The output stage of the switchbox is similar, with the 75ohm resistor replaced by a 56kohm one to present a high impedance to the line. Each input line (four input devices times 4 channels [RGBS] equals 16 BNC connectors) is fed through immediately to its output (16 more BNC connectors). For video switches, the box uses Teledyne RF relays, 712D12. Both the signal and shield are switched. When selected, they are fed into the points marked s and c. The four shields for each input device are connected together inside the switchbox, but each device's shield system is separate, and only the selected system's shield(s) are connected to the Matrix's. The Matrix appeared to have some shunt capacitance, even when set to high- impedance input. Thus our switchbox presents a known high-impedance tap on the video lines, and offers a 75ohm output for the Matrix (which we set to 75ohm input mode). The teledyne RF relays ensured minimum cross coupling. The HP input still does not work correctly; although its output is usable on the Matrix, the HP monitor cannot sync on the output from the switchbox. The HP seems to be working on a current-based specification. Work continues. . . . Hope this helps somebody. If anyone's more interested and I haven't made something clear, send me a note or give me a ring at (416)924-0671x4924. I could arrange to mail copies of the shop's notes and schematics. Chip Campbell VAX System Manager, Physics Division Ontario Cancer Institute Toronto, Ontario bitnet: campbell@utoroci