Hi Jasen

Attached is you suggested circuit redrawn in Workbench. When I looked
at it, I like the idea how D1 and D2 are used to terminate through the
voltage divider powered by -5v and +5v (R1||R2= 75ohms) and then
reverse D1 and D2 to no termination. Very clever. I ran this circuit
on the simulator and toggle J1 between 5v and 0v. Unfortunately, I
saw no change on the right side of R11. Checking on the circuit I
notice that Q5 and Q1 can never turn on regardless if the right side
of R8 is 0v or 5v. The BE (Base- emitter) of Q1 never completely gets
forward bais to turn the transistor. The same is Q5. I can't seem to
see any bipolar configuration for Q2 to switch Q5 and Q1 on or off.
Can you see if I redrew your schematic correct. I hope to get your
schematic to work on Workbench first before I try to bread-board it.
Your idea for the diode to switch on or off the termination make
sense. I just have to get the logic controls to work proper. Please
send me your feedback.

Caesar



On Fri, 03 Feb 2006 07:00:30 -0000, Jasen Betts
wrote:

On 2006-02-02, LRCR wrote:

video source---------------*-------------------Oscilloscope
|
|
75 ohm
|
|
C
0V or 5VDC--10k--------B 2N3904
E
|
|
-5VDC



(view with a fixed-pitch font)

this one may work a bit better

--+--- +5
|
10K
source | 2N3906
| 1N914 |
| +--|--+-------C E-------+---- -5
| | | B |
\ 150R === 100pF |---[33k]-'
\ / | 1K .---[10K]-- control
X --+-- 0V | .2N3906 | (0 or 5V)
/ \ | C ' B
/ 150R === 100pF B---[10K]--+-C E-- 0V
| | | E | `.
| | | |--[33K]-. 47K 2N33904
| | | B | |
| +--|--+-------C E------+--+--- +5
| 1N914 |
scope | 2N3904
10K
|
--+--- -5

TTL-swittched broadband terminator concept.


Bye.
Jasen