chrisathome
New Member
I'd like some help with ttl logic, my electronics is well rusty. I need to explain what I have, and what I want to do with it, sorry for the length.
I volunteer as an engineer for a small UK hospital radio charity (= no money!) and need to rebuild their programme switching circuits.
I have a turnkey audio switching unit, built for me in another life, that has 10 individual removable relay cards, each of which can select one of 4 possible card inputs to its output. Some of the inputs are commoned to each card. Cards can be removed from the unit without affecting operation of the other cards. Each card has 4 latching set/reset relays which survive power loss, and each relay has a signalling contact, active ttl high (e.g. +5v through 150 ohms) when operated. Card logic ensures only one relay per card can be latched on at a time. The cards were built around 74 logic not LS or Cmos.
Card source switching is achieved using a separate select card with 4 momentary buttons and IN4148 diode matrixes to set ttl high on a 3 bit BCD bus common to all cards. A momentary set button on each card transfers the bus state to the card logic when pressed, using AND gating. When no buttons are pressed the bus lines are pulled to 0v by 100k resistors on each card. On each card the bus connects to a 4028 BCD to decimal decoder, then other driving ic's. Both the select card button assembly and the set button on each card are wired to +5v through 150 ohms.
I have brought out the 3 bit bus, card set line and the relay status lines from each card, together with +12v +5v and 0v to an external connector on the unit as I need to extend the switching ability and relay status to other areas in the studio complex. The 3 bit bus obviously has to be bidirectional, the status doesn't.
The station has overall screened multipair cable to all the areas concerned which I can use for this purpose. Is any buffering or level shifting required on the long bus lines which will effectively be star wired out from the router position? Worst case is about 15 metres. I have a couple of 74HCT04 hex inverters available for test purposes but my logic knowledge is 20 years out of date. The multipair will only be carrying steady state voltages, no serial or parallel data.
Also, I need to pick up the +5v relay status lines at the remote end then use them to drive 12v led + resistor indicators in an existing display unit that I cannot modify. I can supply the 12v from the router and thought to use a ULN2003A for this as the each led in the display assembly require grounding pin(s) to turn the led(s) on.
Hope this is enough for someone to understand!
Thanks
Christopher
I volunteer as an engineer for a small UK hospital radio charity (= no money!) and need to rebuild their programme switching circuits.
I have a turnkey audio switching unit, built for me in another life, that has 10 individual removable relay cards, each of which can select one of 4 possible card inputs to its output. Some of the inputs are commoned to each card. Cards can be removed from the unit without affecting operation of the other cards. Each card has 4 latching set/reset relays which survive power loss, and each relay has a signalling contact, active ttl high (e.g. +5v through 150 ohms) when operated. Card logic ensures only one relay per card can be latched on at a time. The cards were built around 74 logic not LS or Cmos.
Card source switching is achieved using a separate select card with 4 momentary buttons and IN4148 diode matrixes to set ttl high on a 3 bit BCD bus common to all cards. A momentary set button on each card transfers the bus state to the card logic when pressed, using AND gating. When no buttons are pressed the bus lines are pulled to 0v by 100k resistors on each card. On each card the bus connects to a 4028 BCD to decimal decoder, then other driving ic's. Both the select card button assembly and the set button on each card are wired to +5v through 150 ohms.
I have brought out the 3 bit bus, card set line and the relay status lines from each card, together with +12v +5v and 0v to an external connector on the unit as I need to extend the switching ability and relay status to other areas in the studio complex. The 3 bit bus obviously has to be bidirectional, the status doesn't.
The station has overall screened multipair cable to all the areas concerned which I can use for this purpose. Is any buffering or level shifting required on the long bus lines which will effectively be star wired out from the router position? Worst case is about 15 metres. I have a couple of 74HCT04 hex inverters available for test purposes but my logic knowledge is 20 years out of date. The multipair will only be carrying steady state voltages, no serial or parallel data.
Also, I need to pick up the +5v relay status lines at the remote end then use them to drive 12v led + resistor indicators in an existing display unit that I cannot modify. I can supply the 12v from the router and thought to use a ULN2003A for this as the each led in the display assembly require grounding pin(s) to turn the led(s) on.
Hope this is enough for someone to understand!
Thanks
Christopher