TTL & Relay Outputs


vijay K. Anthwal

No ,Relay output card would not work. However TTL might work as it is suitable for fast operations. TTL can work on Ac or Dc load for faster frequency than relay however it needs external supply. Best course would be to use 0-10 or 0-12V Dc pulse output if it is available for your PLC.
The TTL output for a DO channels swings between 0.7volts (for logic 0 ) and +5volts ( for logic 1).Generally , they can be connected to devices which operate a low voltages and currents such as LED dispays,LED lamps or TTL input circuits etc.

In case of a relay output the output terminals of the module are connected to the relay contact(s).As such they are usually called a potential free contacts or dry outputs. Generally ,they can drive loads at higher voltages and currents such as motor contactors,solenoid valves etc.

Another diffrence is the duty cycle.TTL outputs can be ON to OFF (or OFF to ON) at much higher rate due to the lower IN(OFF) response times.Thus they can be used in duty cycle contol such as a Thyristor regulator. Life cycle of the TTL output is very high.
In case of relay output, the duty cycle which you can achieve is mainly limited by the lower ON(OFF) response times as well as the shorter number off life cycles beacuse a relay is simply a mechanical device. You can use it to drive a SSR provided that you limit the current and voltage which is required to drive the control (gate signal) of the SSR and the speed of operation is within the rated speed of the output relay.
I hope the above should provide some ideas for your project.

Moore, Scott A

TTL is a transistor logic. ie. 0 or 5 volts at very low current which you must limit. Relay output is nothing more than a switch which passes a source voltage when activated. Current depends on source of supply. Relay output should be fine for SSR's. What type of SSR's are you using?


Jeffrey Eggenberger

Relay outputs use little cube relays that are mounted on the card. The outputs are mechanical contacts in the relays. The advantage is not as
subject to voltage transients.

TTL outputs are solid state instead of contacts. The advantage is not subject to wearing out like mechanical contacts.


Rajesh Kenge

hi aniruddha

what is ssr?? Any way i shall tell the plus and minus of relay o/p's and TTL o/p's

Relay o/ps are used to drive loads like solenoid valves, contactors etc. They provide a means of isolation from the PLC o/p and load.

also the current carrying capacity is more.

The one drawback of using relay o/p's is as electromechanical coils are involved they is a delay of around 250 ms from the o/p begin on from PLC and actual o/p. That's it.

As regards TTL o/p's give I suppose 5 V when the o/p is high. They are used to drive some devices like transistors etc. The current carrying capacity of these o/p's is very low.

The advantage is as no electromechanical devices are there the delay as in relay o/p's is not in TTL o/ps.

Hope this will solve u r problem.

any help u can contact me at [email protected]