Wiegand to OSDP retrofits: Anyone else seeing latency issues on existing old wiring?

[sanhaowangluo]

Morning gents,
 

We’ve been pushing hard to migrate most of our commercial clients away from legacy Wiegand and over to OSDP v2 (Secure Channel) for the obvious encryption benefits.
 

However, I'm running into a frustrating real-world issue. In a perfect lab environment, the RS-485 backbone handles the two-way OSDP handshake beautifully. But on actual retrofits—where we are forced to reuse the client's existing, ancient 22/6 untwisted alarm wire—I'm noticing a slight, but perceptible, latency when swiping the card before the door actually fires.
 

It feels like the reader and controller are struggling with packet loss/retries over the degraded cable before finally authenticating the secure channel.
 

I know the textbook says RS-485 is good for 4,000 feet, but what is your actual safe distance limit when pushing OSDP over crappy legacy wire? Do you guys just bite the bullet and pull new shielded twisted pair, or are there any termination tricks (besides the standard 120-ohm resistor) to clean up the signal on old cables?
 

Curious to hear your field experiences.

[al-yeti]

5 hours ago, sanhaowangluo said:

Morning gents,
 

We’ve been pushing hard to migrate most of our commercial clients away from legacy Wiegand and over to OSDP v2 (Secure Channel) for the obvious encryption benefits.
 

However, I'm running into a frustrating real-world issue. In a perfect lab environment, the RS-485 backbone handles the two-way OSDP handshake beautifully. But on actual retrofits—where we are forced to reuse the client's existing, ancient 22/6 untwisted alarm wire—I'm noticing a slight, but perceptible, latency when swiping the card before the door actually fires.
 

It feels like the reader and controller are struggling with packet loss/retries over the degraded cable before finally authenticating the secure channel.
 

I know the textbook says RS-485 is good for 4,000 feet, but what is your actual safe distance limit when pushing OSDP over crappy legacy wire? Do you guys just bite the bullet and pull new shielded twisted pair, or are there any termination tricks (besides the standard 120-ohm resistor) to clean up the signal on old cables?
 

Curious to hear your field experiences.

Blame it on the electrician and don't worry about it lol

[sanhaowangluo]

On 21/03/2026 at 14:56, al-yeti said:

Blame it on the electrician and don't worry about it lol

Haha, the universal 'get out of jail free' card!
 

I might just have to use that if the client complains about the half-second card read delay. I was secretly hoping someone here had a magical impedance-matching trick for 20-year-old alarm wire, but I guess 'blame the sparky' is the gold standard for a reason!
 

Enjoy the rest of the weekend, mate.

[al-yeti]

3 hours ago, sanhaowangluo said:

Haha, the universal 'get out of jail free' card!
 

I might just have to use that if the client complains about the half-second card read delay. I was secretly hoping someone here had a magical impedance-matching trick for 20-year-old alarm wire, but I guess 'blame the sparky' is the gold standard for a reason!
 

Enjoy the rest of the weekend, mate.

I think the delays are rare tho mostly find they all open , I don't do much access , but go through plenty , ususally where the customer doesn't want to spend much is where maybe this problem will occur amongst other issues 

[james.wilson]

rs485 'should' have termination resistors. Might help if your cable is truly shocking. Ive run rs485 2.5 miles as a test in the ancient past but on twisted pair belden

[sanhaowangluo]

You hit the nail on the head, @al-yeti. It always boils down to the budget. When clients want the high-end encrypted OSDP readers but refuse to pay for a proper cable pull to support them, we end up fighting these exact gremlins. Cheap wire always ends up costing more in labor!
 

@james.wilson - 2.5 miles is absolutely insane! That really speaks to the magic of using proper Belden twisted pair. You are completely right about the termination resistors; I made sure we have the 120-ohm resistors fitted across the bus. It definitely stabilized the connection, but I think the untwisted nature of this 20-year-old alarm wire is just struggling with the constant two-way polling of the OSDP Secure Channel.
 

Appreciate the sanity check from both of you. Guess it's time to have that tough conversation with the client: if they want that instant card read speed, we need to pull new cable. No magic tricks this time.

[james.wilson]

I'm sure you can balance it with different values. I'd test with different values and see but you might be able to calculate the swr. Can you add capicitance on the run? 

Your big issue is going to be reflection. Stick a scope on it and look. Or go back or recable