Title says it all, is the SDI protocol duplex or simplex? I cant seem to find a straight anwser on the web, I suspect it is simplex but am unsure.
I don't know if this is exactly what you are asking, but in the early days of HD video, an early version of a 3 Gbps SDI was created using a pair of cables, each a 1.5 Gbps link. Later (probably when 3 Gbps transceivers became more available), the "3G" links were a single cable. Unfortunately, two incompatible "standards" emerged: level-A and level-B. One was adapted from the 2 x 1.5 Gbps version, and another was a essentially an up-speed to 3 Gbps. Some equipment transmits either, but most (especially older) only transmits one or the other, and it is often difficult to determine which, as the specifications may have been written before each "standard" was even called level-A or level-B. Likewise, some receiving devices accept only one format, some both, and yet others have two inputs and will accept the dual-link version as well as one or the other single link version. Fun stuff.
The 6G (for 4k@30 Hz) and 12G (for 4K@60 Hz) versions are single link and much more likely to be fully compatible between vendors. 3G is a mess.
It is always unidirectional.
I'm asking about this for a product demonstrator. The company I work at has a chipset they like to demonstrate at trade-shows with a video link. One of our old digital guys, no longer working here, built an entire FPGA system that takes HDMI and converts it to a serial datastream so we can transmit it. The FPGA also handles all things like handshaking, since our link is fully one-way - there is no way to do any handshaking.
However, the current demonstrator uses not-even-full-HD resolution, and as a result doesn't even come close to stressing our link. We want to update this to a 4K link, but lost the team that did the FPGA design. What I was no hoping was that we could use these HDMI-SDI converters. However, I've since been told by one of the old guard at the place I work that SDI does a low-speed handshake before going full blast, and that is why we needed the FPGA solution.
I've since been told by one of the old guard at the place I work that SDI does a low-speed handshake before going full blast
They don't know what they are talking about.
SDI is a stream you can disconnect/reconnect at any time and recover the next frame from. Auto-detection of the protocol and rate might make that many frames depending on the implementation, and some receivers are just bad at it needing seconds. The only real limitation with SDI (particularly considering what you describe) is you have to stick within a limited set of broadcast/cinema standard frame rates and resolutions.
SDI is purely unidirectional. You can pass it through a distribution amplifier, split it one-to-many, plug and unplug cables, or whatever. It won't skip a beat.
This represents a major difference to HDMI. With HDMI, the display or sink device communicates its EDID to inform the source what its capabilities are such as resolutions, color formats, bit depth, and more. With SDI, the transmitter just sends the video stream and the receiver must be compatible or it won't work. Again due to no sink-to-source communication, SDI has no HDCP or other content protection. This means that most or all HDMI-to-SDI converters will not perform HDCP negotiation on the HDMI side. That implies that a source such as a Blu Ray player may not transmit video to an HDMI-SDI converter, or limit to 480P if it does work at all. There are ways around this, but it is a headache if you try to use HDMI/SDI conversion outside of (HDCP-free) pro video. Computers typically do not require HDCP, but if it is not present, then some types of content can become restricted, even things like web-based playback of Netflix or Youtube.
It might be the HDCP handshake that necessitated the custom solution your company created.