In-line device turnaround time

[sjackman]

Hi Gerry,

There's two ways that an inline device could be designed. For simplicity, let's consider a repeater that has just one responder port and one command port.

1. A microcontroller sits between the two ports. Each port is connected to a UART of the microcontroller. There is no direct connection between the two ports. The microcontroller receives on one port and transmits to the other.

2. A transceiver is connected to each port, and the two transceivers are connected to each other with nearly nothing in between, except perhaps a little logic. A microcontroller listens on each port to control the transmit/receive direction of the two transceivers, but doesn't transmit bits itself.

I've worked on devices of both types, and each has its advantage. The latter maintains the exact wave form of both DMX and RDM from the controller to devices on the command ports and vice versa.

To borrow some terminology from network switching (which is really what we're doing here) method #1 is a store-and-forward switch (each byte is stored and retransmitted, introducing a delay of 44 µs) and #2 is a switching fabric.

You're correct that a store-and-forward implementation has no timing issue between the transceiver turnaround and the microcontroller transmitting the first byte on the responder port. A switching-fabric implementation must however consider the turnaround delay.

Cheers,
Shaun