Splitting differential output from magnetic scale - Page 1
Forum section: electronics / projects

author: Jester
author_rank: Frequent Contributor
timestamp: February 07, 2024, 01:51:01 am
content: This question relates to the differential signaling used between magnetic scale readers and the DRO's that display the data.Each magnetic reader generates two differential pairs of quadrature data; A+, A-, and B+, B- (RS422 signal levels), these connect to a DRO to display the position of the reader along the magnetic scale.The differential quadrature signals from the read head are in a shielded cable terminating in a DB9 that plugs into the display (DRO).I have an application were I need to feed the reader or pickup signal to both the DRO (as originally designed) as well as to an FPGA card about 2m away in a fairly noisy (electrical) environment.Option 1 is to purchase DB9 splitter cable and then feed the split cable to the FPGA card with a RS422 to TTL converter just before the FPGA board., effectively drive both destinations with the original signal.Option 2 is to make a splitter PCB at the display (DRO) and use a RS422 driver on the splitter board to drive the FPGA via some sort of shielded cable possibly an HDMI cable.Thoughts please, is option 2 overkill with no real benefit? If I do use option 2 not sure if a differential in, differential out (RS422) IC is readily available and inexpensive?

author: ajb
author_rank: Super Contributor
timestamp: February 07, 2024, 02:43:50 am
content: Option 1 is probably fine. RS422 is meant to be a one driver to many receivers arrangement anyway, the only potential issue is down to the locations of the split and the termination resistor. Presumably the DRO has a termination resistor on each of its inputs, so ideally you'd go out from the scale, to a splitter at (as in, plugged directly into) the receiver at the FPGA board, and then from the other leg of the splitter to the DRO. That minimizes the stub length and keeps the termination resistor at the end of the line where it belongs. Any other arrangement of a cable split risks reflections from the end of the line potentially appearing as spurious edges that the DRO/FPGA may interpret as position changes, but with only a couple of meters of cable the risk of that being a problem is relatively low.Either option has the possibility of introducing noise into the system, depending on a lot of factors, via the additional cable to/through the noisy environment, but option two would allow you to introduce galvanic isolation between the two or three systems, which may (or may not!) help.Quote from: Jester on February 07, 2024, 01:51:01 amYou'd just use a receiver back-to-back with a driver -- or more likely, two transceivers with their DE/RE lines strapped appropriately. RS-485 transceivers are widely available and will work fine for RS-422 (and are often marketed for both), dedicated drivers/receivers are less common. If you wanted isolation you would just stick a digital isolator (or optocoupler) in between the receiver and driver with an isolated power supply. Dedicated redriver ICs may exist, but would be a niche product.Quote from: Jester on February 07, 2024, 01:51:01 amIn many cases, shielding isn't even necessary for RS-485/422, after all that's a big reason for them being differential. You could use an HDMI cable if you want, but Cat5 (or Cat-whatever, optionally shielded) cable would be more convenient, and is commonly used for RS-485. You can use two pairs for signal, and both wires in a third pair for common.