Best way to create a DP4T switch for RS-422 (differential, negative voltages)? - Page 1
Forum section: electronics / projects

author: I wanted a rude username
author_rank: Frequent Contributor
timestamp: September 02, 2023, 10:25:04 pm
content: The application is switching simplex RS-422 (transmit only) from one server to four clients. Can't use RS-485 etc., and it needs to survive the +/- 6 V of RS-422. On the plus side, there's a microcontroller in charge of everything, so sequencing break-before-make is straightforward.My usual go-to, USB analogue switches, are not suitable here ... and I'd rather avoid building the switch from discretes if possible.Suggestions for solid-state solutions?

author: artag
author_rank: Super Contributor
timestamp: September 02, 2023, 11:57:07 pm
content: By transmit-only, do you mean that the server provides an RS-422 output signal and it must be sent to only 1 of 4 clients at a time, which also have RS-422 receivers ? And there is no data going the other way, or else it's already controlled appropriately ?

author: ledtester
author_rank: Super Contributor
timestamp: September 03, 2023, 12:15:19 am
content: Would a line driver chip like this work for you?https://www.ti.com/lit/ds/symlink/sn65lbc174a-ep.pdfMight need two of them. Similar chips include the MAX304x series:https://www.mouser.com/datasheet/2/609/MAX3040_MAX3045-3128606.pdf

author: Benta
author_rank: Super Contributor
timestamp: September 03, 2023, 12:34:18 am
content: Vishay/Siliconix DG409 or DG509 should do the job.

author: I wanted a rude username
author_rank: Frequent Contributor
timestamp: September 03, 2023, 03:17:05 am
content: Quote from: Benta on September 03, 2023, 12:34:18 amThis looks like the best option, not least because it will still work when the requirements invariably change to running the transmission in the opposite direction.These multiplexers will clip the input voltage to the supply voltage, right? I.e. if supplied at 5 V (single-ended supply), they will rail a +/- 6 V input down to an output in the range 0 - 5 V? (The datasheet for the similarMAX339is not as shy about stating this.) And the lower the supply voltage, the higher the Rds(on)? Looks like a couple of DC-DC converters are also on the cards then ...

author: AndersJ
author_rank: Frequent Contributor
timestamp: September 03, 2023, 04:42:29 am
content: Quote from: I wanted a rude username on September 02, 2023, 10:25:04 pmWhy is +/- 6V mentioned?RS422 uses 0-5V signals.Use the Driver Enable input of a RS-485 tranceiver to select the destination.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: September 03, 2023, 06:11:06 am
content: Quote from: I wanted a rude username on September 03, 2023, 03:17:05 amADI also have new parts called 'Beyond-the-Rails' analog Muxes.

author: Benta
author_rank: Super Contributor
timestamp: September 03, 2023, 11:37:24 am
content: Quote from: I wanted a rude username on September 03, 2023, 03:17:05 amThey don't consume much power. ICL7660 can make the negative rail.

author: macboy
author_rank: Super Contributor
timestamp: September 03, 2023, 12:58:44 pm
content: Quote from: AndersJ on September 03, 2023, 04:42:29 amYou'll need to look at the spec again. It is +-6 V (max) differential signaling.Quote from: AndersJ on September 03, 2023, 04:42:29 amThat's a great suggestion.

author: artag
author_rank: Super Contributor
timestamp: September 03, 2023, 02:05:58 pm
content: Quote from: Benta on September 03, 2023, 12:34:18 amAre CD4066 switches cheaper ? DG509 always used to be the high end. And a 4066 will run on 15v or +-7.5V

author: AndersJ
author_rank: Frequent Contributor
timestamp: September 03, 2023, 05:10:46 pm
content: Quote from: macboy on September 03, 2023, 12:58:44 pm”Differential”, yes, absolutely.But the OP did not say that,and instead implied negative voltages with respect to signal ground,which is incorrect.Concerning 5/6 vs 5 Volts,normaly Vcc is 5V, and sometimes lower, but rarely above 5.

author: I wanted a rude username
author_rank: Frequent Contributor
timestamp: September 03, 2023, 05:44:24 pm
content: Quote from: AndersJ on September 03, 2023, 05:10:46 pmI implied negative voltages with respect tothe device's ground... which is true, right?The standardstates:> common mode voltage (Vcm) range of +7 volts to −7 volts... and furthermore requires the device survive voltages between -12 and +12 V, with a maximum differential at any time of 12 V.Quote from: PCB.Wiz on September 03, 2023, 06:11:06 amThis seems like the best solution suggested so far. TheMAX14788is a DP4T switch that integrates the charge-pumps required to switch up to +/- 25 V. Single chip (plus three capacitors) for about USD $3.

author: AndersJ
author_rank: Frequent Contributor
timestamp: September 03, 2023, 05:58:53 pm
content: Quote from: I wanted a rude username on September 03, 2023, 05:44:24 pmPoint taken.I did not consider common mode voltages.Sorry ☺️