Differnetial signal recevers, double question - Page 1
Forum section: electronics / beginners

author: Infraviolet
author_rank: Super Contributor
timestamp: August 17, 2024, 07:09:12 pm
content: I've got Two questions about receiving differential signals for which I could use some advice:1. What sort of chip is usually used to receive them? Effectively one is looking for behaviour similar to an op-amp or comparator, but less exacting in some ways and mroe exacting in others.With an op-amp you need analogue linearity, a dfferential receiver can simply output Vcc or Gnd depending on the state of the two incoming signals. An op amp can also be quite slow in terms of slew rate, for a reciever you need a fairly fast rise time (for my situation I'm looking at the output going Vcc to Gnd or Gnd to Vcc, with Vcc=5V, in < 50ns). Op amps aren't too good with being subjected to inputs voltages beyond their rails either (I'm expecting differential signal voltages of 0V to 12V, with a bit of margin beyond that for noise spikes, but also need to be able to cope with differential signals using the 0V to 5V range, again with a risk of spikes going beyond that), often rated in their abs max to only cope with Vcc+0.5V to Gnd-0.5V, using a resistove dvider before an op-amp surely just makes the higher differential voltage smaller and more at risk of being affected by noise?With a comparator you need it to swing its output once the two incoming voltages are only a tiny amount (mV or tens of mV, or less) apart in either direction, if you want hysteresis with one you have to use a resistor network with feedback from the output, a differential receiver would welcome a fairly large hysteresis and wouldn't need it to be precise. For a receiver with Vcc=5V, a 2V +/- 50% hysteresis could do fine, much less exacting a requirement than a comparator which you want to always say whether voltage A is above or below voltage B even with the two only a tiny amount apart.I have seen, on electronics distributor websites, lots of CAN or RS*** protocol specific receivers, but it is my understanding these will naturally interpret particular protocols and behave in special ways if particular states last for too long or if the data coming along the differential wires doesn't conform to the protocols' standards.What then are my options for a fairly generic type of differential receiver which doesn't require a specific protocol, but simply outputs high (5V, or atleast > 4.5V) or low (Gnd, or atleast < 0.5V) depending on which of it's inputs is higher than the other, a small hysteresis is acceptable but a volt or two of hysteresis would be preferred)? In the same way as one can get, from many different manufacturers an SOIC quad op-amp with the same physical pinout (1=outA, 2=in-A,3=in+A,4=Vdd,5=in+B...) and with 10MHz of GBWP and rail-to-rail inputs and outputs, are there differential receivers available from a range of different sources subject to a common set of performance parameters and pinout.2. Differential bus terminations, what happens if you don't?If you want to produce a differential bus which can branch a lot, so you can't really have two opposite ends with termination resistors on them the way CAN does for a long linear topology, and at each receiver the bus wires are attached only to the high impedances of the receiver chip itself, can it work ok? If one is ruling out particularly fast transitions, say edges slowed to take about 50ns each, and the bus distance is fairly short ( <10 metres) do the signal reflection situations which termination resistors damp out cease to matter? Does the lack of termination resistors make the system less resilient to environmental electrical noise in any other ways, or are terminations only ever there to deal with signal reflection?Thanks

author: Benta
author_rank: Super Contributor
timestamp: August 17, 2024, 07:24:29 pm
content: RS-422 and RS-485 receivers were made for his.No reason to reinvent the wheel.

author: Infraviolet
author_rank: Super Contributor
timestamp: August 17, 2024, 09:50:53 pm
content: RS422 and RS485 receivers are protocol free in terms of timings, method of encoding (which is your 1 vs which is your zero) and data structures being transferred?

author: Benta
author_rank: Super Contributor
timestamp: August 17, 2024, 09:56:05 pm
content: Absolutely.Differential inputs, logic output. Nothing else, pure level translation. The absolute classic is the SN75176https://www.ti.com/lit/ds/symlink/sn75176a.pdfBut there are lots of newer types available with better ESD/transient protection or isolation or...

author: PGPG
author_rank: Frequent Contributor
timestamp: August 17, 2024, 10:05:02 pm
content: Didn't tested it, but I believe that without termination you can still communicate provided that you do it 10 times slower (you need to wait until signal stabilizes after few reflections). Termination limits the bus length as constitutes a voltage divider with wire resistance reducing signal level. Because of this RS485 is limited to about 1km. I believe that using standard RS485 drivers without bus termination, reducing communication speed you can communicate at 10km or longer.RS485 standard has integrated in it up to 7V ground difference between source and destination.

author: David Hess
author_rank: Super Contributor
timestamp: August 18, 2024, 12:55:45 am
content: Quote from: Infraviolet on August 17, 2024, 07:09:12 pmA differential line receiver is a voltage comparator but with lower gain allowing higher speed because it is not expected to operate with small voltage differences. The circuit is similar to emitter coupled logic, but with both inputs of the input differential pair brought out. With ECL, one of the inputs is tied to an internal voltage reference.

author: Infraviolet
author_rank: Super Contributor
timestamp: August 18, 2024, 11:58:21 pm
content: ThanksGood to see that the SN75176A has separated driver and receiver pins, so you can connect only the receiver ones in cases where the bus is already driven by other parts. Now I understand they're pure level translators, I'll have a look at other RS485 and RS422 receiver options.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 19, 2024, 12:27:54 am
content: Yeah, someone saying "it's RS-232 compatible" is often used to imply something much more than the literal standard describes. To be fair, later revisions of 232 (or closely related standards, I forget which) did suggest traditional asynchronous serial usage, connector pinouts, and the handshaking signals. But still very general, probably in part to still be inclusive of all the diversity of UARTs and applications extant in the world at the time (and foreseeable future). Like, handshaking signals don't have to be used for that, you can bit-bang them, or even extract power (hence serial mouse, among other things), etc.But the basic core is, and always has been, just the line signaling. What you pulse onto it, and how fast and often, is entirely up to you.The named buses, typically (always?) have some protocol definition, though even then, CANbus transcievers are pretty simple (it's not much different than RS-485 after all, as if TXD were wired to transmit-enable instead) and the protocol is handled in the PHY (usually hardware) hence why it's so convenient to get an MCU with the specific peripheral, if you're going to be working with it.Others, may only define part of the interface. LINbus is, I *think*, similar to CAN, but slower, RS-232 levels (ish?), and uses UART packets. Modbus doesn't define an interface, and indeed can go over anything from UART (RTU) to global internet (TCP). Profinet, EtherCAT, etc. use Ethernet (the standard transmission and routing medium) but use a different transport (and maybe network) layer: they're not TCP or UDP. (Or maybe they are, but it's the next layer up? Not sure.)The OSI protocol stack model is a useful framing device. It's an academically praised concept, but a more practical reading finds it more as another useful tool, than some holy pattern to follow.The stack isn't all that useful or applicable to a lot of cases -- many very common and/or specialized protocols are highly integrated (e.g. Wiznet chips going from media straight to transport or application layer), or simply not many layers are needed (a typical Modbus implementation uses UART PHY, practically a shim of a driver for hardware access, then a simple network layer and that's it). It's somewhat of a relic from the days when whole boards of discrete logic were needed to implement the lower layers, and significant effort went into crafting and proving out software implementations and drivers of the upper layers; and when no one really knew what a good protocol was, so they made each layer general and flexible so everyone can try everything everywhere. Things are pretty settled these days, with lots of excellent protocols to choose for various applications, from inter-MCU communications to global internet, and handy ICs and FOSS projects implementing almost everything.Still, the stack is a useful frame of reference, to be able to call out or identify what parts of a protocol are mandatory, what's variable, how many interface options there are, etc. So, considering the above in this light, try and figure out which aspects are which layers, what are required and what are variable or optional, etc. A lot of standards don't describe themselves in these terms, leaving you to puzzle it out -- and network standards are famously(?) opaque in their adoption of peculiar, unintuitive or even contradictory terminology (what's a client or serverreally, anyway?!), so, it is often a puzzle to read up and down and try and figure out what they're doing in terms of conventional framing like the OSI model. Sometimes this may still not yield unambiguous interpretations, but mostly it works out.Tim

author: PGPG
author_rank: Frequent Contributor
timestamp: August 19, 2024, 01:06:31 am
content: Quote from: Infraviolet on August 18, 2024, 11:58:21 pmI'm not sure if understand well what you mean, but for me transmitter and receiver are more effectively separated in SN75179 than in SN75176.

author: tooki
author_rank: Super Contributor
timestamp: August 19, 2024, 11:48:03 am
content: Quote from: Infraviolet on August 18, 2024, 11:58:21 pmFWIW, in one project I did, I chose the Analog Devices LTC2863-2.I've attached an overview of all the pin-compatible 8-pin full-duplex transceivers I could find. Even if you don't need full-duplex, most of them have half-duplex versions, usually in the same datasheet.

author: Terry Bites
author_rank: Super Contributor
timestamp: August 20, 2024, 04:11:10 pm
content: Why would you leave off the termination?Read up on some basic transmission line theory.