Shared Communications Bus - RS-422 or RS-485

RS-422 is normally a point-to-point interface. It is one line in each direction, but using balanced pairs instead of a TTL signal. You would not normally connect multiple receivers or transmitters to an RS-422 bus, as the standard practice is that each transmitter is always driving the pair it is attached to - there is no multi-drop.

RS-485 is easy from a PC using appropriate USB devices. We make heavy use of FTDI's chips and cables - they handle the drive enables for the RS-485 automatically so that from the PC side you just read and write to the serial port.

The reception of the last byte from a slave is not finished until the stop bit has been properly received by the master - that means at least half-way through the sending of the stop bit. Then there is a delay before the data gets sent back to the host PC, a delay through the kernel and drivers before it reaches the user program, time for the program to handle that message, time for it to prepare the next message, delays through the kernel and drivers before it gets to the USB bus, latency in the USB device that receives the USB message and then starts transmitting. There can be no collision unless all that delay is less than half a bit time. And no matter how fast your computer is, you are always going to need at least one full USB polling cycle for all this, which for USB 2.0 is 0.125 us. That means that if you have a baud rate of 16 kbaud or higher, there is no possibility of a collision.

You need to do some calculations to see if you can get enough telegrams and enough data through a single serial port. You'll be hard pushed to find a USB serial port device of any kind that goes above 3 Mbaud, and you need to be careful about your selection of RS-485 drivers for those kinds of rates. You will also find that much of your bandwidth is taken up with pauses between telegrams and reply latency, unless you make your telegrams quite large.

When we have made testbenches that required serial communication to multiple parallel devices, we typically put a USB hub in the testbench and use multiple FDTI USB to serial cables. You only make one (or possibly a few) of the testbenches - it's much cheaper to use off-the-shelf parts than to spend time designing something more advanced. You can buy a /lot/ of hubs and USB cables for the price of the time to design, build and program a custom card for the job. It also makes the system more scalable, as the communication to different devices runs in parallel.

We have also done systems where there is a Raspberry Pi driving the hub and multiple FTDI converters. The PC is connected to the Pi by Ethernet (useful for galvanic isolation), and the Pi runs forwarders between the serial ports and TCP/IP ports.

To be fair, I don't recall any testbenches we've made that needed more than perhaps 8 serial ports. If I needed to handle 80 lines, I would probably split things up - a Pi handling 8-10 lines from a local program, communicating with a PC master program by Ethernet.

However this depends on the speed of slave too, because it could be slow to move *its* direction from TX to RX. If the master starts transmitting after the stop bit from the slave, but *before* it changes *its* direction from TX to RX, the first bytes could be corrupted.

Unfortunately, not all UARTs in MCUs are able to drive automatically the DE (Drive Enable) signal, so it sometimes happens that DE is a normal GPIO. If you are lucky, you have the TXC (transmit complete) interrupt that fires *after* stop bit is transmitted, a safe time to move DE signal.

In this case interrupt delay is short, but you could have other active interrupts that occasionally could delay the TXC interrupt for some time.

True, and it that is an important point.

I think the OP has FPGA's for the slave side of the equation, so there should not be a delay in switching their drivers off after the last byte is sent. Even if it is a microcontroller and has no hardware control for the DE line, pretty much any half-decent microcontroller from this century has a TXC interrupt and can react and turn off the driver within a few microseconds. I am assuming he is not trying to do this project using PIC's or 8051's !

If this kind of thing is a risk, then it's not hard to put a short delay on the PC side between receiving a reply and sending out the next telegram. But it's good that you brought it up, so that the OP can decide if it /is/ a risk.

That is simply not true. Data sheets for RS-422 devices often show multidrop applications and how to best terminate them.

I've yet to be convinced of this. Admittedly, my last interaction with RS-485 was many years ago, but there were some four or five different devices being integrated with a PC, and no two of them handled it the bus the same way. The PC in particular, would cut on and off the driver in the middle of bits. No one put a bias on the bus, so it was indeterminate when no one was driving. It was a horrible failure.

Every time I've seen this discussed, the driver control has been an issue.

That's not sufficient. Everyone's halfway is a bit different and start bit detection may not be enabled on some device when the next driver outputs a start bit, or the last driver may not be turned off when the next driver starts.

If your numbers are accurate, that might be ok, but I'm looking for data rates closer to 1 Mbps. Admittedly, I have not done an analysis of what will actually be required, but 128 UUT, or possibly 256, can do a lot of damage to a shared bus. At 1 Mbps, 128 UUT results in an effective bit rate maximum of 7.8 kbps. With 256 UUTs, that's 3.9 kbps. No, I don't think this will work properly at much slower speeds than 1 Mbps. At 16 kbps, the effective rate to each UUT is just 62.5 bps, not kbps.

I assume by "telegrams", you mean the messages. They will be small by necessity. The protocol is interactive with a command message and a reply message. Read a register, write a register.

USB hubs are a last resort. I've found many issues with such devices, especially larger than 4 ports.

There is a possibility of using an rPi on an Ethernet cable to the PC with direct comms to each test fixture board, but that's more work that I'm interested in.

That's the advantage of the shared bus. No programming required, other than extending the protocol to move from "selecting" a device on the FPGA, to selecting the FPGA as well.

RS-422 is not multidrop. Occasionally you will see multiple receivers on a bus, but not multiple transmitters.

Of course the same driver chips can be used in different combinations of wiring and drive enables. An RS-422 driver chip can be viewed as two RS-485 driver chips - alternatively, a RS-485 driver can be viewed as an RS-422 driver with the two differential pairs connected together. Really, all you are talking about is a differential driver and a differential receiver.

So yes, you can do multidrop using an RS-422 driver chip. But it is not RS-422, which is a point-to-point serial bus standard.

I can tell you it works perfectly with FTDI's RS-485 cables - every time, every OS, regardless of the software. Some RS-485 drivers rely on RTS for the drive enable - this was the standard for RS-232 to RS-485 converters from the old days of 9-pin and 25-pin serial ports on PC's. With such drivers, it is certainly possible to get things wrong. With the drive enable handled directly by the UART hardware on the USB chip, it is /far/ harder to get it wrong.

I would expect there to be many alternatives to FTDI that work similarly well, but that's the ones we generally use.

"At least half-way" means "at least 50% of the bit time". As long as the start bit from the next message is not sent until at least 50% of a bit time after the stop bit is detected, it will not conflict and all listening devices will be ready to see the start bit. (Devices that needed two stop bits haven't existed in the last 50 years.)

You asked specifically about bus turnaround at the host side - I assume that is because on the slave devices, you have control of the drive enables and bus turnaround happens with negligible latency.

USB serial ports generally use the 48 MHz base USB reference frequency as their source clock to scale down by a baud rate divisor, and common practice is 16 sub-bit clocks per line bit (so that you can have multiple samples for noise immunity). Thus baud rates of integer divisions of 3 MBaud are common. Certainly the FTDI chips handle 1, 2 and 3 MBaud. (I haven't had need of such speeds with RS-485, but have happily used the common 3v3 TTL cables at 3 MBaud.)

As long as you are /above/ 16 kbaud, you should be fine (at the PC side). At 1 Mbaud, you do not need to worry about the PC starting a new telegram before the last received stop bit is completed.

Telegram, message, packet - whatever term you prefer. At faster baud rates, the inevitable pauses between messages take proportionately more of the total bandwidth. Longer messages will be more efficient. But you'll have to do the sums yourself to see what rates you need, and whether or not this will be an issue.

We find they work fine - I have very rarely seen any issues with off-the-shelf hubs, regardless of the number of ports. (They are almost all made with 1-to-4 hub chips, which is why hubs are often found in sizes of 4 ports, 7 ports, or 10 ports.)

A key complication with multiple serial ports on hubs is if you are using Windows, it can be a big pain to keep consistent numbering for the serial ports. You may have to use driver-specific libraries (like FTDI's DLL's) to check serial numbers and use that information. It's far easier on Linux where you can make a udev configuration file that gives aliases to your ports ordered by physical tree address.

Or you could use one Pi for a set of boards - whatever is physically convenient.

If you are familiar with socat, the Pi doesn't necessarily need any programming either. (In our case we wanted some extra monitoring and logging, which was more than we could get from socat - so it was a couple of hundred lines of Python in the end.)

Rather than make a huge shared bus, I wonder if you could move the test controller from a PC to a small microprocessor board that controls two FPGA's or whatever. Then use a bunch of separate boards of that type, communicating with a PC using some method that doesn't have to be especially fast. The microprocessor board could be something like a Raspberry Pi Pico, which costs $4 and can run Mecrisp, if that is what your software is written with. It is quite a powerful little board.

Not sure of your point. Multi-drop is multiple receivers on a single transmitter. Multi-point is multiple drivers and receivers. Look at a few references. Even wikipedia says, "RS-422 provides for data transmission, using balanced, or differential, signaling, with unidirectional/non-reversible, terminated or non-terminated transmission lines, point to point, or multi-drop. In contrast to EIA-485, RS-422/V.11 does not allow multiple drivers but only multiple receivers."

Sure, but the point is, nothing in RS-422 precludes multiple receivers, and in fact, every reference I've found (not paying for the actual spec) shows multi-drop receivers.

I don't believe that is correct. If you have a copy of the spec to share, I'd love to look at it. I might have one myself, but it would be a paper copy somewhere unknown. The diagrams showing multi-drop RS-422 is so ubiquitous, I expect they are from the standard itself.

So, what range of speeds have you used? It is actually the UART hardware that *does* get it very wrong by working off the transmitter empty signal, which changes in the middle of the bit. The control has to be specially designed to transition at the *end* of the stop bit of the transmitted character. Knowing when it is ok to enable the driver has the same problem. The data is "received" in the middle of the stop bit. So the enable has to be a half bit time later, at the end of the stop bit.

None of this matters to me really. I'm going to use more wires, and do the multi-drop from the PC to the slaves on one pair and use RS-422 to multi-point from the slaves to the PC. Since the slaves are controlled by the master, they will never collide. The master can't collide with itself, so I can ignore any issues with this. I will use the bias resistors to assure a valid idle state. I may need to select different devices than the ones I use in the product. I think there are differences in the input load and I want to be sure I can chain up to 32 units.

You don't seem to understand that there is nothing timing from the start of the bit. The timing is from the first detected low of the start bit. From there, all timing is done by an internal clock. Check the math, you don't get 50% of the stop bit, guaranteed. That's why they call it "asynchronous" serial.

I know the master has the most trouble with this. The slaves tend to not have a problem because they are operated by MCUs and can wait a bit time before replying, or even a character time. I suppose they don't have any magic on turning off the driver though, but early is the easy way and generally doesn't cause a problem. The master has trouble on both ends of it's message, needing to be careful to not turn on the driver too soon and not turning it off too late to clobber the reply.

At some point you have to worry with the line waveforms. So too fast can cause problems when using *lots* of receivers.

Not entirely. The master has to turn *off* the driver before the slave replies. At higher speeds that's a problem. But it all depends on how it is being done. This is why I'm going with two busses, one for master transmit and one for master input.

Not sure what delays you are talking about. Every message is either selecting a slave, or reading a register or writing a register. You are probably thinking like a code banger where you have to worry with software delays. The protocol at the interface to the UUT is serial, at 30 MHz and the turn around is so quick, I had to use a mux to select the first bit and shift the rest of the data from a shift register. All in all it is under a μs for a transfer, and the data can be sent to the PC in ASCII Hex format which even at 1 Mbps is much slower. I can't say what the delays on the PC are. It's never been of interest, but I can't imagine it's much.

Exactly, and I find combining them like that has issues.

Yet another reason to avoid such complications. The reality is there's no gain. The multi-drop is the right way to go here.

But it's yet another piece to keep working. Much easier to just use the multi-drop. I will keep that idea as a backup plan. But getting RS-422 on an rPi is a hassle. That would need to be a hat, or a shield or whatever they call daughter cards on rPis. Last time I checked, it was hard to find rPis. They are part of the unobtainium universe now, it seems.

A couple hundred lines I'd rather not write.

Thanks for the comments.

I'm lost. How is this any better? The data collection is running on the PC, so there still has to be communications. If I want to make changes to the test program, it's one place, not 32 places, or 128 places. All they would be doing is acting as a serial port concentrator. They went out 40 years ago!

Well, I always like doing stuff in software instead of hardware. Serial port concentrators are still around and I posted a link to one in your other thread. It seems like a reasonable approach too. Oddly, a quick web search doesn't find any big cheap serial to ethernet ones, but maybe you could use USB hubs and FTDI-like cables.

The idea of using an MCU is to move almost everything speed critical away from the PC. The MCU only has control the FPGA's and transfer transfer digested data upwards to the PC. By having some fanout you could potentially control 1000s of UUT instead of 80 from a single PC, without having to build anything special. It would just mean plugging some off the shelf boxes together, and writing some code.

You're more comfortable with hardware than I am, so maybe that approach has less attraction for you than it does for me.

What hardware can you replace with software??? You use *different* hardware and then have to add new software. That's not an improvement.

I'm getting tired of discussing this. Your added hardware solves no problems. Having 32 serial ports just makes the software more complex and saves nothing.

What hardware??? It's a serial port either way. You want to add a middle man that accomplishes nothing. You talk about controlling 1000's of UUTs, but there is no need for that.

What I need at this point, is the mechanical details of how to design a board to fit into a Eurocard chassis and how to figure out all the bits that go with it.

Do you mean that RX interrupt triggers in the middle of the stop bit and not at the end? Interesting, but are you sure this is the case for every UART implemented in MCUs?

I wouldn't be surprised if the implementation was different for different manufacturers.

In my experience, termination resistors at each end of the line could introduce other troubles if they aren't strictly required (because of signal integrity on long lines at high baud rates).

The receiver input impedance of all the nodes on the bus are in parallel with the two terminators. If you have many nodes, the equivalent impedance on the bus is much small and the partition with bias resistors could reduce the differential voltage between A and B at idle to less than 200mV.

If you don't use true fail-safe transceivers, a fault start bit could be seen by these kind of receivers.

Of course I'm not sure - there are a /lot/ of MCU manufacturers!

UART receivers usually work in the same way, however. They have a sample clock running at 16 times the baud clock. The start bit is edge triggered to give the start of the character frame. Then each bit is sampled in the middle of its time slot - usually at subbit slots 7, 8, and 9 with majority voting. So the stop bit is recognized by subbit slot 9 of the tenth bit (assuming 8-bit, no parity) - the voltage on the line after that is irrelevant. (Even when you have two stop bits, receivers never check the second stop bit - it affects transmit timing only.) What purpose would there be in waiting another 7 subbits before triggering the interrupt, DMA, or whatever?

I've seen a bit of variation, including 8 subbit clocks per baud clock, wider sampling ranges, re-sync of the clock on edges, etc. And of course you don't always get the details of the timings in datasheets (and who bothers measuring them?) But the key principles are the same.

RS-485 requires them - you want to hold the bus at a stable idle state when nothing is driving it. You also want to have a bit of load so that you have some current on the bus, and thereby greater noise immunity.

Receiver load is very small on modern RS-485 drivers.

No, I have not tested every MCU UART ever made. This was the case when I tried using RS-485 many years ago and was in every UART chip available. I forget the number, but there was a particular part made by Western Digital that became the "standard". It blossomed into a family of devices with small improvements in each (typically adding a FIFO and the size of the FIFO). I never saw one of that family which changed this "feature". It's because the purpose of this signal was not to control a driver, but to signal to a CPU the buffer condition. The UART timing is to first align to the middle of the start bit, then continue marking the time for bit centers. When it times the middle of the stop bit, the receiver or transmitter is done and flags the received character is available or the transmitter register is empty. The stop bit is the default value of the line, so nothing further has to be done in the UART, other than the receiver entering the start bit hunt mode.

If a modern UART has a separate signal that flags the end of the stop bit time, that would be great, but I have no reason to think this is available in every case, or any particular case, unless it is documented, *well* documented. Have you seen any parts that specifically indicate they flag the end of the stop bit of characters received or transmitted?

I recall the 8251 USART by Intel was full of bugs and this flag was no exception.

Of course. The trouble is knowing which ones have what!

That depends on the details of the receivers and drivers. I have control over that other than the USB cable. To assure I'm using appropriate RS-422 devices, I could use a TTL cable and on the first test fixture board use a separate connector with TTL level signaling. This would then be buffered on this first board to RS-422 for the rest of the chain. I could do all this with RS232, but it doesn't provide for tristate on the outputs. Not that I recall anyway.

Sorry, I don't know what you mean by "fail-safe" transceivers. Are you talking about the driver or the receiver? Do you mean the internal bias some receivers have, so with zero volts across the input pair they are in a defined state? That can be done through external resistors as well.

I'm expecting to use shorting jumpers for setting various modes on these test fixtures. Do they make any shorting jumpers that are more than just two pins? I've never seen that. I suppose I could make one using a connector and wire.

I think my biggest problem is going to be the mechanical parts of the chassis. I need the boards to be very strong to withstand many insertion removal cycles of the UUTs from these boards. We currently do burn in using production Eurocard format units. They are very stiff with a rail on the front. I just realized, they probably get a lot of stiffening from the massive Eurocard connectors on the back and similar connectors on the front panel. I was planning to use a front panel, but maybe I need to add board stiffeners as well. In the "old" days, when DIPs roamed the earth, these were a combination of power decoupling capacitor and board stiffener. They probably don't even make them anymore. I really need the board to be solid so it doesn't suffer damage from the strain of repeated use. Maybe I just need to bolt a hunk of metal to the card.

The idea is to avoid BUILDING hardware. Plugging cables into a box is not building. There is no soldering iron, oscilloscope, or anything like that in the picture. If you already avoid that, then great.

I wonder if you can use some kind of low-force connector or cable on the card, instead of plugging and unplugging the card from a chassis.

E.g. something like

depending on how many conductors are needed.

You seem to misunderstand. I AM building hardware. There's no choice in that matter. You want me to add other hardware and also software that adds nothing, improves nothing, and just creates more potential problems. This all seems to be because you can't understand the very, very simple nature of a master-slave shared bus and a polling protocol.

The bottom line is, you don't know much about the application, but think you can design it better. Why are you doing this?

The only way to "improve" this, might be to use RS-232 instead of RS-422. But that loses the noise immunity of the differential signaling and would require adding analog switches to the slave transmit outputs. It is also very unspecified, so I'd have to do more engineering to be sure it will work. So RS-422 is looking pretty good to me.

The connectors will be RJ-45 8P8C connectors. They are cheap and easy to plug/unplug. They mount securely to the board and can even be integrated into the front panel rather than hanging out the back. I still need to figure out how to distribute power. It's probably going to be plugs like they use on PC laptop supplies. This will most likely be supplied by a laptop supply, so I'll need a board that splits out the one PSU to 16 power cables. Sounds like a job for perf board and a small, plastic case. The supply on my laptop is 300W. That should do the trick easily.

How about something like this?

That's what I'm going to use. I'd run the power through it as well, but even with the low power I'll be using, 28 ga is a bit small. I'm just not a big fan of the typical barrel power connectors. I'd be happy if they had a detent so they won't fall out. The nylon shell connectors are a pain to remove.

Hi Rick - I have an RS-485 system on my desk using an implementation of the old Intel BitBus. Works fine for a handful of nodes, limited distance, and very simple cabling - but only 62.5kbaud. Good solid technology for 1994 when I designed it...

Why would you use RS-485 instead of CAN? A million chips out there support CAN with no fuss, works at decent speeds over twisted pair, not hard to use.

BTW, another option for interfacing to RS-485 from USB is XR21B1411 which is what I happen to have on my desk.

Hope that helps! Best Regards, Dave

You asked for suggestions and I gave some.