32 UART on a single system

Question

I need to add 32 serial ports to an SBC, for my tests I'm using a raspberry CM4, but it could also be a similar SBC.

I'm trying to figure out what solution is best for having 32 serial ports on my system. From a first analysis I understood that the only usable interfaces are USB and PCIe.

The problem is that all USB-Serial or PCI-Serial bridge chips I could find have at most 4 serial ports.

So my idea is to use 8 of these chips, for example 8 x FT4232H chips. Speed isn't critical, definitely a baud rate under 1Mbps, but I can go even slower if needed.

NOTE: Everything should then be done on a single PCB; therefore, the reasoning must be done at the chip level.

The problem now is, how to connect these 8 chips to my system, I came up with these solutions:

• 8 port USB hub and connect a FT4232 CHIP on each port. But it's not easy to find chips to make 8-port USB hubs.

• Chain of USB HUBs, for example a 4-port USB hub and another 2-port hub on each port. And return to a situation similar to the first one.

• 8-port PCIe hub, but I couldn't find any suitable chip for this. Anyway, then on each PCI port put a PCIe-to-4xUART converter.

• Mixed solution, that is, a 2 port PCie hub, then a PCie-to-4xUSB converter on each port, and I go back to having the 8 USB ports of the first solution.

In your opinion, what could be the best solution?

Are there any other solutions that I haven't come up with?

Answer 1

As for products with more than 4 serial ports on a quick search found:

1. 16-Port Low-Profile Serial Card - RS232 - PCI Express which is a PCIe card with 16 RS232 ports. The chipset is listed as:
   • ASMedia - ASM1083 which is a PCI Express-to-PCI forward bridge
   • SystemBase - SB4002A which connects Local Legacy Bus and PCI Bus
   • SystemBase - SB16C1058 which is an Octal-UART with 256 Byte Transmit/Receiver Deep FIFOs. There are two of these Octal-UARTs on the board.
2. SIO8BX2V which is a PCIe card with Eight Channels with Multi-protocol Transceivers supporting RS422 (V.11)/RS485, RS232 (V.28). Independent Transmit and Receive FIFOs for each Serial Channel 32k byte each channel. General Standards user FPGAs to manage FIFOs which are deeper than than on standard UARTs, which might help in your application.

Another thought is that a FPGA could be used to create a custom design with multiple serial ports.

Answer 2

Disclaimer: I never used analog multiplexers to carry digital signals, so all assumptions below are based on some documents/websites (including manufacturers sites) stating that you can use analog switches/muxes for both analog and digital signals. It would maybe be better to wait for someone more experienced with that kind of multiplexing...

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You could try to use some switches or multiplexers.

TI should release the TMUX9832 sooner or later, which is a 32:1 switch, meaning that this IC allows you to redirect up to 32 data lines into a single one. By putting two of them in parallel, one for your Tx & one for Rx, you may be able to multiplex all of your 32 sensors. However, it will only available in VQFN or NFBGA form factors, which isn’t really suitable for hand soldering (although TI may also produce an "evaluation board" in the future, with all required components already mounted, as many manufacturers do).

You could also use more classic multiplexer ICs such as CD4051B and CD4052B, but you will need to combine several of them. I can think of a combination of one CD4052 & 8 CD4051, which would "only" need 5 GPIO out of your SBC plus Rx and Tx. This combination should, in theory, work (I have never tested this myself, but maybe I'll give it a try). Here is the theory:

On this picture (not a proper schematic, but quite understantable), inputs A,B and C (let's call them A2, B2 and C2) on CD4052B can be shared (so all 4052B have their A pin tied to the same GPIO, B pin to another...); A&B (A1 & B1) on CD4051B are "unique". Connect INH pins to GND. With all A1, B1, A2, B2, C2 low, you should be able to reach the first sensor. Switch A2 to high to reach the 2nd one, etc. Once at the "end", switch A2, B2, C2 to low and switch A1 to high, and now you reach sensor number 9. Iterate until you reached all your sensors.

One thing I like with these CD4050X is that they are available as DIP packages, so can easily be installed on breadboards (for testing) or perfboards. Also, they are quite inexpensive (around $0.5/Unit for CD4052B, $0.45/U for CD4051B), so for less than 10 bucks you should have enough ICs to try out this solution, should you want to.