Modbus over Point to Point Radio MDS SD4s


Thread Starter


I am trying to decide which is better for me with my 9600 link. Do I use Mobus RS-232 or do I use ModbusTCP?

I currently use RS-232, but with these new SD4 radios that do RS-232/RS-485/Ethernet, that I might want to move to an ethernet connection.

Or will all the ethernet Modbus TCP overhead make it actually slower than my RS-232 link at 9600?

I do like the convenience of just using the ethernet connection on the radios, but if it only gets me an effective throughput of 2400bps with all the overhead, it might not be worth it.

Btw, does anyone have any experience with GE/MDS SD family of ethernet radios?

Lynn August Linse

I can't comment on the radio you mention, however you will find that most modern radios (those without extreme +25 mile distances at least), that the over-the-air packets are moving at between 100K and 250k baud. So the time to move 12 bytes instead of 8 has little impact in end-to-end time.

Your use of 9600 on the serial side is likely unrelated to the RF/over-the-air speeds.
Unfortunately, that is the speed of my over the air packets. I am expecting about 9600 baud of total throughput, as I will be operating on a slim 12K or 6.25K channel.

Maybe I using the wrong radios. I am looking for 450MHz or below, going about 8 miles.

Lynn August Linse

> Unfortunately, that is the speed of my over the air packets.

In that case, the Ethernet-route would have huge overhead (but not the 6 byte header). All TCP/IP streams (even over PPP serial-radio) will require various socket open/close, TCP-level ACKs and so on. Over cellular, the TCP 'overhead' can approach 85% or more of the data moved quite fast.

My own gut feeling is that for 8 miles you should focus on a 'good radio signal' and live with what ever speed you get :) Do be aware that at 8 miles, the height of the antenna will be critical, so don't expect to place the radios at chest-level with common antenna and expect the 8 miles distance. At your frequency I'm not sure the frensel-zone height, but the higher your antenna is from the ground the better.