What kind of gsm modems?
What kind of microcontroller?
How long is the communication distance?
- You could use an IC like MAX14830. "Quad Serial UART with 128-Word FIFOs". This is not a cheap IC.
- Or, You could use small microcontrollers to communicate with each gsm modem and then connect them to a main micrcocontroller using i2c or spi. Not sure if this is cheaper solution than the MAX14830, depends what microcontrollers you use.
- Or.. maybe some kind of wireless link that can multiplex.. zigbee or bluetooth.
- Or, buy a microcontroller with enough UARTS.. I think 4 uarts is the maximum you can find, so you could buy two of those and make them communicate with each other via i2c or SPI.
But, there might bee even easier solution.. depending on what kind of GSM modules you have. Maybe there is an interface that you can use.
PS. Why do you need so many GSM modems?